Multi-Channel Vaginal Cylinder Brachytherapy: Impact of Tumor Size and Location on Dose to Organs at Risk

Abstract

Multichannel vaginal brachytherapy (MCVB) allows dose to be preferentially delivered to particular portions of the vaginal side wall while sparing other regions. For superficial lesions MCVB has been used as an alternative to interstitial brachytherapy and can be thought of as an extension of intracavitary brachytherapy. Due to the exponential dose fall off associated with brachytherapy, treating lesions of increasing depth requires exponentially higher doses to more proximal tissue. The aim of this study was to investigate the dosimetric consequences of treating lesions of varying size and location with a multi-channel vaginal cylinder. We identified patients previously treated with MCVB and randomly selected one patient for each of six categories based on location (lateral, anterior, or vaginal cuff/apex) and size of cylinder (2.5 cm or 3.0 cm). Based on MRI, each patient’s target lesion was extended circumferentially into theoretical high risk clinical target volumes (HRCTV) measuring 5 mm, 7 mm, and 10 mm in depth. Rectum and bladder were contoured for each patient. Using Nucletron Oncentra version 4.0 (Nucletron, Veneendal Netherlands), image-based brachytherapy treatment plans for each of the six patients’ three target volumes were generated. Each plan was required to have a D90 of 100%. Total dose EQD2 dose was calculated using an EBRT dose of 45 Gy in 25 fractions in conjunction with a high dose rate (HDR) brachytherapy dose of 25 Gy in five fractions. Vaginal surface was contoured by creating a 1 mm expansion from the cylinder surface and subtracting out the cylinder. Maximum EQD2 vaginal surface doses in gray for 5 mm/7 mm/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, apex-2.5 cm: 349/461/706. This represents an average increase of 121 Gy to the vaginal surface when moving from a 5 mm thick lesion to a 10 mm thick lesion and treating with a 3 cm cylinder and 221 Gy for a 2.5 cm cylinder. Across all 18 plans, total EQD2 dose to 2 cc of the rectum ranged from 53.9-67.2 Gy. There was an average 3.44 Gy (range=0.83-6.01 Gy) absolute increase in rectal 2 cc EQD2 total dose when target depth increased from 5 mm to 10 mm. Total EQD2 bladder 2 cc dose ranged from 51.5-71.2 Gy. There was an average 5.24 Gy (range=0.66-9.99 Gy) absolute increase in total EQD2 bladder 2 cc dose when target depth increased from 5 mm to 10 mm. The vaginal surface dose appears to be the dose-limiting structure for anterior, lateral, and apical vaginal lesions. Caution should be taken when treating apical lesions and lesions greater than 5mm in depth especially with cylinders smaller than 3.0 cm in diameter. In such cases, interstitial brachytherapy should be given strong consideration.