Impact of renal cell cancer histology on outcomes of microscopic positive surgical margins and implications on post-surgical follow-up.

519 Background: Positive surgical margins (PSM) are associated with worsened survival outcomes in Renal Cell Carcinoma (RCC). Current AUA guidelines recommend intensified post-surgical follow-up and clinical vigilance in patients with PSM. We sought to evaluate outcomes of different RCC-histologies with microscopic-PSM. Methods: We performed a retrospective analysis of patients undergoing partial nephrectomy (PN) or radical nephrectomy (RN) for RCC who had microscopic-PSM. The cohort was divided according to RCC-Histology [Chromophobe-RCC (chRCC); Clear Cell-RCC (ccRCC); Papillary-RCC (pRCC) for descriptive and survival analyses. Primary outcome was all-cause mortality (ACM)/overall survival (OS). Secondary outcome was cancer-specific mortality (CSM)/cancer-specific survival (CSS). Cox proportional hazards multivariable analysis (MVA) was used to elucidate predictive factors for ACM and CSM. Kaplan-Meier Analysis (KMA) was performed to analyze 5-year OSS and CSS. Results: A total of 8100 patients were analyzed, and we identified 312 patients with microscopic-PSM [26 (8.3%), 236 (75.6%), and 50 (16.0%) of patients had chRCC, ccRCC, and pRCC respectively]. Cox-regression revealed ccRCC to be associated with worsened ACM (versus chRCC [referent] HR=7.8, p=0.042), and worsened CSM (versus chRCC [referent] HR=2.97, p=0.033); pRCC did not demonstrate worsened ACM (p=0.148) and CSM (p=0.201). Comparing chRCC, ccRCC, and pRCC, KMA revealed a 5-year OS of 91%, 87%, and 63%, (p=0.024) and 5-year CSS of 93%, 86%, and 65%. KMA comparing microscopic-PSM versus negative surgical margins (NSM) noted no significant difference in 5-year OS for chRCC (85% vs. 86%, p=0.365) or pRCC (80% vs. 84%, p=0.952), but significantly lower 5-year OS in microscopic-PSM in ccRCC (77% vs. 85%, p<0.001). Conclusions: In patients with microscopic-PSM, we noted significant differences according to histology, with ccRCC being associated with significantly worsened survival and mortality outcomes when compared to chRCC and pRCC; microscopic-PSM was also associated with significantly worsened survival compared to NSM in ccRCC, but not chRCC and pRCC. Microscopic-PSM can be considered to represent a higher risk subgroup in ccRCC but not chRCC or pRCC. Intensified post-surgical follow up compared to NSM patient is appropriate in ccRCC, while microscopic-PSM in chRCC and pRCC can be followed similarly to NSM patients with the same histology.