Comprehensive analysis of dosimetry correlation and clinical outcomes in image-guided brachytherapy vs. intensity-modulated radiotherapy photon boost after whole breast hypofractionated radiation in post-breast conserving surgery: A 5-year median follow-up data from a tertiary care institution in India

Purpose:
This study evaluated dosimetry and clinical correlations between intensity modulated radiation therapy (IMRT) boost and image-guided multicatheter interstitial brachytherapy (MIBT) boost, after whole breast hypofractionated radiation post-breast conserving surgery.

Material and methods:
A mono-institutional randomized prospective controlled trial was conducted in the Department of Radiotherapy and Clinical Oncology at PGIMER, Chandigarh, India, among fifty patients (n = 50) enrolled between September 2018 and January 2020. Each patient received 3D-CRT whole breast radiotherapy (WBRT), 40 Gy in 16 fractions was administered, and delivered within 3.5 weeks. Subsequently, either an IMRT boost (n = 25) with 16 Gy in 8 fractions, or a MIBT boost (n = 25) with 15 Gy in 5 fractions (bid) were delivered. Follow-up visits were scheduled at 6 months, 1 year, 2 years, and 5 years post-treatment to assess toxicity and cosmesis. Cumulative EQD₂ and boost phase EQD₂ doses for dosimetry comparisons were computed. Toxicity was evaluated using RTOG grading, whereas cosmesis was assessed with patient-reported number scores and physician-reported standards based on Harvard criteria. Dosimetry was compared using t-tests, toxicity and cosmesis with a likelihood ratio test, and loco-regional recurrence (LRR) was estimated using Fisher’s exact test. Progression-free survival (PFS) and overall survival (OS) were analyzed with Kaplan-Meier method and log-rank test.

Results:
The MIBT group had significantly lower mean cumulative skin and rib D_max (54.3 Gy and 57.4 Gy) than the IMRT group (62.8 Gy and 64.5 Gy), with a p-value of less than 0.001. This also applied to all organs at risk (OARs) EQD₂ doses during the boost phase, with a p-value of less than 0.001, except for NTB V₉₀ and V₅₀. Nonetheless, there were no significant differences in the cumulative EQD₂ doses of other OARs, or acute toxicity, late toxicity, patient- and physician-reported cosmesis as well as LRR, PFS, and OS.

Conclusions:
In left-sided breast cancer, image-guided MIBT outperforms photon boost by reducing the cardiac dose. Despite no statistical significance in cosmesis differences, MIBT exhibited marginally enhanced cosmesis than IMRT boost.