Purpose

High-dose-rate (HDR) brachytherapy for gynecologic cancers requires repeated organ at risk (OAR) contouring for each fraction, which is a time-consuming task susceptible to variability. This study evaluated the use of patient-specific adaptive rings to streamline OAR contouring without compromising dosimetric accuracy and patient safety.

Material and methods

Fifty HDR brachytherapy tandem and ovoid (T&O) fractions from twenty-five previously treated patients were analyzed. Two adaptive rings were investigated: one generated based on margins from clinical target volume (CTV) to 50% isodose line (IDL), and the other based on margins from CTV to 75% IDL, referred to as the 50% IDL and 75% IDL adaptive rings, respectively. Bladder, rectum, small bowel, and sigmoid were contoured within these two rings, and D2cc values based on these new contours were compared with clinical D2cc values from full-organ contours generated by physician. Contouring times were measured for 10 fractions using full OAR delineation and each ring strategy.

Results

Errors in D2cc between ring-based and full-organ contours were less than 3% for all OARs at clinically relevant dose levels. Contouring time decreased by 50-60% using adaptive rings (p < 0.001), with the 75% IDL ring giving the largest reduction in time due to the smaller ring size.

Conclusions

Adaptive IDL-based rings provide an efficient strategy for OAR contouring in HDR gynecologic brachytherapy. Using adaptive rings to limit OAR contouring allows for fraction-specific adaptation, reduces contouring workload, and maintains dosimetric accuracy.