PV QA 4 - Poster Viewing Q&A 4
Purpose/Objective(s): The purpose of this study was to reconfirm our previous findings that rectal dose-volume parameters and source strength both affect late rectal morbidity in patients with computed tomography (CT)-based image-guided brachytherapy (IGBT) for cervical cancer.
Materials/Methods: Between March 2012 and October 2016, 113 patients with cervical cancer who underwent high-dose rate (HDR) intracavitary brachytherapy (ICBT) and were followed up for 1 year or more were analyzed. All patients were treated with CT-based IGBT in all brachytherapy sessions. The rectum was contoured from the bottom of the ischial tuberosity to the sigmoid flexure using the external wall contour. The minimal dose received by the 0.1-cc, 1-cc, 2-cc, and 5-cc volumes with the highest irradiation (D0.1cc, D1cc, D2cc, and D5cc, respectively) was determined. To determine the dose from combined external beam radiotherapy (whole pelvic irradiation dose, excluding the fractions with central shielding) and ICBT, the total dose was calculated as the biologically equivalent dose in 2-Gy fractions (EQD2). Data for the Ir-192 source strength were collected on each day of the HDR-ICBT session, and the average source strength was calculated over 2-4 ICBT sessions. The association between the respective rectal EQD2 or average source strength and late rectal morbidity was then analyzed.
Results: The median follow-up period was 36 months (range 12-70 months). Twenty-five patients (23%) developed late rectal morbidity, including 19 (17%) with Grade 1 toxicity, 4 (4%) with Grade 2, and 2 (2%) with Grade 3. Patients with rectal morbidity had significantly greater D0.1cc, D1cc, D2cc, and D5cc, and received a greater average source strength. Multivariate analysis was performed with D2cc, average source strength, and 3 other clinical parameters that were judged to be potential risk factors for late rectal morbidity: age, smoking, and BMI. Of these parameters, D2cc and source strength emerged as independent predictors of late rectal morbidity. The logistic regression curve indicated that D2cc of 67 Gy and a source strength of 2.8 cGy•m2•h-1 resulted in a 20% probability of late rectal morbidity.
Conclusion: Rectal D2cc and source strength are independent predictors of late rectal morbidity in patients with CT-based IGBT for cervical cancer.
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