HDR Endorectal/Endoluminal Brachytherapy Boost in Rectal Organ Preservation: A Systematic Review and Meta-Analysis

Background and purpose: Organ-preservation strategies are increasingly being incorporated into rectal cancer management, but the role of HDR endorectal/endoluminal brachytherapy boost remains less well defined than that of broader non-operative treatment pathways. Existing literature is frequently mixed with contact X-ray brachytherapy series, neoadjuvant protocols with planned surgery, or heterogeneous watch-and-wait cohorts, limiting the interpretation of this specific strategy. Materials and methods: We performed a PROSPERO-registered systematic review and meta-analysis of studies evaluating definitive-intent external beam radiotherapy (EBRT), with or without chemotherapy, followed by HDR endorectal/endoluminal brachytherapy boost in histologically confirmed rectal adenocarcinoma managed without planned surgery. Pooled analyses were performed for clinical complete response (cCR) and late grade ≥3 gastrointestinal (GI) toxicity. Regrowth/local failure outcomes were synthesized descriptively because of heterogeneity in endpoint definitions, denominator selection, and follow-up structure. Results: Six studies were included in the quantitative evidence base, with one additional small feasibility report summarized narratively. The pooled cCR proportion was 69.2% (95% confidence interval CI, 43.7–86.6). The pooled proportion of late grade ≥3 GI toxicity was 18.1% (95% CI, 10.9–28.6). Reported regrowth/local failure outcomes were not suitable for formal pooling because of inconsistent definitions, differing denominator structures, and non-uniform follow-up frameworks across studies. Conclusion: Current evidence suggests that EBRT plus HDR endorectal/endoluminal brachytherapy boost may represent a selective organ-preservation strategy for carefully chosen patients with rectal adenocarcinoma, particularly where surgery is not feasible or not desired. Its broader clinical use remains limited not only by the small size of the evidence base, but also by fragmented endpoint definitions, inconsistent denominator reporting, and insufficiently standardized durable local-control outcomes. These findings support cautious interpretation of the current evidence and highlight priorities for future prospective studies in rectal cancer management.