Multichannel vaginal cylinder brachytherapy-Impact of tumor thickness and location on dose to organs at risk

Purpose: Multichannel vaginal cylinder brachytherapy (MCVCB) has the potential to sculpt dose distribution, although this is typically reserved for lesions <5-mm thick. The aim of this study was to investigate the dosimetric consequences of treating lesions with MCVCB of varying locations, ≥5 mm in thickness.

Methods and materials: Patients previously treated with MCVCB were randomly selected to each fill one of six categories based on location (lateral, anterior, or vaginal cuff and/or apex) and size of cylinder (2.5 or 3.0 cm). Based on magnetic resonance image, each patient's target lesion was extended circumferentially into theoretical high-risk clinical target volumes measuring 5, 7, and 10 mm in thickness. Image-based brachytherapy treatment plans for each of the six patients' three target volumes were generated. Total 2 Gy per fraction equivalent dosages (EQD2) were calculated using an external beam radiation therapy dose of 45 Gy in 25 fractions in conjunction with a high-dose-rate brachytherapy dose of 25 Gy in five fractions.

Results: Maximum EQD2 vaginal surface doses in gray for 5-, 7-, and 10-mm targets were as follows (location-cylinder size): lateral-3.0 cm: 122/153/210, lateral-2.5 cm: 145/195/301, anterior-3.0 cm: 115/135/197, anterior-2.5 cm: 132/173/283, apex-3.0 cm: 173/241/367, and apex-2.5 cm: 349/461/706. Total rectal EQD2 D 2 cc ranged from 53.9 to 67.2 Gy. Total bladder EQD2 D 2 cc ranged from 51.5 to 71.2 Gy.

Conclusions: The vaginal surface dose seems to be the dose-limiting structure for anterior, lateral, and apical vaginal lesions. Caution should be taken when treating lesions >5 mm in depth, with particular attention to vaginal surface dose, especially for apical lesions and with smaller cylinders. In such cases, interstitial brachytherapy should be given strong consideration.