Comparison of outcomes between rectal squamous cell carcinoma and adenocarcinoma.

Large, population-based analyses of rectal squamous cell carcinoma (SCC) have not been previously conducted. We assessed patterns of care, prognostic factors, and outcomes of rectal SCC and adenocarcinoma (AC) in population-based cohorts. Surveillance, Epidemiology, and End Results (SEER) registry searches were performed (1998-2011), producing 42,308 nonmetastatic rectal cancer patients (999 SCC and 41,309 AC). Patient, tumor, and treatment characteristics were compared. Based on risk factors, SCC/AC groups were subdivided into low-, intermediate-, and high-risk groups. Overall survival (OS) was compared between histological and risk groups using Kaplan-Meier method and log-rank test. Multivariate logistic regression models evaluated prognostic factors for 5-year survival. Cox regression modeling was performed on propensity-matched data. Rectal SCC, more common in females and associated with larger tumors of higher grade, was more often treated with radiotherapy (RT) than surgery. Surgery was associated with higher OS in AC but not SCC, and RT had proportionally greater benefits in SCC. These effects of RT and surgery were retained when stratified into risk groups (particularly high/intermediate-risk). Favorable prognostic factors for survival included younger age, non-black race, SCC histology, size ≤3.9 cm, localized stage, lower grade, surgery, and RT. For SCC, race, tumor grade, and surgery were not prognostic factors for survival. Cox regression modeling of propensity-matched data showed that AC histology increased risk of death versus SCC. In the largest analysis of rectal SCC to date, and in the notable absence (and unlikelihood) of prospective data, nonsurgical and RT-based treatment is recommended.