WE‐A‐BRB‐03: A Fiber Optic Dosimeter to Calibrate P‐32 Conformal Brachytherapy Sources

Abstract

Purpose: Conformal brachytherapy plaques are used to irradiate the dura for paraspinal tumors. The plaques consist of Phosphorus‐32 chemically bonded to a flexible polymeric film. Radiochromic film has been successfully used to calibrate P‐32 plaques, but this method is time consuming due to the film characteristics. We investigate using a commercial fiber optic dosimetry system as a more efficient calibration method. Methods: A calibration curve for the radiochromic film was determined using an ADCL calibrated Strontium‐90 source, and this film is used to measure the dose rate of a P‐32 plaque. The Orbital Therapy system consists of a small cylindrical plastic scintillator coupled to a fiber optic cable. The dosimeter, controlled by a LabView‐based user interface, counts scintillation photons during a set time period and provides an immediate readout. Custom plastic phantoms hold the dosimeter probe in contact with a P‐32 plaque. The counts from the fiber optic dosimeter are plotted against the expected surface dose and modeled with a linear function to extract the dose deposited per count. Different measurement designs are considered to select a measurement time with accurate and reproducible results. Results: The fiber optic dosimeter is observed to have a linear dose response curve to both P‐32 and Sr‐90 sources. For a P‐32 plaque each count corresponds to 1.5 microGy, while for a Sr‐90 source each count corresponds to 1.16 microGy. We average 10 consecutive 6‐second measurements of the P‐32 plaque for a standard deviation of 0.1% and precision within 1%. Conclusions: The Orbital Therapy fiber optic dosimeter is acceptable for calibrating P‐32 conformal brachytherapy plaques. Initial calibration of the dosimeter requires a calibrated source of the same isotope, as the dose response depends on the beta energy spectrum. Once calibrated, the dosimeter is faster, simpler, and less error‐prone than using film for P‐32 plaque calibration.