2.1

Cohort selection

Conduct of this study was in accordance with ethical guidelines and received Institutional Review Board (IRB) approval. Prospectively collected data was utilized from an institutional registry of patients who underwent radical prostatectomy (RP) from 1985 to 2017. This study included open and robotic laparoscopic approaches. Eligibility for inclusion was pathologic confirmation of prostate adenocarcinoma with localized disease. Patients were excluded if they had any pathologic variant, had previously undergone any prostate procedure (including transurethral resection of the prostate, prostatic urethral lift, photoselective vaporization of the prostate, and simple prostatectomy), or had received any form of radiation therapy. Additionally, patients presenting with metastatic disease at the time of prostatectomy were not considered. Body mass index (BMI) was used to define obesity. BMI was binned in line with the CDC/WHO guidelines: Underweight: BMI <18.5, Normal weight: BMI 18.5 to 24.9, Overweight: BMI 25.0 to 29.9, Obesity (Class 1): BMI 30.0 to 34.9, Obesity (Class 2): BMI 35.0 to 39.9, Extreme Obesity (Class 3): BMI ≥ 40.0 [ ].

2.2

Oncologic outcomes of interest

The primary oncologic outcomes of interest were to assess the association of BMI with BCR, metastasis free survival (MFS), prostate cancer specific survival (CSS), and overall survival (OS). In addition, nonlocalized disease (defined as pT3+) was also examined. Postoperative follow-up, including physical examination and serum PSA measurement, was generally performed quarterly for the initial 2 years, semiannually for the next 2 years, and annually thereafter. BCR was defined as a single postoperative PSA of 0.4ng/mL or greater [ , ]. For men receiving follow-up elsewhere, the Prostatectomy Registry monitors outcomes annually by soliciting both provider and patient correspondence.

2.3

Nononcologic outcomes of interest

Secondary outcomes of interest were nononcologic postoperative outcomes thought to be relevant to a patient’s body habitus and the corresponding difficulty of performing surgery [ ]. These included positive surgical margins, complications and functional outcomes. Any positive surgical margin regardless of length on final pathologic section was considered as positive.

Early complications (≤ 30 days post – RP) captured in the registry: included hemorrhage, urethral stricture, extravasation, hernia, wound infection, ureteral obstruction, sepsis, renal failure, UTI, vesicourethral anastomotic contracture, urinary retention, aortic occlusion, DVT, evisceration, lymphocele, myocardial infarction, pulmonary embolism, lymphedema, allergy, respiratory, gastrointestinal tract, and central nervous system complications.

For functional outcomes: continence and potency were studied. Continence at 1 year postoperatively (evaluated only among patients with preoperative continence) was defined as 0 pads per day or seldom wet. Potency at 1 year postoperatively (evaluated only among patients with sufficient preoperative erectile function) was defined as the ability to achieve an erection sufficient for intercourse regardless of use of PDE-5 inhibitors.

2.4

Covariates for adjustment

Additional clinicopathologic features recorded included age at surgery, year of surgery (treated as a continuous variable with an effect size per 10 year intervals), preoperative PSA, history of preoperative ADT, clinical tumor stage according to the American Joint Committee on Cancer staging [ ], pathologic TNM stage, Gleason score at RP, margin status, tumor volume, and receipt of adjuvant radiotherapy or ADT.

2.5

Statistical analyses

Continuous features were summarized with means and standard deviation and compared using analysis of variance (ANOVA). Categorical features were summarized with frequency counts and percentages compared using the χ2 test for categorical variables. Long term oncologic outcomes were compared across BMI categories with multivariable Cox regression models using nonprostate cancer mortality as a competing risk. Given prior reports of diabetes modifying the effect of BMI [ ], subset analysis was conducted between obesity status (BMI 30 or greater) and diabetes status for the subset of patients with known diabetes status. Multivariable logistic regression models were used for all nontime dependent outcomes: pathologic T stage (T3+), positive margins, complications, and functional outcomes. The multivariable models incorporated the following covariates: age (continuous), surgical year (continuous), surgical approach (open vs. laparoscopic), receipt of adjuvant hormonal therapy or adjuvant radiation, log-base-2 PSA, pathologic stage (this was clinical stage for pT3 outcome), pathologic Gleason score (8–10 vs. <8). For functional outcomes, nerve-sparing status was an additional covariate. Statistical analysis was performed using SAS © (9.4). All tests were 2-sided, with P < 0.05 considered statistically significant.

3.1

Cohort description

Among the cohort of 21,602 patients ( Table 1 ), 6,252 (28.9%) were obese (BMI > 30) and 222 (1.0%) were morbidly obese (BMI 40+). The number of obese patient undergoing surgery has increased over the 30 year study period: 19.5% obese in 1985 to 38.9% obese in 2017 ( Fig. 1 ).Obese patients were more likely to undergo conversion from laparoscopic to open surgery compared to nonobese patients (0.26% vs. 0.13%, P < 0.001). The chances of bilateral nerve sparing was highest for the BMI 18.5 – 24.9 group (65.1%) compared to the BMI <18.5 (51.9%) and BMI 40+ (61.8%) ( P < 0.001) groups. Operative time increased with higher BMI: from a mean of 234mins in BMI 0 to 25 to 280mins in BMI 40+. Similarly, median blood loss increased with higher BMI: 379.1 cc in BMI <18.5 to 547cc in BMI 40+.

BMI composition of annual prostatectomy cases.

3.2

Oncological outcomes

The median follow up of patients was 12.2 years (IQR: 6.4 to 18.5). BMI was significantly associated with all-cause mortality ( Table 2 ), with BMI over 30 having increased risk of death relative to BMI 18.5 to 24.9. For the obese patients, the hazard ratio of death increased for higher severities of obesity: BMI 30 to 34.9 (HR 1.30, P < 0.001), BMI 35 to 39.9 (HR 1.43, P < 0.001), and BMI > 40 (HR 2.20, P < 0.001). Given the increased risk of all-cause mortality with BMI, nonprostate cancer death was treated as a competing risk in multivariable models ( Table 2 ). BMI was not significantly associated with biochemical failure, metastasis, or cancer specific mortality. Fig. 2 demonstrates regression adjusted competing risk estimates of nonprostate cancer and prostate cancer mortality by BMI category to show that the former varies with BMI while the latter does not. Importantly, more than 84% of radical prostatectomy patients remained alive 10 years after surgery for all BMI categories.

Table 2

Multivariable cox proportional hazard models for key longitudinal oncologic outcomes, including a competing risk model

	Biochemical failure		Metastasis		Cancer specific mortality		All-cause death		CSS competing risk
	HR	P Value	HR	P Value	HR	P – Value	HR	P -Value	HR	P value
Body mass index		0.02		0.001		0.01		< 0.0001		0.09
0–18.5 vs. 18.5–25	1.24 (0.66, 2.30)	0.50	1.05 (0.15, 7.52)	0.96	1.88 (0.26, 13.43)	0.53	1.60 (0.93, 2.76)	0.09	1.02 (0.13, 7.73)	0.99
25–30 vs. 18.5–25	1.07 (1.01, 1.13)	0.03	1.00 (0.87, 1.16)	1.00	0.96 (0.80, 1.15)	0.65	1.05 (0.99, 1.11)	0.14	0.95 (0.79, 1.14)	0.56
30–35 vs. 18.5–25	1.13 (1.05, 1.21)	0.001	1.20 (1.02, 1.41)	0.03	1.20 (0.98, 1.47)	0.08	1.30 (1.21, 1.39)	< 0.0001	1.14 (0.92, 1.41)	0.23
35–40 vs. 18.5–25	1.10 (0.99, 1.23)	0.08	1.51 (1.20, 1.91)	0.001	1.47 (1.08, 2.01)	0.01	1.43 (1.28, 1.61)	< 0.0001	1.33 (0.97, 1.83)	0.08
40+ vs. 18.5–25	1.19 (0.96, 1.47)	0.11	1.03 (0.64, 1.67)	0.90	0.83 (0.39, 1.77)	0.62	2.20 (1.73, 2.80)	< 0.0001	0.68 (0.32, 1.47)	0.33
Age at surgery	1.00 (0.99, 1.00)	0.14	1.00 (0.99, 1.00)	0.32	1.01 (1.00, 1.02)	0.02	1.10 (1.09, 1.10)	< 0.0001	0.99 (0.98, 1.00)	0.07
Pathological T34 vs. T2	1.76 (1.67, 1.85)	< 0.0001	3.10 (2.73, 3.51)	< 0.0001	2.90 (2.46, 3.41)	< 0.0001	1.24 (1.18, 1.31)	< 0.0001	2.82 (2.38, 3.36)	< 0.0001
Log 2 (preoperative PSA)	1.23 (1.20, 1.26)	< 0.0001	1.15 (1.10, 1.21)	< 0.0001	1.09 (1.03, 1.15)	0.005	1.02 (1.00, 1.04)	0.06	1.07 (1.00, 1.14)	0.04
Pathological Gleason score	1.73 (1.67, 1.79)	< 0.0001	2.98 (2.74, 3.24)	< 0.0001	3.04 (2.74, 3.37)	< 0.0001	1.25 (1.20, 1.29)	< 0.0001	2.95 (2.66, 3.28)	< 0.0001
LRP vs. RRP	0.89 (0.83, 0.95)	0.001	1.35 (1.13, 1.61)	0.001	1.02 (0.76, 1.37)	0.88	0.99 (0.88, 1.10)	0.81	0.96 (0.72, 1.28)	0.76
Year of surgery (every 10 y)	0.73 (0.71, 0.76)	< 0.0001	0.95 (0.87, 1.03)	0.21	0.92 (0.82, 1.04)	0.17	1.07 (1.03, 1.12)	0.002	0.82 (0.73, 0.92)	0.0006
Positive surgical margin	1.70 (1.62, 1.79)	< 0.0001	1.18 (1.05, 1.33)	0.005	1.37 (1.18, 1.59)	< 0.0001	1.04 (0.99, 1.10)	0.14	1.36 (1.17, 1.59)	< 0.0001
Adjuvant radiotherapy	0.50 (0.45, 0.57)	< 0.0001	0.78 (0.63, 0.96)	0.02	0.68 (0.52, 0.89)	0.005	1.04 (0.93, 1.15)	0.52	0.67 (0.51, 0.89)	0.006
Adjuvant hormonal therapy	0.47 (0.43, 0.50)	< 0.0001	0.79 (0.68, 0.91)	0.001	1.04 (0.88, 1.23)	0.66	1.04 (0.97, 1.12)	0.30	1.05 (0.88, 1.25)	0.57

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