Dosimetric comparison between intra-cavitary breast brachytherapy techniques for accelerated partial breast irradiation and a novel stereotactic radiotherapy device for breast cancer: GammaPodTM

The GammaPod (TM) device, manufactured by Xcision Medical Systems, is a novel stereotactic breast irradiation device. It consists of a hemispherical source carrier containing 36 Cobalt-60 sources, a tungsten collimator with two built-in collimation sizes, a dynamically controlled patient support table and a breast immobilization cup also functioning as the stereotactic frame for the patient. The dosimetric output of the GammaPod (TM) was modelled using a Monte Carlo based treatment planning system. For the comparison, three-dimensional (3D) models of commonly used intra-cavitary breast brachytherapy techniques utilizing single lumen and multi-lumen balloon as well as peripheral catheter multi-lumen implant devices were created and corresponding 3D dose calculations were performed using the American Association of Physicists in Medicine Task Group-43 formalism. Dose distributions for clinically relevant target volumes were optimized using dosimetric goals set forth in the National Surgical Adjuvant Breast and Bowel Project Protocol B-39. For clinical scenarios assuming similar target sizes and proximity to critical organs, dose coverage, dose fall-off profiles beyond the target and skin doses at given distances beyond the target were calculated for GammaPod (TM) and compared with the doses achievable by the brachytherapy techniques. The dosimetric goals within the protocol guidelines were fulfilled for all target sizes and irradiation techniques. For central targets, at small distances from the target edge (up to approximately 1 cm) the brachytherapy techniques generally have a steeper dose fall-off gradient compared to GammaPod (TM) and at longer distances (more than about 1 cm) the relation is generally observed to be opposite. For targets close to the skin, the relative skin doses were considerably lower for GammaPod (TM) than for any of the brachytherapy techniques. In conclusion, GammaPod (TM) allows adequate and more uniform dose coverage to centrally and peripherally located targets with an acceptable dose fall-off and lower relative skin dose than the brachytherapy techniques considered in this study.