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

Background: Endorectal brachytherapy is an attractive option in the neoadjuvant setting for locally advanced rectal cancer, but it is not considered standard of care.
Objective: 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.
Design: This study is a retrospective chart review.
Settings: This study was conducted in a single large tertiary academic colorectal surgery practice in Canada.
Patients: Adult patients with MRI-staged T3 rectal adenocarcinoma treated with neoadjuvant radiotherapy followed by total mesorectal excision from 2007 to 2016 were included.
Interventions: Neoadjuvant radiotherapy was delivered by high-dose-rate endorectal brachytherapy or conventional external beam radiotherapy.
Main Outcome Measures: 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.
Results: 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).
Limitations: This study was limited by its retrospective nature and modest sample size.
Conclusions: 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.