Is the Pathologic Response of T3 Rectal Cancer to High-Dose-Rate Endorectal Brachytherapy Comparable to External Beam Radiotherapy?

Abstract

Endorectal brachytherapy is an attractive option in the neoadjuvant setting for locally advanced rectal cancer, but it is not considered standard of care. This study aimed to compare pathologic outcomes of patients with clinical T3 rectal cancer who underwent high-dose-rate endorectal brachytherapy with those who underwent conventional external beam radiotherapy. This study is a retrospective chart review. This study was conducted in a single large tertiary academic colorectal surgery practice in Canada. Adult patients with MRI-staged T3 rectal adenocarcinoma treated with neoadjuvant radiotherapy followed by total mesorectal excision from 2007 to 2016 were included. Neoadjuvant radiotherapy was delivered by high-dose-rate endorectal brachytherapy or conventional external beam radiotherapy. Primary outcome was pathologic complete response, defined as ypT0N0. Secondary outcomes included tumor (T stage) and lymph node (N stage) downstaging and tumor regression grade. Ninety-nine patients were identified as having clinical T3 rectal cancer based on blinded pretreatment MRI review. Mean age was 66.2 years (± 6.2) and 59 patients (59.6%) were male. Thirty-three patients were clinically node negative (33.3%), 45 had c-N1 disease (45.5%), and 21 had c-N2 disease (21.2%). Sixty-four patients (64.6%) underwent high-dose-rate endorectal brachytherapy and 35 (35.4%) underwent external beam radiotherapy. The high-dose-rate endorectal brachytherapy group had a lower median mesorectal depth of invasion (4 mm vs 5 mm, p = 0.010); all other preoperative tumor characteristics were similar in both groups. Eighteen patients (18.2%) achieved pathologic complete response: 12 in the high-dose-rate endorectal brachytherapy group and 6 in the conventional external beam radiotherapy group (18.8% vs 17.1%, p = 0.84). High-dose-rate endorectal brachytherapy was superior to conventional radiotherapy for tumor (T stage) downstaging (59.4% vs 28.6%, p = 0.0030) but not for lymph node (N stage) downstaging (35.9% vs 51.4%, p = 0.14). This study was limited by its retrospective nature and modest sample size. Neoadjuvant treatment of T3 rectal cancer with high-dose-rate endorectal brachytherapy appears to achieve equivalent rates of pathologic complete response and superior T-stage downstaging compared with conventional external beam radiotherapy. See Video Abstract at http://links.lww.com/DCR/A905.