1 evidence supporting the use of adjuvant RT for non-rectal colon cancer in the modern chemotherapy era, there are studies that suggest a local control benefit. Given this limited evidence, adjuvant RT in colon cancer is uncommon and quite controversial. Even with T4 disease, NCCN guidelines say only to "consider RT for T4 with penetration to a fixed structure." We believe that this treatment modality is underutilized, particularly in T4 tumors, and seek to determine if there is an underappreciated benefit to adjuvant RT. Due to the limited number of patients with non-rectal, nonmetastatic colon adenocarcinoma who receive post-operative RT, a national cancer treatment database was searched to provide sufficient patient numbers to power a survival analysis. Materials/Methods: A retrospective analysis using the Surveillance, Epidemiology, and End Results (SEER) database from the National Cancer Institute was performed using data up to and including 2015. Patients were included if they met the following criteria: non-rectal colon cancer, AJCC 6 th or 7 th edition T4, AJCC 6 th or 7 th edition M0, who had oncologic resection, and 1st cancer site. Patients were excluded if they had RT prior to or during surgery, or if the sequence of RT relative to surgery was unknown. Using a Cox proportional hazard model, the relative risk of cause-specific mortality for "RT after surgery" versus "No RT" was calculated. Results: 21,789 pts were identified who met the inclusion criteria. Of these, only 1000 received adjuvant RT and 20,683 received no adjuvant RT; 46% were male (52% RT vs. 46% no RT), 46% were age 70 or older (27% RT vs. 47% no RT), and 64% node-positive (53% RT vs. 65% no RT). When comparing RT vs. no RT, after adjusting for sex, age, N stage, and grade, we determined the relative risk of death from any cause was 0.8849 (95% CI: 0.8008, 0.9779) with a p-value of 0.0165. In other words, adjuvant RT decreased the relative risk of death from colon cancer by 11.5% at 5 years. Conclusion: Adjuvant RT is not routinely utilized for definitive treatment of T4 non-rectal colon cancer, but this analysis shows a significant benefit in cause-specific survival that warrants attention. The limitations of conducting a randomized trial and small patient numbers were mitigated with the use of this large database, however, use of chemotherapy, performance status and physician bias were unknown. This data provides impetus for the oncology community to re-evaluate the use of RT in locally-advanced non-rectal colon cancer. With advances in systemic therapy long term survivorship is common and local control with adjuvant RT will play a more profound role in the definitive treatment of resectable T4 colon cancer.
