Purpose

To analyze and compare the clinical outcomes of boost modality choice after external beam radiation therapy (EBRT) or chemoradiotherapy (CRT) for anal canal cancers (ACC).

Material and methods

162 patients with ACC were treated in our institution with two different boost modalities: EBRT boost (EBRTb) or high-dose-rate (HDR) interstitial brachytherapy boost (ISBT). Local relapse-free survival (LRFS), disease-free survival (DFS), overall survival (OS), colostomy-free survival (CFS), and toxicity rates were analyzed.

Results

With a median follow-up (FU) of 66 months, thirteen (8%) local recurrences were reported, six in the ISBT and seven in the EBRTb groups. The 5-year LRFS rate for the entire cohort was 87% (80-92%), without differences in both groups: 88% in ISBT vs. 86% in EBRTb group (p = 0.7). The 5-year DFS, OS, and CFS rates for the overall population were 84% (78-89%), 93% (88-96%), and 92% (88-95%), respectively, without significant differences between ISBT and EBRTb groups. Multivariate analysis for gastrointestinal (GI) toxicity found a non-significant tendency between EBRTb and GI toxicity grade ≥ 2: odds ratio (OR) = 1.82 (0.88-3.78), p = 0.1060. Univariate analysis for fecal incontinence revealed that EBRTb was significantly associated with fecal incontinence grade ≥ 2 (p = 0.0005), and this tendency was confirmed in the multivariate analysis (p = 0.0012). Sensitivity analysis, excluding patients with theoretical brachytherapy contraindications, confirmed these results. Univariate and multivariate assessments found ISBT as an independent prognosis factor for better sphincter function: OR = 5.44 (1.57-18.91), p = 0.0077.

Conclusions

CRT provide excellent rates of tumor control and colostomy-free survival. Compared with EBRTb, interstitial brachytherapy boost demonstrates a favorable profile in GI toxicity, with a low impact on fecal incontinence.