Development of a water calorimetry‐based standard for absorbed dose to water in HDR I 192 r brachytherapy

Purpose: The aim of this article is to develop and evaluate a primary standard for HDR I192 r brachytherapy based on 4 ° C stagnant water calorimetry. Methods: The absolute absorbed dose to water was directly measured for several different Nucletron microSelectronI192 r sources of air kerma strength ranging between 21 000 and 38 000 U and for source‐to‐detector separations ranging between 25 and 70 mm. The COMSOL MULTIPHYSICS ™ software was used to accurately calculate the heat transport in a detailed model geometry. Through a coupling of the “conduction and convection” module with the “Navier–Stokes incompressible fluid” module in the software, both the conductive and convective effects were modeled. Results: A detailed uncertainty analysis resulted in an overall uncertainty in the absorbed dose of 1.90% ( 1 σ ) . However, this includes a 1.5% uncertainty associated with a nonlinear predrift correction which can be substantially reduced if sufficient time is provided for the system to come to a new equilibrium in between successive calorimetric runs, an opportunity not available to the authors in their clinical setting due to time constraints on the machine. An average normalized dose rate of 361 ± 7 μ Gy / ( h U )at a source‐to‐detector separation of 55 mm was measured for the microSelectronI192 r source based on water calorimetry. The measured absorbed dose per air kerma strength agreed to better than 0.8%( 1 σ )with independent ionization chamber and EBT‐1 Gafchromic film reference dosimetry as well as with the currently accepted AAPM TG‐43 protocol measurements. Conclusions: This work paves the way toward a primary absorbed dose to water standard inI192 r brachytherapy.