Introduction

INTRODUCTION

Thyroid cancer is the most common cancer of the endocrine system. It is the fifth most common cancer diagnosed in females in the United States and is increasing in incidence among both males and females, with an estimated 62,500 new cases diagnosed in 2016 (Fig. I-1).¹,²,³ The primary histologic subtypes include differentiated thyroid cancer (DTC: papillary and follicular), medullary thyroid cancer, and anaplastic thyroid cancer. In recent years, both the American Thyroid Association (ATA) and the National Comprehensive Cancer Network have published guidelines on the initial evaluation of patients with thyroid nodules and thyroid cancer. These have included indications for fine-needle aspiration (FNA) biopsy, use of molecular testing for cytologically indeterminate thyroid nodules, indications for thyroidectomy and lymphadenectomy, and postoperative surveillance strategies.⁴,⁵,⁶,⁷

Given the scope of these previously published works, the intent of this section is to focus on the preoperative and intraoperative aspects of the evaluation and treatment of a patient who is undergoing surgery for a biopsy-proven papillary thyroid cancer (PTC), which is the most common subtype. In view of the low mortality rates associated with PTC, recurrence has emerged as the primary outcome of interest when determining the optimal surgical treatment algorithm in the majority of patients. Therefore, the ultimate goals of initial and reoperative surgery are to minimize rates of locoregional recurrence and to limit operative complications. Primary among these complications are permanent hypoparathyroidism and recurrent laryngeal nerve injury, which may substantially affect patients’ quality of life.⁸,⁹ Multiple studies have demonstrated that these complications occur less frequently when operations
are performed by experienced surgeons and that the extent of surgery can increase the risk of complications, irrespective of a surgeon’s case volume.⁴,¹⁰,¹¹,¹² This work will not focus on patients with papillary thyroid microcarcinoma (cancers <1 cm), given the ongoing controversy regarding the clinical significance of the majority of these tumors.¹³,¹⁴,¹⁵

FIGURE I-1 New cases of thyroid cancer by histology per year in the United States (1993-2012). APC is significantly different from zero (p < 0.05). APC = annual percent change.

Clinical Staging

Using the American Joint Committee on Cancer tumor-node-metastasis (TNM) staging system (Table I-1)¹⁶ is strongly encouraged for all thyroid cancers because it provides prognostic information that can be used to guide treatment and surveillance and

because it also allows for risk-stratified classification of disease that can be consistently communicated to other providers. In light of emerging data about outcomes of patients with low-risk thyroid cancer, the eighth edition of the American Joint Committee on Cancer staging system modified definitions of primary tumor and regional lymph node metastases. In addition, the age cut-off for determining disease-specific survival was changed from 45 to 55 years.¹⁶

TABLE I-1 American Joint Committee on Cancer Staging Criteria for Thyroid Cancer, 8^th Edition

TABLE I-1A Definition of Primary Tumor (T)

Papillary, Follicular, Poorly Differentiated, Hürthle Cell, and Anaplastic Thyroid Carcinoma
Category	Criteria
TX	Primary tumor cannot be assessed
T0	No evidence of primary tumor
T1	Tumor ≤2 cm in greatest dimension limited to the thyroid
T1a	Tumor ≤1 cm in greatest dimension limited to the thyroid
T1b	Tumor >1 cm but ≤2 cm in greatest dimension limited to the thyroid
T2	Tumor >2 cm but ≤4 cm in greatest dimension limited to the thyroid
T3	Tumor >4 cm limited to the thyroid, or gross extrathyroidal extension invading only strap muscles
T3a	Tumor >4 cm limited to the thyroid
T3b	Gross extrathyroidal extension invading only strap muscles (sternohyoid, sternothyroid, thyrohyoid, or omohyoid muscles) from a tumor of any size
T4	Includes gross extrathyroidal extension beyond the strap muscles
T4a	Gross extrathyroidal extension invading subcutaneous soft tissues, larynx, trachea, esophagus, or recurrent laryngeal nerve from a tumor of any size
T4b	Gross extrathyroidal extension invading prevertebral fascia or encasing the carotid artery or mediastinal vessels from a tumor of any size
Note: All categories may be subdivided: (s) solitary tumor and (m) multifocal tumor (the largest tumor determines the classification).

TABLE I-1B Definition of Regional Lymph Node (N)

Category	Criteria
NX	Regional lymph nodes cannot be assessed
N0	No evidence of locoregional lymph node metastasis
N0a	One or more cytologically or histologically confirmed benign lymph nodes
N0b	No radiologic or clinical evidence of locoregional lymph node metastasis
N1	Metastasis to regional nodes
N1a	Metastasis to level VI or VII (pretracheal, paratracheal, prelaryngeal/Delphian, or upper mediastinal) lymph nodes. This can be unilateral or bilateral disease.
N1b	Metastasis to unilateral, bilateral, or contralateral lateral neck lymph nodes (levels I, II, III, IV, or V) or retropharyngeal lymph nodes

TABLE I-1C Definition of Distant Metastasis (M)

Category	Criteria
M0	No distant metastasis
M1	Distant metastasis

TABLE I-1D AJCC Prognostic Stage Groups-Differentiated Thyroid Cancer

When age at diagnosis is…	And T is…	And N is…	And M is…	Then the stage group is…
<55 years	Any T	Any N	M0	I
<55 years	Any T	Any N	M1	II
≥55 years	T1	N0/NX	M0	I
≥55 years	T1	N1	M0	II
≥55 years	T2	N0/NX	M0	I
≥55 years	T2	N1	M0	II
≥55 years	T3a/T3b	Any N	M0	II
≥55 years	T4a	Any N	M0	III
≥55 years	T4b	Any N	M0	IVA
≥55 years	Any T	Any N	M1	IVB
Note: American Joint Committee on Cancer staging and TNM classification for differentiated thyroid cancer.¹⁶ Used with permission of the American Joint Committee on Cancer (AJCC), Chicago, Illinois. The original and primary source for this information is the AJCC Cancer Staging Manual, Eighth Edition (2017) published by Springer International Publishing.