Experimental and theoretical dosimetry of the RIC-100 phosphorus-32 brachytherapy source for implant geometries encountered in the intraoperative setting

Purpose: Experimental and theoretical dosimetry of the RIC-100 phosphorus-32 brachytherapy source is presented for implant geometries that may occur in an intraoperative setting during treatment of localized spinal tumors with temporary superficial radiation. Dose variation, due to source shape and size, is evaluated, and nonideal implant conditions are simulated. Methods and Materials: Calibration, depth dose, and dose profiles were evaluated for several implant geometries and source sizes. Experimental measurements were performed using EBT3 gafchromic film. Theoretical calculations were performed using dose point kernel (DPK) formalism, which simulates isotropic, monoenergetic point sources distributed uniformly throughout the source and emitting electrons radially outward. Results: Calibration and depth dose for RIC-100 are independent of source size for diameters >1 cm. Sources should be ordered with physical dimensions ~0.2 cm larger than the target size, in all dimensions, to deliver >90% prescription dose to target edges. Relative dose profile shape is approximately constant as a function of target depth. Air gaps between the source and target cause narrower dose profile widths and shallower depth dose in the therapeutic range. DPK for RIC-100 agrees with published P-32 kernels, and DPK calculations agree with measurement (within 5%) for many depths and geometries. Conclusions: Intraoperative placement and measurement dosimetry of RIC-100 require careful setup due to steep dose gradients. Physical source dimensions should be chosen carefully based on treatment site dimensions, and air gaps between source and target should be minimized, to prevent underdosing the target in the lateral extent. Radiological scaling should be used to calculate expected dose when nonwater materials are used in experimental measurements, such as calibration or depth dose.