Protocol for the measurement of the absorbed dose rate to water for a planar ³²P beta emitting brachytherapy source:...

This is a multi-institutional report on inter-observer and inter-instrument variation in the calibration of the absorbed dose rate for a planar ³²P beta emitting brachytherapy source. Measurement accuracy is essential since the dose profile is steep and the source is used for the treatment of tumors that are located in close proximity to healthy nervous system structures.

An RIC-100 ³²P source was calibrated by three institutions using their own equipment and following their standard procedures. The first institution calibrated the source with an electron diode and EBT3 film. The second institution used an electron diode. The third institution used HD810 film. Additionally, each institution was asked to calibrate the source using an electron diode and procedure that was shared among all institutions and shipped along with the radiation source. The dose rate was reported in units of cGy*min^-1 at a water equivalent depth of 1 mm.

Close agreement was observed in the measurements from different users and equipment. The variation across all diode detectors and institutions had a standard deviation of 1.8% and maximum difference of 4.6%. The observed variation among two different diode systems used within the same institution had a mean difference of 1.6% and a maximum variation of 1.8%. The variations among film and diode systems used within the same institution had a mean difference of 2.9% and a maximum variation of 4.3% CONCLUSIONS: The absorbed dose rate measurement protocol of the planar beta-emitting ³²P source permits consistent dosimetry across three institutions and five different electron diode and radiochromic film systems. The methodologies presented herein should enable measurement consistency among other clinical users, which will help ensure high quality patient treatments and outcomes analysis.