2747 - Dosimetric Evaluation of Urethral Dose and Toxicity in Image-Guided Brachytherapy (IGBT) for Gynecological Cancers: Implications for Organ-at-Risk (OAR) Consideration
Presenter(s)
A. P. E. Galerani, T. Ali, M. A. Gurmani, N. Babu, and M. Al Balushi; Sultan Qaboos Comprehensive Cancer Care and Research Centre, Muscat, Oman
Purpose/Objective(s):
While guidelines recommend considering urethral contouring in cases with significant vaginal involvement, no specific dose constraints have been established. The toxicity profile of urethral irradiation remains unclear, making it challenging to assess its clinical impact. Understanding and incorporating urethral dose contribution from the external beam is complex and highlights the need for more investigation. This study aims to dosimetrically evaluate the urethral doses in patients with gynecological cancers treated with Image-Guided Brachytherapy (IGBT) and correlate to early and late genitourinary (GU) toxicities.
Materials/Methods:
Retrospective dosimetric analysis was conducted for primary gynecological cancer patients who underwent IGBT between October 2021 and June 2023. All patients had a CT or MRI planning for all fractions. In each fraction, urethra was uniformly contoured with a 1 cm static brush, extending from the bladder base and caudally to the introitus. Parameters were individually calculated: mean dose, D0.1cm³, D0.2cm³, D0.5cm³, and D1cm³. Cumulative doses from all fractions per patient were summed, and mean EQD2 was calculated. Demographics, applicator type, dose, and fractionation were collected. Genitourinary subcategories for early and late toxicity were assessed using CTCAE version 5.0.
Results:
A total of 19 patients with primary cancers of the cervix (47%), endometrium (43%), and vagina (10%) were analyzed, totaling 63 image sets. The mean age was 59 years (range: 37–77). The average dose and fractions were 6.83 Gray (Gy) (range: 5–8 Gy), and 3.4 fractions (range: 2–5). Applicators included single-channel (17%) and multichannel cylinders (27%), Fletcher (27%), and interstitial implant (29%). The average EQD2 Mean dose was 20.5 Gy (range: 7.6–56.7 Gy). Mean urethral dose parameters were D0.1cc: 41.9 Gy (15.1–87.6 Gy), D0.5cc: 34.7 Gy (13.4–77.4 Gy), D1cc: 29.6 Gy (11–70.5 Gy), and D2cc: 39.2 Gy (14.5–84 Gy). Acute grade 1 or 2 toxicity was observed in 5 patients (26%), while late grade 1 or 2 toxicity occurred in 4 patients (21%). Mean urethral doses were significantly higher, approximately threefold, in patients who developed genitourinary toxicity.
Conclusion:
This dosimetric analysis demonstrates that urethral doses, regardless of the applicator type, are not negligible and should be considered clinically significant. The doses observed in this study are comparable to those received by other organs at risk, emphasizing the importance of consistent urethral contouring. The average dose from brachytherapy and its associated toxicity profile remains to be established. In the era of highly conformed doses and IGBT, urethra should be recognized as an OAR, and future studies should aim and validate dose constraints to minimize toxicity.