When Should a Positive Surgical Margin Ring a Bell?  An Analysis of a Multi-Institutional Robot-Assisted Laparoscopic Radical Prostatectomy Database. 
The impact of positive surgical margin (SM) on cancer control outcomes in prostate cancer patients is a subject of continuous debate.  We test the hypothesis that the impact of SM on clinical recurrence (CR) rate may vary based on the other clinical/pathologic characteristics of the tumor.  We focused on 5290 patients treated with robot-assisted radical prostatectomy and pelvic node dissection, between 2002 and 2013, at three tertiary care centers.  Regression tree analysis stratified patients into risk groups based on their tumor characteristics and the corresponding CR rate.  Kaplan-Meier log-rank and multivariable Cox regression models tested the relationship between SM status and CR rate in each tree-generated risk group.  Mean (median) follow-up time was 47.7 (39.0) months.  Regression tree analysis that considered all available covariates, except SM status, divided patients based on their CR risk into the following risk groups: (1) high risk (any pT3b/pT4 disease); (2) intermediate risk (≤pT3a disease and pGS 8-10); (3) low risk (≤pT3a, pGS ≤7, and prostate-specific antigen [PSA] >9 ng/mL); and (4) very low risk (≤pT3a, pGS ≤7, and PSA ≤9 ng/mL).  Positive SM had a significant detrimental impact on CR risk only in two groups: intermediate risk (p < 0.001) and high risk (p = 0.01).  These observations were confirmed by multivariable analyses.  Our findings show that positive SM had a detrimental impact on CR only in a minority of patients (15%), specifically in those with advanced pathologic stage and/or pathologically poorly differentiated tumor.  For all the remaining patients (85%), positive SM by itself did not increase the risk of CR.