Dosimetric characteristics of a low-kV intra-operative x-ray source: implications for use in a clinical trial for treatment of low-risk breast cancer.

The dosimetric characteristics of a low-kV x-ray device for performing intra-operative irradiations, the Intrabeam (Photoelectron Corporation, Lexington, MA), are examined. Two dosimetric models are considered--an analytical model considering only the primary x-ray beam, and a Monte Carlo model utilizing the EGSnrc code and a spherical simulation geometry. Both models prove reliable for verifying measured dose distributions for the device. The Monte Carlo model is necessary for examining spectral variations and the influence of inhomogeneities. The predictions of the Monte Carlo model are utilized to examine points of consideration for a multi-center clinical trial using the Intrabeam in the intra-operative, single fraction post-resection treatment of low-risk breast cancer. Predicted differences in radiological equivalence of breast tissue and water suggest a 3-5% under-dose of breast tissue (in a 50 kV beam) when dose fall-off data in water is used. A substantial dose enhancement in bone (i.e., ribs) adjacent to the treatment site is predicted though, based on published clinical data for radiation-induced rib fracture, it is concluded that induction of radiation-induced rib fracture would not pose a significant risk. Dose-volume changes with size of the treatment area (defined by the size of the resection volume) are examined indicating large variations in dose-volume characteristics across the range of possible "target" volumes.