Near-catheter dosimetry of a HDR brachytherapy source using Gafchromic film

High dose rate intraluminal brachytherapy treatments can be delivered using as few as one or two afterloading catheters, delivering doses of up to 10 Gy at 10 mm, leading to high dose gradients and extreme hot spots close to the catheter. These conditions have the potential to damage the patient's health tissues, possibly leading to necrosis, or even death from uncontrolled bleeding. Ionisation chambers and solid state detectors are limited in their usefulness for near-catheter dosimetry because of their physical size and in some cases energy and dose rate dependence. In contrast, radiochromic film has a large dose-response range, excellent spatial resolution, near-energy independence for megavoltage photons and the ability to measure dose in two dimensions, making it ideal for this application. The aim of this study was to measure the location and relative magnitude of any dosimetric hot spots produced by a typical endobronchial treatment plan. The study also investigated the effect of the step size of the (192)Ir source on both the dose hot spots and dose distribution. Our measurements show that for a typical single catheter treatment with 2.5 mm step size the maximum dose hot spots at the catheter surface are up to 37 times the prescription dose, up to 40 times for a 5 mm step size, and up to 46 times for a 10 mm step size. It is important that brachytherapy clinicians and physicists understand that hot spot magnitude increases with source step sizes and are aware of the risks associated with this form of brachytherapy treatment.