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A Fiber Optic Dosimeter to Calibrate P-32 Conformal Brachytherapy Sources

J Pursley

J Pursley1*, T Mauceri2, A Sliski3, (1) Brigham and Women's Hospital, Boston, MA, (2) Massachusetts General Hospital, Boston, MA, (3) Orbital Therapy LLC, Bedford, MA

WE-A-BRB-3 Wednesday 8:00:00 AM - 9:55:00 AM Room: Ballroom B

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.

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