A new water calorimeter for modern radiotherapy

Abstract

Accuracy in radiotherapy is of vital importance for individual patient treatment and for clinical trials on which patient treatments. There is a continuous demand for improvement of dosimetry methods and adaption of dosimetry (primary) standards for new applications, including the introduction of new treatment modalities (e.g. FFF) and reference conditions (e.g. magnetic fields in MRgRT). For primary standard dosimetry laboratories (PSDLs), it is generally not feasible to acquire such modalities. Therefore, it is strongly desired that Dw measurements with primary standards can be performed on-site in clinical beams for new treatment modalities. In this study, a transportable absorbed dose to water primary standard, a water calorimeter, was developed for operation on-site in various beam geometries including MRI-incorporated treatment machines. The relative combined standard uncertainty (k = 1) for Dw measurements with the new standard in 60Co and MV-photons was 0.37 % (k = 1). Calibrations were carried out with a standard uncertainty of 0.42 % and kQ-factors are determined with a standard uncertainty of 0.40 %. An international key comparison with the BIPM (BIPM.RI(I)-K6) resulted in registration of related calibration measurement capabilities in the BIPM key comparison database (KCDB). This provided the required evidence for international acceptance of the water calorimeter as the Dutch absorbed dose to water primary standard for accelerator photon beams, ensuring acceptance of its measurement results through the CIPM Mutual Recognition Arrangement, CIPM MRA. Measurements with the new primary standard were performed in FFF-cFF beam pairs at 6 MV and 10 MV of an Elekta Versa HD for a selection of three different Farmer-type ion chambers. Based on these measurements and with the equipment described, it was concluded that the kQ factors provided by current codes of practice can be applied for flattening filter free beams without additional correction. However, an ion chamber volume averaging correction must be applied. Commissioning of the calorimeter in the 7 MV-photon beam of a pre-clinical MRI-linac in a 1.5 T magnetic field showed that the calorimeter corrections are either independent of magnetic field or can be determined in a magnetic field. The uncertainty for measurement of Dw with a water calorimeter in a 1.5 T magnetic field was estimated to be the same as under conventional reference conditions. Therefore, the VSL water calorimeter can be applied as a primary standard for Dw in magnetic fields and is currently the only primary standard operable in a magnetic field that provides direct access to the international traceability framework. For the first time ion chamber correction factors, kQ and kB, were directly measured in an MRI-linac. It was concluded that calculated and measured results in existing literature agree with the new values. The values in this study are currently the only available measured values for kQ and kB in an MRI-linac that are directly linked to the international traceability framework for the quantity absorbed dose to water, Dw. The new primary standard and its application stimulates the development of new measurement methods and contributes to a safe introduction of future radiation techniques.