Direct measurement of absorbed dose to water in HDR I192r brachytherapy: Water calorimetry, ionization chamber, Gafchromic film, and TG‐43

Purpose: Gafchromic film and ionometric calibration procedures for HDRI192r brachytherapy sources in terms of dose rate to water are presented and the experimental results are compared to the TG‐43 protocol as well as with the absolute dose measurement results from a water calorimetry‐based primary standard. Methods: EBT‐1 Gafchromic films, an A1SL Exradin miniature Shonka thimble type chamber, and an SI HDR 1000 Plus well‐type chamber (Standard Imaging, Inc., Middleton, WI) with an ADCL traceable Sk calibration coefficient (following the AAPM TG‐43 protocol) were used. The Farmer chamber and Gafchromic film measurements were performed directly in water. All results were compared to direct and absolute absorbed dose to water measurements from a 4°C stagnant water calorimeter. Results: Based on water calorimetry, the authors measured the dose rate to water to be 361±7μGy/(hU) at a 55 mm source‐to‐detector separation. The dose rate normalized to air‐kerma strength for all the techniques agree with the water calorimetry results to within 0.83%. The overall 1‐sigma uncertainty on water calorimetry, ionization chamber, Gafchromic film, and TG‐43 dose rate measurement amounts to 1.90%, 1.44%, 1.78%, and 2.50%, respectively. Conclusions: This work allows us to build a more realistic uncertainty estimate for absorbed dose to water determination using the TG‐43 protocol. Furthermore, it provides the framework necessary for a shift from indirect HDRI192r brachytherapy dosimetry to a more accurate, direct, and absolute measurement of absorbed dose to water.