Daily Reproducibility of Normal Structure Dosimetry in Single Insertion, Multi-Day Interstitial Gynecologic Brachytherapy
Faculty and Abstracts
Purpose: Single insertion, muti-fraction high dose rate brachytherapy (HDR) has emerged as a solution to several barriers to quality care including COVID-19 and geographic disparities.1 For single insertion gynecologic HDR at our institution, interfraction anatomic reproducibility is assessed by visually comparing the target and organs at risk (OARs) between the initial planning scan and subsequent pre-treatment scans. Patients typically receive 4 fractions treated BID over multiple days. With steep dose gradients and mobile pelvic anatomy, we sought to confirm OAR dosimetry safety in a cohort of treated patients passing the initial cognitive fusion test.
Methodology: We retrospectively assessed OAR dosimetry for 15 consecutive single implant, multi-fraction gynecologic HDR brachytherapy patients. OARs including bladder, small bowel, sigmoid colon, rectum, and urethra were contoured on each Day 3 scan. The first fraction dose distribution was rigidly registered onto the Day 3 simulation scan and OAR planning metrics were calculated. These were compared to initial plan metrics for safety and statistically evaluated using a paired T-test.
Results: 15 consecutive patients were assessed including 10 Kelowna, 4 Tandem-Ring interstitial hybrid, and 1 Tandem-Ovoid interstitial hybrid applicators. As shown in table 1, most OAR’s experienced modest average increases in dose after two days of anatomy changes with several experiencing what may be considered clinically significant changes. Sigmoid colon dose became more favorable over this time period likely due to rectal filling.
Conclusions: While most patients experienced only modest changes in OAR dose at Day 3, a general trend toward increased dose was observed. Importantly, a minority of patients experienced sizeable dose increases that could be clinically significant. These increases were not caught on visual inspection of repeat simulation anatomy. While bowel gas is known to effect dosimetry, 2 more subtle shifts in anatomy appeared to bring the bladder and urethra into higher doses as well. Based on these findings, we plan to implement a more robust OAR dosimetry evaluation process for subsequent treatment days.
References: 1Mahantshetty U et al. Single Application Multifractionated Image Guided Adaptive High-Dose-Rate Brachytherapy for Cervical Cancer: Dosimetric and Clinical Outcomes. Int J Radiat Oncol Biol Phys. 2021;111(3):826-834. 2Damast S et al. Single application hybrid interstitial brachytherapy for cervical cancer: an institutional approach during the COVID-19 pandemic. J Contemp Brachytherapy. 2022;14(1):66-71.