Purpose: The present work reports effects of source step sizes on dose distribution in patients treated with cobalt-60 (Co-60) high-dose-rate afterloading brachytherapy in carcinoma cervix (Ca-cx).
Methods and materials: The retrospective study is based on data of 15 patients of Ca-cx treated with Co-60 high-dose-rate intracavitary brachytherapy with dose of 21 Gy in three fractions with source step size of 2.5 mm after external beam radiotherapy of 46 Gy. The effect of source step size on overall treatment procedure was evaluated from prescribed dose volume, dose to organ at risks, and treatment time for source step sizes of 1 mm, 2.5 mm, 5 mm, and 10 mm for each patient.
Results: The mean dose to bladder point for 1 mm, 2.5 mm, 5 mm, and 10 mm source step sizes was found to be 3.37 Gy (SD: 1.36), 3.44 Gy (SD: 1.38), 3.54 Gy (SD: 1.41), and 3.74 Gy (SD: 1.46), respectively. Similarly, the mean dose received by rectum point for these source step sizes were 2.86 Gy (SD: 0.64), 3.02 Gy (SD: 0.67), 3.25 Gy (SD: 0.71), and 3.63 Gy (SD: 0.73), respectively. The treatment time and prescribed dose coverage volume were both found to be gradually increasing with increase in step size.
Conclusions: Our results on Ca-cx brachytherapy using Co-60 source indicate that the prescribed dose volume gradually increases from smaller source step to larger source step size. This results in increase of dose to the bladder and rectum and may lead to increase in toxicity and reduces quality of life. The study recommends that step size more than 5 mm should not be used for uterine cervix intracavitary application using Co-60 source.
Keywords: Computed tomography; High dose rate; Intracavitary brachytherapy; Step size.
Copyright © 2018 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.