Dosimetric Impact of Interfractional Anatomical Changes In Syed HDR Brachytherapy: Is Adaptive Planning Required for Every Fraction?
Abstract
Purpose
To quantify the dosimetric impact of interfractional anatomical changes in Syed-based high-dose-rate (HDR) interstitial brachytherapy for locally advanced cervical cancer by comparing a single-plan (non-adaptive) versus multi-planned adaptive strategy.
Methods
Eighteen patients with stage III–IV cervical cancer previously treated at our institution were retrospectively analyzed. All patients received Syed-based high-dose-rate interstitial brachytherapy delivering 28 Gy in 4 fractions. Two planning strategies were compared. In the single-plan strategy, the treatment plan generated for the first fraction was assumed to be delivered for all four fractions without accounting for interfractional anatomical changes. In the adaptive replanning strategy, each fraction was independently planned using daily CT imaging, and cumulative dose was derived from all four delivered plans. Equivalent EQD2 doses for tumor and organ at risks (OARs) were computed and compared between the two strategies. Target metrics included D90 and D98 for high-risk clinical target volume (CTV_HR), D98 for intermediate-risk clinical target volume (CTV_IR), while OAR metrics included D2cc for bladder, rectum, bowel, and sigmoid. Paired dosimetric comparisons were performed using the Wilcoxon signed-rank test.
Results
Significant dosimetric differences were observed between the single-plan and multi-planned adaptive strategies for multiple OARs. The multi-planned adaptive approach provided statistically improved sparing for bladder, bowel, and sigmoid compared with the single-plan strategy (p<0.05 for all). No statistically significant differences were found for target coverage, including CTV_HR D90, CTV_HR D98, and CTV_IR D98, demonstrating that both strategies achieved comparable tumor dose.
Conclusion
Interfractional anatomical variations significantly affect organ-at-risk dosimetry in Syed HDR brachytherapy. Compared with a single-plan dose calculation, multi-fraction adaptive replanning provided superior sparing of bladder, bowel, and sigmoid without compromising target coverage. These findings support routine adaptive planning across fractions to mitigate high-dose exposure to mobile pelvic organs while maintaining adequate tumor dose in cervical cancer HDR brachytherapy.