Poster - Thur Eve - 64: A Water Calorimetry-Based Dosimetry Standard for Direct Measurement of Absolute Absorbed Dose in Scanning Proton Beam Delivery

Abstract

The aim of this work is to develop a novel water calorimetry‐based primary standard in scanned proton beams. A relatively homogeneous dose distribution was painted with 15 layers of proton energies ranging between 128–150 MeV. A 4°C Domen‐type stagnant water calorimeter with a parallel‐plate vessel was used to directly measure the absolute absorbed dose to water D w on the central axis of the beam, at a point of minimal dose non‐uniformity (peak‐to‐trough variation < 0.25%). With the water calorimeter, the dose contributions of the individual layers to the point of measurement as well the total dose delivered were analyzed. In order to validate the calorimeter, absolute dosimetry in scattered beams (250 MeV) was performed and all calorimetric results were compared with D w determined using an Exradin T1 Mini Shonka chamber following TRS‐398 protocol. The overall 1‐sigma uncertainty on the absorbed dose determination was 0.38% (scattered) and 0.64% (scanned), compared to an estimated 1.9% uncertainty obtained with TRS‐398. The agreement between the calorimetric results and TRS‐398‐based ion chamber results was better than 0.14% (scattered) and 0.32% (scanned). The feasibility of water calorimetry in proton therapy in general and scanned beam delivery in particular has been shown both experimentally and numerically. Relative to current protondosimetry protocols (such as TRS‐398), not only would a water calorimetry‐based primary standard measure the absolute D w directly, but it would also improve the uncertainty on the overall dose measurement by a factor of 3.