Locoregional relapses in the ACCORD 12/0405-PRODIGE 02 study: Dosimetric study and risk factors.

PURPOSE: The aim of this study is to correlate locoregional relapse with radiation therapy volumes in patients with rectal cancer treated with neoadjuvant chemoradiation in the ACCORD 12/0405-PRODIGE 02 trial. PATIENTS AND METHODS: We identified patients who had a locoregional relapse included in ACCORD 12's database. We studied their clinical, radiological, and dosimetric data to analyze the dose received by the area of relapse. RESULTS: 39 patients (6.5%) presented 54 locoregional relapses. Most of the relapses were in-field (n = 21, 39%) or marginal (n = 13, 24%) with only six out-of-field (11%), 14 could not be evaluated. Most of them happened in the anastomosis, the perirectal space, and the usual lymphatic drainage areas (presacral and posterior lateral lymph nodes). Only patients treated for a lower rectum adenocarcinoma had a relapse outside of the treated volume. 2 patients with T4 tumors extending into anterior pelvic organs had relapses in anterior lateral and external iliac lymph nodes. CONCLUSIONS: Lowering the upper limit of the treatment field for low rectal tumors increased the risk of out of the field recurrence. For very low tumors, including the inguinal lymph nodes in the treated volume should be considered. Recording locoregional involvement, treated volumes, and relapse areas in future prospective trials would be of paramount interest to refine delineation guidelines.