Sci-Fri AM(2): Brachy-02: Realizing Absorbed Dose Directly for HDR Ir-192 Brachytherapy: Water Calorimetry and comparison to Ion Chamber, Gafchromic Film, and TG-43

Abstract

A new standard based on direct measurement of absolute absorbed dose to water in HDR 192 Ir brachytherapy using water calorimetry is discussed. The calorimetry results are compared with the results from the in‐water measurement of absorbed dose using ionization chamber and EBT Gafchromic film reference dosimetry, as well as TG‐43. The water calorimetry measurements were performed using a spring‐loaded catheter holder system that mounts onto a parallel‐plate calorimetric vessel. The COMSOL MULTIPHYSICS™ software was used to perform the numerical heat‐transport calculations. The calorimetric measurements were performed with three different 192 Ir sources with nominal S k ranging from 31600–36940 U (i.e. 7.6–9.2 Ci activity) over a one‐year period. A source‐to‐detector separation d src ‐ det ranging between 51–67 mm, and an irradiation time ranging between 200–300 s were used. Based on water calorimetry, an average dose rate to water of 361±11 μGy/(h.U) at d src ‐ det =55 mm was measured. Chamber reference dosimetry resulted in a dose rate measurement of 366±9 μGy/(h.U), while EBT Gafchromic film reference dosimetry resulted in 368±11 μGy/(h.U). From TG‐43 protocol, we obtain a value of 364±15 μGy/(h.U), a 0.8% difference from water calorimetry. All Results agree well to within uncertainty. Our work paves the way to successful primary absorbed dose determination for 192 Ir using calorimetric techniques.