Hereditary risk factors

Inherited or familial pancreatic cancers represent approximately 5–10% of all pancreatic cancers.⁷ At-risk patients for familial pancreatic cancer include those with a minimum of two first-degree relatives with pancreatic cancer.⁸ Although the genetic mutations responsible for the majority of clustering of pancreatic cancer in families have not been identified, several pancreatic cancer genes that are associated with either a known cancer syndrome or chronic inflammatory diseases have been identified (Table 1).⁷

Susceptibility variants

Three genome-wide association studies (GWAS) have been conducted in the United States with a majority of the study population of European ancestry.^25–27 Polymorphic variants of the ABO, NR5A2, LINC-PINT, PDX1, BCAR1/CTRB1/CTRB2, ZNRF3, and TERT genes and a nongene chromosome region 13q22.1 have been identified as susceptibility factors for pancreatic cancer with genome-wide significance (p < 5 × 10⁻⁸). Two additional GWAS have also been conducted in Chinese²⁸ and Japanese populations.²⁹ Post GWAS analyses suggest that genes involved in the pancreas development pathway may play an important role in modifying the risk of pancreatic cancer.^30–32 Initial examination on the interactions of genes with known etiological factors, such as smoking,³³ obesity, and diabetes,^{34, 35} has generated interesting clues to the genetic factors that predispose individuals with such exposure to pancreatic cancer. Identification of the causal alleles and functional characterization of the genes or variants may involve many genes with modest effect on the phenotype.^{36, 37}

Tobacco and alcohol

The risk factor most firmly associated with pancreatic cancer is cigarette smoking. It is estimated that approximately 25% of cases of pancreatic cancer are due to cigarette smoking with a 1.5- to 2.0-fold increased risk.^38–41 The risk increases as the amount and duration of smoking increase. Long-term smoking cessation (>10 years) reduces the risk by approximately 30% relative to current smokers.^{38, 42} Whether smokeless tobacco products, pipe or cigar smoking increases risk remains controversial.^{43, 44}

The mechanisms of smoking-induced pancreatic cancer involve tobacco carcinogen-induced DNA damage and pancreatic tissue injury.⁴⁵ Tobacco carcinogens can be metabolically activated to cause DNA damage and gene mutations.^{46, 47} Nicotine also plays an important role in tumor promotion and progression via nicotinic acetylcholine receptor and adrenergic receptor-mediated signaling in multifactorial events that lead to pancreatic injury.^{45, 46, 48}

The association between alcohol consumption and PDAC has also been investigated. In general, heavy alcohol consumption (30–40 g alcohol or ≥3 drinks/day) but not light or moderate consumption has been associated with increased risk.^49–52 It appears that alcohol may account for 2–5% of all pancreatic cancer.

Obesity

Similar to tobacco, it has been estimated that obesity accounts for another 25% of the pancreatic cancer cases in the US population.⁵³ Positive associations between obesity and pancreatic cancer risk have been reported in several large prospective studies and meta-analyses.^54–62 In addition to BMI (body mass index), abdominal fat, a measured by waist to hip ratio, has been related to increased risk of pancreatic cancer, especially in women.^{60, 61} Obesity at younger age has been associated with a higher risk of pancreatic cancer than weight gain at older age. Furthermore, obesity has been associated with a younger age of disease onset and reduced overall survival (OS).⁶³

Biological mechanisms linking obesity to pancreatic cancer are insulin resistance and inflammation.⁶⁴ Studies using prediagnostic blood samples have shown positive associations of PDAC risk and markers for hyperglycemia and insulin resistance.^65–68 Insulin and insulin-like growth factors are key regulators for cellular metabolism and growth and their signaling transduction networks play an important role in tumor development and progression.^69–71 The mechanisms driving pancreatic carcinogenesis may have similarities to other inflammatory conditions associated with an increased risk of cancer. Fatty infiltration of the pancreas may lead to steatopancreatitis, suggesting a potential link among obesity, nonalcoholic fatty pancreatic disease, nonalcoholic steatopancreatitis, and PDAC.^{72, 73}

Diabetes mellitus and antidiabetic medication

Diabetes mellitus has been implicated as both an early manifestation of pancreatic cancer and a predisposing factor.⁷⁴ A consistent association of long-term diabetes and risk of pancreatic cancer has been reported from three large-scale meta-analyses^75–77 and three pooled data analyses^{68, 78, 79} with each involved more than 1500 PDAC cases. Of note, PDAC can induce peripheral insulin resistance⁸⁰ and type 3c (pancreatogenic) diabetes.⁸¹ It was observed that hyperglycemia could occur 2 years before clinical diagnosis of pancreatic cancer, and one in 125 new-onset diabetics would develop PDAC in 3 years.^{82, 83} Biomarkers that could help to distinguish type II diabetes mellitus from type 3c diabetes are being pursued.^{84, 85}

Since the discovery that biguanides, the most commonly used antidiabetic medications, inhibited pancreatic tumor development in experimental animals,⁸⁶ the role of antidiabetic therapy in PDAC risk reduction has been intensely studied. Other experimental evidence supports the antitumor effect of metformin.^{87, 88} Metformin has diverse regulatory effects and the major molecular targets of metformin are the liver kinase B1 (LKB1)-AMP-activated protein kinase (AMPK) signaling and mammalian target of rapamycin (mTOR) pathways, which are central in the regulation of cellular energy homeostasis, cell division, and cell proliferation.⁸⁹ Metformin also inhibits the inflammatory response associated with cellular transformation and selectively targets the cancer stem cell.^{90, 91} In humans, eight cohort studies^92–98 and three case–control studies^99–101 have examined the impact of metformin use on risk of PDAC. Meta-analysis of these studies estimates RR of 0.63.¹⁰² Two additional studies have reported that metformin use in patients with pancreatic cancer and concurrent diabetes is associated with longer survival and reduced risk of death.^{103, 104}

Pancreatitis and other medical conditions

Chronic pancreatitis is a risk factor for pancreatic cancer, and new onset pancreatitis could be a sign of occult pancreatic cancer.¹⁰⁵ Data pooled from 10 case–control studies with adjustment for obesity and alcohol consumption, demonstrated that the risk of pancreatic cancer was nearly threefold for patients with an interval of >2 years between the pancreatitis and pancreatic cancer diagnoses.¹⁰⁶

Infectious diseases

Helicobacter pylori may cause subclinical pancreatitis or increased gastrin levels, with resultant trophic effects on the pancreas. Several studies have examined the association between H. pylori and pancreatic cancer; with large meta-analyses being showing a weak yet statistically significant association between prior H. pylori infection and pancreatic cancer.¹⁰⁷ It has been hypothesized that ABO genotype/phenotype status influences the behavior of H. pylori that in turn affects gastric and pancreatic secretory function, and these ultimately influence the pancreatic carcinogenicity of dietary- and smoking-related N-nitrosamine exposures, and thus risk of PDAC.¹⁰⁸

Hepatitis B may also be a risk factor for pancreatic cancer.¹⁰⁹ Subsequent studies were conducted mostly in Asian and a meta-analysis of nine such studies reported that the pooled RR was 1.39 (95% CI: 1.22–1.59) in chronic HBV (hepatitis B virus) carriers and 1.41 (95% CI: 0.06–1.87) in past exposure to HBV.¹¹⁰ Hepatitis B surface antigen (HBsAg) has been detected in pancreatic and bile juice,¹¹¹ and there is evidence of HBV replication in pancreatic cells. HBV results in damage to pancreatic exocrine and endocrine epithelial cells via an inflammatory response.^{112, 113}

Dietary factors

Various dietary factors have been suspected to play a role in the development of pancreatic cancer. Generally, high intakes of fat or meat increase the risk; whereas high intakes of fruits and vegetables reduce the risk.⁵ Less is known about which type of fatty acids pose the greatest risk.¹¹⁴ Several studies suggest that the method of meat preparation (deep fried, grilled, or barbequed), and subsequent intake of food mutagens may contribute to the development of pancreatic cancer.^115–117

The associations of dietary carbohydrates, refined sugars, and glycemic index or load with pancreatic cancer have been investigated in many studies but the results are inconsistent. A recent meta-analysis of data from eight prospective cohort studies concluded that there was no association among diets high in glycemic index, glycemic load, total carbohydrates or sucrose, and pancreatic cancer risk.¹¹⁸

Decreased pancreatic cancer rates have been associated with the high consumption of vegetables, citrus fruits, fiber, and vitamin C.⁵ It remains uncertain whether vitamin D has a protective effect on PDAC.¹¹⁴

Occupational exposures

The role of occupational or industrial factors in pancreatic cancer has been investigated extensively. Increased risk of pancreatic cancer has been associated with exposures to some chemicals (e.g., organochlorines, chlorinated hydrocarbons, and formaldehyde) or some specific occupations (e.g., stone miners, cement workers, gardeners, and textile workers).^{4, 88} The strongest and most consistent occupational risk factors associated with pancreatic cancer to date are chlorinated hydrocarbons and polycyclic aromatic hydrocarbons.⁸⁸

In summary, cigarette smoking and obesity may each be responsible for as many as 25% of the cases of pancreatic cancer implying that half of pancreatic cancer cases may be preventable.^{119, 120} Conversely, the proportion of pancreatic cancer cases attributable to known inherited pancreatic cancer syndromes is small. Identification of individuals with excessively high risk of pancreatic cancer may allow for focused screening.^121–123 Thus far, proved benefit to screening remains elusive, except in highly selective populations such as high-risk familial pancreatic cancer kindreds.¹²⁴ Currently, for those individuals determined to have structural,¹²⁴ or blood-based abnormalities,¹²⁵ suggesting premalignancy or early stage neoplasm, management remains extremely controversial. Current options range from close observation to aggressive surgical intervention with a variety of approaches in evolution.^126–128

Histopathology of exocrine pancreatic neoplasms

PDAC accounts for approximately 95% of all neoplasms arising in the pancreas.¹³³ On histological examination of surgically resected tumors, it is not uncommon to find neoplastic cells infiltrating into lymphatic structures or into the many nerve endings enervating the pancreatic bed, which may explain the pain associated with PDAC. Another defining histological feature of PDAC is an exuberant host stromal reaction to the cancer (desmoplasia), wherein smooth muscle actin expressing myofibroblasts generate abundant extracellular collagen that surrounds infiltrating neoplastic glands (Figure 2).¹³⁴ Other components of this host desmoplastic response include secreted glycosaminoglycans (e.g., hyaluronan) that contribute to high interstitial pressures and impede the passive efflux of chemotherapeutics from the circulation into the peritumoral milieu.¹³⁵

PDAC arises through a stepwise histological progression of well-recognized precursor lesions. The most common precursor lesions are microscopic, and known as pancreatic intraepithelial neoplasia or PanIN, which are almost always present in the pancreatic parenchyma surrounding an invasive cancer. PanINs vary from innocuous low-grade PanIN-1 lesions to the ominous highest grade PanIN-3 (previously known as carcinoma in situ), wherein the constituent cells morphologically resemble PDAC, except for their containment within a basement membrane. PanIN-3 lesions share many of the genetic aberrations found in infiltrating cancer. Of note, while PanIN-1 and -2 lesions can occur incidentally (with autopsy studies reporting as many as 50% of elderly individuals with low-grade PanINs in their pancreas),¹³⁶ PanIN-3 is essentially never found in the absence of a synchronous PDAC.¹³⁷ Thus, the emergence of high-grade PanIN-3 lesions likely represents the critical inflection point in the multistep progression of PDAC. By definition, and in contrast to cystic precursor lesions of PDAC (see below), isolated PanINs are not seen on abdominal imaging, although individuals with germ line mutations or a familial predisposition may have structural pancreatic changes discernible on endoscopic ultrasound (EUS).¹²⁸

Cystic precursor lesions of PDAC comprised two distinct entities—intraductal papillary mucinous neoplasms (IPMNs) and mucinous cystic neoplasms (MCNs).¹³⁸ IPMNs arise in either the main duct or one of the side branches of the pancreas and are characterized by broad “finger-like” papillae and secretion of abundant extracellular mucin. MCNs are less common and occur in women with significantly greater propensity than men (9 : 1 ratio). In contrast to IPMNs, MCNs have no communication with the ductal system. Akin to PanINs, the epithelial lining of cystic precursor lesions also demonstrates histological progression from low-grade (adenoma) through high-grade (carcinoma in situ) to invasive cancer. Approximately 25–33% of surgically resected cystic lesions are associated with an invasive adenocarcinoma, and the onset of invasion is associated with a significant reduction in median OS.^{139, 140} The precise timeline of progression from noninvasive cyst to invasive carcinoma is not known, although the median age of presentation for cystic lesions is about a decade earlier than the median age for PDAC, suggesting that this might reflect the period required for progression. Of note, the explosion in the use of abdominal imaging for symptoms not related to the pancreas has led to a veritable “man-made” epidemic of incidentally discovered cystic lesions of the pancreas. As the overwhelming majority of these asymptomatic cysts are not likely to progress to invasive cancer, radiological criteria have been proposed that help distinguish “high risk” from “indolent” cysts.^{141, 142} Intensive efforts are underway to identify molecular markers from cyst fluid or blood that would further refine this distinction and help prevent unnecessary surgeries and frequent monitoring.¹²³

Other uncommon exocrine tumors include (1) acinar cell carcinoma, which bears morphological resemblance to pancreatic acini, wherein the neoplastic cells express acinar enzymes, such as trypsin and chymotrypsin;¹⁴³ (2) solid pseudopapillary neoplasm, a cystic tumor of uncertain histogenesis that often arises in the tail of the pancreas in young adults, and typically has a favorable prognosis;¹²² (3) serous cystic neoplasm, a cystic tumor characterized by serous (watery) secretion, in contrast to the mucinous contents of IPMN and MCN, and which does not harbor a risk of progression to PDAC;¹⁴⁴ and (4) pancreatoblastoma, the most common pancreatic tumor in children.¹⁴⁵

Approximately 2% of primary tumors arise from the endocrine compartment (islets of Langerhans) and are known as pancreatic neuroendocrine tumors or PanNETs or “islet cell tumors.”¹⁴⁶ PanNETs can be nonfunctional in up to a quarter of cases, or they can express a variety of islet cell hormones such as insulin or glucagon, which may lead to characteristic syndromes owing to systemic hormonal excess. PanNETs do not form glandular structures and are typically not associated with an exuberant host desmoplastic reaction. PanNETs can be readily identified based on expression of neuroendocrine markers such as chromogranin or synaptophysin in tissue sections.

Molecular pathology of exocrine pancreatic neoplasms

The advent of next-generation sequencing technology has enabled considerable advances over the last 5 years in elucidating the genomic landscape of PDAC,¹⁴⁷ as well as that of other variant neoplasms of the exocrine and endocrine pancreas.^{148, 149} The International Cancer Genome Consortium (ICGC) has sequenced hundreds of PDAC, where the sequencing data is publicly available.¹⁵⁰

By far, the most common genetic abnormality in the multistep pathogenesis of PDAC happens to be oncogenic mutation of KRAS, located on chromosome 12p, which encodes for Ras, a membrane-bound small GTPase.¹⁵¹ Somatic mutations of KRAS are reported in >90% of PDAC cell lines and patient-derived xenografts, although the frequency is somewhat lower (75–90%) in primary tumor samples. The majority of KRAS mutations cluster at codon 12, with mutations also found at codons 13 or 61, respectively. Somatic KRAS mutations result in constitutive activation of the GTPase activity of the encoded Ras protein, which, in turn, activates multiple downstream effector pathways.¹⁵² Somatic KRAS mutations are present even in the lowest grade PanIN lesions, underscoring its status as an early genetic “hit.”¹⁵³ In genetically engineered mouse models (GEMMs), expression of a mutant KRAS in the pancreas results in preinvasive neoplasia closely recapitulating human PanINs, with a minority of animals progressing to full-fledged invasive cancer.¹⁵⁴ Additional genetic events, such as expression of mutant p53 or biallelic deletion of the Ink4a/Arf locus, significantly accelerate the natural history of Ras-induced pancreatic neoplasia.^{155, 156} Conversely, in GEMMs with an inducible mutant KRAS allele, turning off Ras expression in established PDAC, leads to variable tumor regression.^{157, 158} Not surprisingly, a multitude of avenues have been explored toward pharmacological inhibition of aberrant Ras activity in PDAC and other KRAS-mutant tumors,^159–161 but the clinical results overall have been disappointing. Nonetheless, Ras remains an attractive target, and a large federally funded “RAS Initiative” is currently underway at the National Cancer Institute to identify strategies to block Ras in PDAC and other cancers (http://www.cancer.gov/researchandfunding/priorities/ras).

Somatic inactivation of the CDKN2A/INK4A gene, which encodes for the cell cycle regulator p16, is another early genetic event in the multistep progression of PDAC. The p16 protein inhibits the cyclin D1/Cdk-4 complex that normally acts to constrain the retinoblastoma (Rb) protein via phosphorylation.¹⁶² In approximately 95% of PDAC, the CDKN2A/INK4A gene is inactivated by one of several mechanisms, including homozygous deletion, the loss of one allele combined with an intragenic mutation in the other allele, or epigenetic inactivation via promoter methylation. A potential therapeutic vulnerability that emerges in the setting of p16 loss pertains to the dependence on Cdk4 activity for unregulated proliferation, which can be targeted using small molecule inhibitors of Cdk4 function.¹⁶³

In the multistep progression model of PDAC, loss of the TP53, SMAD4/DPC4, and BRCA2 tumor suppressor genes are considered later genetic events.¹⁶⁴ Somatic mutations of TP53 are found in approximately 75% of human PDAC, while nuclear accumulation of p53 protein on immunohistochemistry, indicative of an underlying mutation, occurs in the majority of PanIN-3 lesions.¹⁶⁵ The SMAD4/DPC4 tumor suppressor gene on chromosome 18q21 encodes for an intracellular transducer of the transforming growth factor-β signaling pathway that inhibits cell growth under physiological conditions. The SMAD4/DPC4 gene is inactivated in approximately 55% of human PDAC through either homozygous deletions or the loss of one allele coupled with an intragenic mutation in the second allele.¹⁶⁶ Epithelial Dpc4 expression is nearly always retained in low-grade PanIN lesions and in noninvasive cystic precursor lesions of PDAC, reiterating that SMAD4/DPC4 abnormalities generally occur concomitant with progression to invasive cancer.^{167, 168} Overall, KRAS, CDKN2A/INK4A, TP53, and SMAD4/DPC4 are the “big four” peaks in the genomic landscape of PDAC, each mutated in over 50% of cases. Patients that harbor in all four genes tend to have a significantly worse OS than those with mutations in 1 or 2 of these genes, underscoring their role in prognosis.¹⁶⁹

The BRCA2 gene, which has been discussed in the context of familial predisposition to PDAC, is also somatically mutated in approximately 5% of cancers. The BRCA2 protein plays a critical role in homologous recombination-mediated repair (HRR), and neoplastic cells with BRCA2 or related gene defects in the HRR pathway are exquisitely sensitive to double-strand breaks induced by agents such as cisplatin or mitomycin C.¹⁷⁰ Another newer class of agents that is being selectively evaluated in tumors with BRCA2 mutations is PARP ([poly (ADP-ribose) polymerase]) inhibitors, which inhibits a second mechanism of DNA repair in cells already compromised in HRR. The concurrent PARP inhibition in BRCA2-mutant PDAC is expected to create a “synthetic lethality” vis-à-vis DNA repair that can be exploited toward a therapeutic advantage.¹⁷¹

Although a comprehensive tabulation of genetic abnormalities in PDAC is beyond the scope of this book, significant molecular aberrations are observed within every interrogated cellular component, including DNA methylation, histone marks, coding and noncoding RNAs (latter incorporating both long noncoding RNA and microRNAs), and protein. Many of these abnormalities, such as DNA mutations, aberrant DNA methylation, or deregulated microRNAs, are considered candidate biomarkers for the diagnosis of PDAC in surrogate biospecimens such as blood or pancreatic juice.^172–175 Newly emerging areas of biomarker discovery for early diagnosis of PDAC, and in monitoring for recurrence postresection, include circulating cell-free mutant DNA (cfDNA) and DNA contained within extracellular microvesicles (exosomes),^{176, 177} which can be detected using ultra-sensitive PCR techniques.¹⁷⁸

Molecular biology of exocrine pancreatic neoplasms: Translational implications

Below are a few selected examples of recent advances made in understanding the tumor biology of PDAC that have translational relevance within the field of cancer medicine.

A genetic timeline for PDAC progression: The overwhelming majority of PDAC patients present with advanced disease, and early detection remains one of the most promising avenues for improving OS. In a seminal report of PDAC autopsy cases that compared the genomic aberrations present in primary versus synchronous metastatic lesions, investigators calculated that almost two decades might be required for progression from the initial mutant clone(s) to terminal metastatic disease.¹⁷⁹ No more than 2 years of this prolonged genetic timeline occurs during the symptomatic phase of disease, reiterating the importance of elucidating clinically applicable biomarkers that can identify emerging PDAC at a preclinical stage.

PDAC stroma: friend or foe? Traditionally, the cancer-associated myofibroblasts within the exuberant desmoplastic stroma in PDAC have been considered an ally of the invasive cancer.^{180, 181} One pathway that emerged as particularly relevant in facilitating tumor-stroma crosstalk is the Hedgehog signaling pathway,¹⁸² and depletion of stromal myofibroblasts in PDAC GEMMs using short-term therapy with Hedgehog inhibitors was shown to enhance chemotherapy delivery in preclinical studies.¹⁸³ This prompted a randomized clinical trial of an inhibitor of Hedgehog signaling in combination with gemcitabine, but rather unexpectedly, the inhibitor to paradoxical disease progression (www.businesswire.com/news/home/20120127005146/en/Infinity-Reports-Update-Phase-2-Study-Saridegib). A more recent series of studies using both genetic and pharmacological ablation of myofibroblasts in PDAC GEMMs have been instrumental in implicating a tumor-constraining role for stroma.^184–186 Specifically, stromal depletion led to tumor dedifferentiation and increased metastases, with reduced median survival of mice, mimicking the clinical findings. These paradigm-shifting reports have led to a reassessment of the role of stroma in PDAC. One approach that has been proposed in preclinical models is a shift away from stromal ablation to stromal “reprogramming,” wherein myofibroblasts are still retained in situ, but pharmacological strategies are used to revert their transcriptional profiles to a basal, resting state.¹⁸⁷

Overcoming immune tolerance in PDAC: Cancer immunotherapy has emerged as one of the most promising therapeutic strategies over the last 5 years with agents known as checkpoint inhibitors resulting in dramatic improvements in survival for patients with melanoma.¹⁸⁸ To date, these successes have not translated to PDAC.^{189, 190} Some of these failures have been attributed to the unique tumor microenvironment of PDAC. In contrast to certain “immunogenic” cancers such as melanoma, the immunological infiltrate in the PDAC microenvironment is sparse in effector T cells, and populated by cells that dampen tumor rejection, including regulatory T cells, macrophages, and suppressor cells of myeloid origin.^191–193 This immunosuppressive milieu contributes to a state of immune tolerance and facilitates tumor progression. As immunotherapy of PDAC continues to evolve, there is increasing recognition of the importance of combination regimens, such as vaccines to induce an antigen-specific T-cell response in conjunction with blockade of checkpoints,¹⁹⁴ or adoptive therapy using antigen-specific engineered T cells in conjunction with checkpoint inhibitors.¹⁹⁵

Metabolic vulnerabilities in PDAC: As proposed by Otto Warburg nearly a century ago,¹⁹⁶ cancer cells are known to mostly rely on aerobic glycolysis of exogenous glucose for their energy needs. In addition to glucose, exogenous glutamine is a key elemental fuel for PDAC survival and proliferation.¹⁹⁷ Upon uptake, glutamine is converted by the enzyme glutaminase into glutamate. Not surprisingly, mutant Ras appears to be a critical orchestrator of the intracellular reprogramming. Mutant Ras rewires the metabolic machinery such that glutamate is oxidized via a noncanonical pathway distinct from normal cells, without increasing deleterious intracellular reactive oxygen species levels.¹⁹⁸ In preclinical models, this unique glutamine utilization mechanism is essential for cancer cell survival and has generated enthusiasm about targeting glutaminase or other enzymes of this pathway in PDAC. Second, mutant Ras has been shown to promote dependence on autophagy, a nutrient recycling mechanism wherein lysosomal degradation of damaged proteins, macromolecules, and organelles, generates key intermediates for bioenergetics.¹⁹⁹ Autophagy can be blocked using agents such as chloroquine and has shown promising results in preclinical settings.²⁰⁰ Finally, mutant Ras leads to profound increase in macropinocytosis, a primitive nutrient uptake mechanism that functions via endocytosis of extracellular fluid and nutrients via the so-called macropinocytic vesicles originating from the cell membrane.²⁰¹ Macropinocytosis in PDAC cells enhances uptake of albumin and other extracellular proteins that are catabolized into essential amino acids channeled into various anabolic pathways. Thus, inhibition of macropinocytosis can be leveraged as potential therapeutic avenue. Alternatively, the propensity of PDAC cells to uptake extracellular albumin can be used as a “Trojan horse” strategy, by conjugating therapeutic agents with albumin. In fact, there is some evidence to suggest that the enhanced efficacy of nano-albumin-conjugated paclitaxel might be, at least in part, due to the macropinocytosis-induced uptake of the drug within KRAS-mutant cancer cells.

Taken together, these results provide greater understanding of the complexity of PDAC’s underlying biology with protracted genomic evolution, a dynamic microenvironment, and distinct differences in metabolism between normal and malignant cells. These insights suggest strategies to modulate the tumor’s cellular and extracellular defense mechanisms. A few specific examples translating these recent laboratory-based discoveries to the clinical arena will be discussed in the section titled “Future Directions” at the end of this chapter.

Classification of clinical stage

It is critically important to use standardized, objective radiologic criteria for clinical staging. Modern imaging techniques have revolutionized the clinical staging of pancreatic cancer. Precise and objective anatomic radiographic criteria are used to determine the extent of the tumor–vascular relationship and to categorize clinical staging. The clinical stage of pancreatic cancer can be broadly divided into patients with inoperable disease (metastatic or locally advanced) and localized disease (borderline resectable or resectable); see Table 3 for specific radiographic criteria. The majority of patients will present with metastatic disease, as evidenced by ascites/peritoneal implants, liver, or lung metastases. Confirmation of metastatic disease can be achieved with percutaneous biopsy of the metastatic lesion (preferred) or at the time of endoscopy (EUS-FNA (fine needle aspiration)). In the absence of metastatic disease, the clinical stage is determined by the relationship of the primary tumor to adjacent vasculature. Radiographic findings of a potentially resectable pancreatic cancer (AJCC stages I or II) are (1) the absence of tumor-arterial abutment or encasement and (2) <50% involvement of the superior mesenteric vein (SMV)/portal vein (PV) (Figure 3). As a general rule, any tumor abutment (≤180° tumor-vessel interface) or encasement (>180°) of the celiac axis, common hepatic artery, or superior mesenteric artery (SMA) should be considered a contraindication to immediate surgery. A patient is deemed to have locally advanced, unresectable disease (AJCC stage III) when (1) the tumor encases the SMA or celiac axis, as defined by >180° of the circumference of the vessel (Figure 4), or (2) there is occlusion of the SMPV (superior mesenteric-portal vein) confluence without the possibility for venous reconstruction. Patients who have tumor abutment, without encasement, of the SMA or celiac axis, or short segment encasement of the hepatic artery are considered to have borderline resectable pancreatic cancer. In addition, patients with tumors that cause >50% narrowing or short segment occlusion of the SMV/PV that may be amenable to reconstruction are also considered to be borderline resectable. There is emerging consensus that even more subtle tumor-vein abutment may be best considered borderline resectable (especially as neoadjuvant therapy becomes more widely accepted).

Imaging

As accurate characterization of tumor–vascular relationship is critical for clinical staging, interventions such as EUS and ERCP (endoscopic retrograde cholangiopancreatography, which can induce pancreatitis and interfere with accurate assessment of the extent of disease) should be deferred briefly until high-quality imaging is obtained. The single most important imaging tool for the detection and staging of pancreatic cancer is computed tomography (CT).^{202, 203} Current multidetector protocols utilize dual-phase technique. A rapid injection of intravenous contrast allows for the maximal enhancement of the pancreas and mesenteric vasculature.²⁰⁴ The first (arterial) phase is used for visualization of the primary tumor and optimal assessment of the tumor–arterial relationship. The second (venous) phase aids definition of the relationship of the tumor to the surrounding venous structures (SMV, PV, and splenic vein) and may uncover metastases to locoregional lymph nodes and distant organs (particularly to the liver).

If a low-density mass is not seen on CT, the presence of biliary or pancreatic duct obstruction remains concerning for malignancy. Additional imaging with magnetic resonance imaging (MRI) can be considered. Several studies have demonstrated that small pancreatic lesions (<2 cm) are more conspicuous on MRI relative to CT.^{205, 206} As in CT imaging, the hypovascular nature of pancreatic cancer may make tumors best seen on early phase (arterial) sequences.²⁰⁴ Ultimately, patients with suspicion of pancreatic cancer without evidence of a mass should undergo EUS. In addition, EUS combined with FNA may provide tissue confirmation of malignancy. Rarely, patients may present with radiographic features concerning for pancreatic cancer without an identifiable mass on CT, MRI, or EUS. If the patient is jaundiced, an ERCP may provide additional radiographic information to include evidence for a double duct sign or smooth tapering more suggestive of sequelae of pancreatitis (Figure 5). In these settings, biliary brushings can provide material for cytologic evaluation. Importantly, pancreatic cancer remains the major diagnostic consideration in patients (without a history of recurrent pancreatitis or alcohol abuse) who have a malignant-appearing stricture of the intrapancreatic portion of the common bile duct or the presence of pancreatic duct dilation without radiographic evidence of a mass. In the presence of an elevated CA 19-9 level, repeat EUS should be considered and if a cytologic diagnosis cannot be confirmed, upfront surgery would be the obvious next step assuming that the patient is otherwise healthy with no contraindications for major abdominal surgery.

Tissue acquisition

Confirmation of malignancy is required in all patients with locally advanced or metastatic disease before treatment with systemic therapy or radiotherapy. For patients with radiographic evidence of metastatic disease, percutaneous biopsies of the metastatic site can usually be obtained with either US- or CT guidance. For patients with localized disease that may be amenable to surgical resection, we prefer EUS-guided FNA biopsy to minimize the risk of tumor seeding. Furthermore, EUS-guided biopsy is much safer and more accurate than percutaneous CT-guided biopsy for tumors that are smaller or difficult to access using CT guidance.²⁰⁷ A negative cytology from EUS-guided FNA should not be considered proof that a malignancy does not exist, and repeat EUS-guided FNA may improve diagnostic accuracy.²⁰⁸ Of note, FNA biopsy should only be performed if a mass is visualized; blind biopsy of the pancreas is inappropriate.

Laparoscopic staging

In the past, laparoscopy was performed in patients who had radiologic evidence of localized pancreatic cancer.²⁰⁹ However, with the advent of high-quality contrast-enhanced CT, the role of laparoscopy in staging (when performed under a separate anesthesia induction) appears to be decreasing. Moreover, the yield of staging laparoscopy may be improved in those patients with very high levels of serum carbohydrate antigen (CA) 19-9, and acquisition of preoperative CA 19-9 levels is recommended.^{210, 211} If the suspicion of extra-pancreatic disease is high, or if the patient is a marginal surgical candidate based on medical comorbidities, laparoscopy as a stand-alone procedure may be appropriate.

Biliary drainage

In patients with localized pancreatic cancer, placement of endobiliary stents before surgery relieves the symptoms of biliary obstruction and facilitates the normalization of liver function. The latter is particularly important for patients who will be treated with neoadjuvant therapy, as hyperbilirubinemia prevents the safe delivery of systemic therapy. If neoadjuvant therapy is planned, biliary complications, including stent occlusion and cholangitis, can be minimized with insertion of a short metal stent after cytologic confirmation of cancer.²¹² For patients with locally advanced or metastatic pancreatic cancer, an endobiliary self-expandable metal stent has superior long-term patency compared with plastic stents.^{213, 214}

Certain groups of patients with symptomatic distal bile duct obstruction should not undergo endobiliary metal stent placement. Although covered metal stents can also be removed endoscopically,²¹⁵ they should rarely be used to provide temporary biliary decompression in a patient without a tissue diagnosis of malignancy.

Surgical considerations

If the primary tumor cannot be resected completely (gross complete resection), surgery for pancreatic cancer (pancreaticoduodenectomy) offers no survival advantage. Patients who undergo a gross incomplete resection have an expected median survival of <1 year.²¹⁶ Even microscopically positive surgical margins have a negative impact on OS and the ability to obtain a margin negative resection should be viewed as necessary for long-term survival.²¹⁷

Upfront surgery and adjuvant therapy

Currently, the standard of care for early-stage, resectable disease is surgery followed by adjuvant therapy. Owing to recent advances in operative technique, anesthesia, and critical care, the 30-day in-hospital mortality rate is 2% or less in patients who undergo pancreaticoduodenectomy performed by experienced surgeons.^218–220 Data have also demonstrated that higher hospital volume is associated with lower surgery-related mortality.²²¹ Upfront surgical resection followed by adjuvant therapy is potentially curative. However, median survival times of 22–24 months have remained stagnant for the last 30 years using this approach.^{222, 223}

Pancreaticoduodenectomy

The standard surgical procedure for neoplasms of the pancreatic head and periampullary region is pancreaticoduodenectomy. The current technique of pancreaticoduodenectomy has evolved from the procedure first described by Whipple and colleagues in 1935.²²⁴ and incorporates selected aspects of the traditional Whipple procedure and emphasizes the importance of removing all soft tissue to the right of the SMA. The surgical resection is divided into six clearly defined steps (Figure 6); the most oncologically important and difficult part of the operation is step six, during which the pancreas is divided and the specimen is removed from the SMPV confluence and the right lateral border of the SMA.²²⁵

The high incidence of local recurrence after standard pancreaticoduodenectomy requires particular attention to the SMA margin, the soft tissue margin along the right lateral border of the proximal SMA (Figure 7a).²²⁶ It is critical that this margin be identified for the pathologist and assessed histologically; the residual disease status (termed “R” factor) cannot be determined if the retroperitoneal margin is not assessed histologically.^{220, 227}

Vascular resection

The goal of vascular resection is to obtain an R0 resection, which requires complete removal of the tumor from the SMV/PV, and exposure of the SMA to allow sharp dissection of the tumor off this artery. Venous resection and reconstruction should be performed for tumors adherent to the SMV or SMPV confluence (Figure 8) or in the presence of short segment occlusion of the SMV/PV with no tumor encasement of the SMA or celiac axis.^{220, 225}

A very select proportion of patients with locally advanced disease (as currently defined) may be considered for surgery (after neoadjuvant therapy) if (1) tumor encasement of the celiac axis or common hepatic artery can be removed with an Appleby procedure (extended distal pancreatectomy with celiac resection) and (2) there is evidence of radiographic, biochemical (CA 19-9), and physiologic response (improved PS) to preoperative therapy. In these patients, neoadjuvant chemoradiation is recommended and surgical candidacy is reassessed upon the completion of therapy. The most common operation performed for such patients has been a distal pancreatectomy, splenectomy, and en bloc resection of the celiac axis (Appleby procedure). In the small series of patients where resection and arterial reconstruction has been performed, results have been encouraging.^{228, 229}

Pylorus preservation

Currently, available findings from randomized trials suggest no difference in perioperative factors or patient outcome between standard and pylorus-preserving pancreaticoduodenectomy.^230–232 Most investigators would agree that pylorus preservation should not be performed in patients who have bulky tumors of the pancreatic head or duodenal tumors involving the first or second portions of the duodenum.

Minimally invasive pancreatectomy

Interest in applying minimally invasive techniques to pancreatic resection stems from the hypothesis that open surgical resection is associated with substantial physiologic stress and morbidity. While the advantages and appropriateness of minimally invasive approaches are still controversial, from 1998 to 2009, the rates of minimally invasive pancreatectomy have risen from 2.4% to 7.3%.²³³ Minimally invasive procedures include both laparoscopic and robotically assisted procedures.

Laparoscopic distal pancreatectomy is the most common minimally invasive pancreatic operation performed as there is no requirement for anatomic reconstruction after the tumor is removed. Although no prospective trials have been performed comparing open versus laparoscopic distal pancreatectomy, multiple retrospective studies have reported favorable perioperative outcomes, with decreased blood loss, complications rates, and shorter hospital stays associated with the laparoscopic approach.^{234, 235} The oncologic impact of open versus laparoscopic surgery has been less rigorously reported. Only three studies have described the oncologic outcomes of patients undergoing laparoscopic distal pancreatectomy.^236–238 The largest study was a multicenter, retrospective matched cohort analysis; no differences in R0 resection status (74% vs 66%), lymph nodes examined (14 vs 12), or median OS (16 months vs 16 months) were observed between open and laparoscopic distal pancreatectomy.

In contrast to laparoscopic distal pancreatectomy, which is offered in most major centers, minimally invasive approaches to pancreaticoduodenectomy for pancreatic cancer have not yet achieved widespread acceptance. Few centers have a reported experience with 30 or more patients.^{239, 240} Among these studies, conversion to an open procedure occurred in approximately 6–10% of cases.

Robotic-assisted pancreaticoduodenectomy has also been reported with perioperative complication rates that appear comparable to the laparoscopic approach.^241–243 Similar to distal pancreatectomy, the oncologic outcomes for minimally invasive pancreaticoduodenectomy are limited. In one of the largest series of minimally invasive pancreaticoduodenectomy for cancer, 108 patients were reported and their survival duration was no different than the larger cohort of patients who underwent an open pancreaticoduodenectomy. The authors suggested that with a minimally invasive approach, more patients may receive adjuvant therapy and in a more timely manner. However, time to the receipt of adjuvant therapy (duration of time from the date of surgery to the start of systemic therapy) has recently been shown to be less important than whether or not the patient receives all intended postoperative therapy.²⁴⁴

Pathologic (surgical) staging

The staging system of the AJCC and International Union Against Cancer is given in Table 4. The modifications to the TNM staging system in the sixth edition allow for the accurate staging of patients even if they do not undergo pancreatic resection. The T4 (and stage III) designation is reserved for locally advanced unresectable primary tumors in the absence of distant metastases.²²⁶

Source: Greene 2002.²²⁶ Reproduced with permission of Springer.

When the (pathologic) pancreaticoduodenectomy specimen is evaluated, the surgeon and pathologist should first evaluate frozen sections of the common bile duct transection margin and the pancreatic transection margin. The SMA margin (the soft tissue margin directly adjacent to the proximal 3–4 cm of the SMA—Figure 7b) is then evaluated on permanent sections by inking the margin and sectioning the tumor perpendicular to the margin.²²⁰ The R status of resection cannot be determined if the SMA margin is not assessed histologically. Most importantly, the surgeon and pathologist should classify this margin after integrating the operative findings and the histologic assessment of this margin. All pancreatic resections should be classified according to the R status: R0, no gross or microscopic residual disease; R1, microscopically positive surgical margins with no gross residual disease; and R2, grossly evident residual disease. The R designation should appear in the final operative report and should be consistent with the pathology report. The final pathologic evaluation of the permanent sections should include a description of the tumor histology and differentiation, gross and microscopic evaluation of the tissue of origin (pancreas, bile duct, ampulla of Vater, or duodenum), and a measurement of the maximum transverse tumor diameter, involved lymph nodes and total lymph nodes examined, and the presence or absence of perineural, lymphatic, and vascular invasion.

Prognostic factors

Investigators have examined pathologic factors of the resected tumor in an effort to establish reliable prognostic variables associated with decreased survival.^{218, 245} Regional lymph node metastases, poorly differentiated histology, and increased size of primary tumors have all been associated with decreased survival duration. Nevertheless, heterogeneity in patient outcomes remains. For example, in one study of 45 patients with pancreatic adenocarcinoma who underwent complete resection with both negative resection margins and negative regional lymph nodes, median survival was 32 months with a 5-year survival of 40%.²⁴⁶ However, another group reported on 12 five-year survivors (following complete resection); 4 had poorly differentiated histology, 5 had metastatic disease in regional lymph nodes, 9 had histologic evidence of malignant extrapancreatic soft tissue extension, and 10 had evidence of perineural invasion.²⁴⁷ Similarly, in a report of actual 5-year survivors, 36% were node positive, and 20% of all node positive patients survived 5 years; this last report utilized preoperative therapy before surgical resection.²⁴⁸