Commissioning of a water calorimeter as a primary standard for absorbed dose to water in magnetic fields

Abstract

MRI guided radiotherapy devices are currently in clinical use. Detector responses are affected by the magnetic field and need to be characterized in terms of absorbed dose to water, D w , against primary standards under these conditions. The aim of this study was to commission a water calorimeter, accepted as the Dutch national standard for D w in MV photons and to validate its claimed standard uncertainty of 0.37% in the 7 MV photon beam of a pre-clinical MRI-linac in a 1.5 T magnetic field.To evaluate the primary standard on a fundamental basis, realisation of D w at 1.5 T was evaluated parameter by parameter. A thermodynamic description was given to demonstrate potential temperature effects due to the magneto-caloric effect (MCE). Methods were developed for measurement of depth, variation in detector distance and beam output in the bore of the MRI-linac. This resulted in D w measurements with a magnetic field of 1.5 T and, after ramp-down, without magnetic field.It was shown that the measurement of ΔT w and calorimeter corrections are either independent of or can be determined in a magnetic field. The chemical heat defect, h, was considered zero within its stated uncertainty, as for 0 T. Evaluation of the MCE and measurements done during magnet rampdown, indicated no changes in the specific heat capacity of water. However, variations of the applied monitor system increased the uncertainty on beam output normalization.This study confirmed that the uncertainty for measurement of D w with a water calorimeter in a 1.5 T magnetic field is estimated to be the same as under conventional reference conditions. The VSL water calorimeter can be applied as a primary standard for D w in magnetic fields and is currently the only primary standard operable in a magnetic field that provides direct access to the international traceability framework. PAPER Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

Medical physics challenges in clinical MR-guided radiotherapy

“…Official guidelines for absolute dosimetry are however not currently established, placing a burden on early adopters. Primary standards labs and research groups have also recently worked towards MR-compatible water calorimeters [116][117][118] and graphite [118,119] calorimeters. This allows the direct measurement of ion chamber correction factors [120], which should soon lead to additional literature.…”

Water calorimetry in MR‐linac: Direct measurement of absorbed dose and determination of chamber

“…De Prez et al determined that the correction needed to account for this effect was insignificant at 1.00002 for two thermistors perpendicular to a 1.5 T field 20 . A varying magnetic field can also cause an adiabatic change in the temperature of the water through the magnetocaloric effect 18,32 . As calorimetric measurements were performed in a static 1.5 T field, this effect can be ignored.…”

“…This combined with the unique geometry constraints of the bore poses a challenge to primary standard laboratories as many of the water calorimeters currently in use may require a redesign to fit inside the bore and be fully MR‐compatible. Currently, the Van Swinden Laboratory (VSL) Dutch National Measurement Institute has completed construction of a portable MR‐compatible water calorimeter and used it to calibrate ionization chambers in the presence of a magnetic field 18,20,21 . In addition, the German National Measurement Institute (Physikalisch‐Technische Bundesanstalt) has also constructed an MR‐compatible water calorimeter and has used it to calibrate ionization chambers in a Viewray MRIdian 22,23 .…”

First‐stage validation of a portable imageable MR‐compatible water calorimeter

“…A probe-format graphite calorimeter called Aerrow was used in dosimetry of high energy photon beams in the presence of a magnetic field, 22 while the Van Swinden Laboratory (VSL), the Dutch national metrology institute, has constructed a portable MR-compatible water calorimeter that has been successfully used in the presence of magnetic fields. [23][24][25] While several graphite calorimeters exist for direct determination of absorbed dose in IMRT delivery methods, 26,27 to the best of the authors' knowledge there have not been any other calorimeters developed for other volumetric delivery techniques including volumetric arc therapy (VMAT) and Gamma Knife ® ICON TM .…”

“…Calorimetry efforts in the area of MR‐integrated linacs have been limited. A probe‐format graphite calorimeter called Aerrow was used in dosimetry of high energy photon beams in the presence of a magnetic field, 22 while the Van Swinden Laboratory (VSL), the Dutch national metrology institute, has constructed a portable MR‐compatible water calorimeter that has been successfully used in the presence of magnetic fields 23–25 …”