Evaluation of rectal volume correlation with dosimetric parameters during optimized intracavitary high-dose-rate brachytherapy in cervical cancer

Purpose
Brachytherapy (BRT) is a cornerstone in cervical cancer treatment, with the ultimate goal to maximize the tumor dose while sparing organs at risk (OARs), such as rectum. Several studies evaluated the effect of rectal volume on rectal doses, but the results are inconsistent. This study aimed to evaluate the rectal volume and dose-volume histogram (DVH) relationship in high-dose-rate (HDR) brachytherapy in locally advanced cervical cancer.

Material and methods
Planning computed tomography of 65 patients who underwent HDR brachytherapy boost as a component of definitive radiotherapy from March 2016 to February 2018 were reviewed. OARs and target volume were re-delineated by a single physician to decrease interobserver variation. Two sets of plan were generated; in the first set, the dose was prescribed to point A with Manchester system loading pattern, while in the second set, the dose was prescribed to high-risk clinical target volume (HR-CTV) D90 with inverse planning optimization. The DVH values for rectum, sigmoid, and HR-CTV were generated and correlated with rectal or sigmoidal volume variation.

Results
Dose to 2cc (D2cc), 1cc (D1cc), and 0.1cc (D0.1cc) of rectum and sigmoid showed a significant decrease in optimization vs. point A planning (p < 0.0001). HR-CTV D90 coverage was significantly higher in optimization vs. point A planning (p = 0.041). Rectal volume showed a significant correlation with D2cc (rs, 0.302, p = 0.014), D1cc (rs, 0.310, p = 0.012), and D0.1cc (rs, 0.283, p = 0.02) of rectum in optimization planning.

Conclusions
Larger rectal volumes are associated with higher rectal dose parameters during HDR brachytherapy using inverse planning optimization. This method spares OAR, while producing reasonable HR-CTV D90. Prospective studies are needed to find appropriate technique of rectal volume reduction.