P-32 Dosimetry: A Comparison Between Diodes and Gafchromic Film

Abstract

The RIC-100 conformal P-32 source (R.I. Consultants, Hudson, NH) is used for intraoperative brachytherapy. The manufacturer specification of activity as well as previous literature (Cohen et al., 2014) is based on diode dosimetry. Gafchromic film was explored as an alternative dosimeter. Measurements were taken of a 2.3 cm × 3.8 cm rectangular P-32 film with a statement of activity of 18.6 mCi. P-32 has a maximum energy of 1.71 MeV with an expected range in water of approximately 7 mm. The half-life of this source is 14.28 days. The manufacturer’s specified dose rate for P-32 is 38 cGy/min per mCi/cm2 at 1 mm, the clinically relevant depth. Pre-treatment dosimetry of this source was performed with two dosimeters: an electron diode (IBA Electron Field Detector EFD3G) and EBT3 film (Ashland Inc, Covington, KY). The diode methodology followed the procedure previously published by Cohen et. al. This involves calibrating the diode in a 6 MeV electron beam and taking a series of measurements with varying thicknesses of solid water. These measurements establish the PDD and the output at 1 mm under the assumption of an effective depth of 0.7 mm for the detector front surface. EBT3 film was calibrated in an analogous fashion by using a 6 MeV electron beam to create a film calibration curve. Measurements were then taken using the same thicknesses of solid water as were used with the diode. To calculate the output at 1 mm, a depth of 0.144 mm, scaled by a density of 1.335 g/cm3, was used for the EBT3 film’s active layer based on the manufacturer’s specification. In addition, profile measurements and a continuous PDD were taken using EBT3 film, providing additional information difficult to obtain through diode measurements. The resulting output values at 1 mm were compared with the manufacturer’s statement of activity, and the PDD compared against published values (Cohen et al., 2014). Agreement of the source activity to within 5% of the manufacturer’s specification was considered acceptable. Using the published conversion factor of 38 cGy/min per mCi/cm2, the diode dosimetry resulted in a source activity of 19.38 mCi and the film dosimetry gave a source activity of 18.18 mCi. These were respectively 4.3% hotter and 2.3% colder than the manufacturer’s specification of 18.6 mCi. The measured step wise and continuous PDDs were also found to be consistent with expectation (Figure 1). In addition, profiles were analyzed and the source output found to have a standard deviation of 6.5%. EBT3 film is a viable alternative to diodes for pre-treatment dosimetry of P-32 sources. Additional benefits to film dosimetry include the ability to measure profiles and continuous PDDs.