Absolute dosimetry of a 1.5 T MR‐guided accelerator‐based high‐energy photon beam in water and solid phantoms using Aerrow

Abstract

Purpose In this work, the fabrication, operation, and evaluation of a probe‐format graphite calorimeter — herein referred to as Aerrow — as an absolute clinical dosimeter of high‐energy photon beams while in the presence of a B = 1.5 T magnetic field is described. Comparable to a cylindrical ionization chamber (IC) in terms of utility and usability, Aerrow has been developed for the purpose of accurately measuring absorbed dose to water in the clinic with a minimum disruption to the existing clinical workflow. To our knowledge, this is the first reported application of graphite calorimetry to magnetic resonance imaging (MRI)‐guided radiotherapy. Methods Based on a previously numerically optimized and experimentally validated design, an Aerrow prototype capable of isothermal operation was constructed in‐house. Graphite‐to‐water dose conversions as well as magnetic field perturbation factors were calculated using Monte Carlo, while heat transfer and mass impurity corrections and uncertainties were assessed analytically. Reference dose measurements were performed in the absence and presence of a B = 1.5 T magnetic field using Aerrow in the 7 MV FFF photon beam of an Elekta MRI‐linac and were directly compared to the results obtained using two calibrated reference‐class IC types. The feasibility of performing solid phantom‐based dosimetry with Aerrow and the possible influence of clearance gaps is also investigated by performing reference‐type dosimetry measurements for multiple rotational positions of the detector and comparing the results to those obtained in water. Results In the absence of the B‐field, as well as in the parallel orientation while in the presence of the B‐field, the absorbed dose to water measured using Aerrow was found to agree within combined uncertainties with those derived from TG‐51 using calibrated reference‐class ICs. Statistically significant differences on the order of (2–4)%, however, were observed when measuring absorbed dose to water using the ICs in the perpendicular orientation in the presence of the B‐field. Aerrow had a peak‐to‐peak response of about 0.5% when rotated within the solid phantom regardless of whether the B‐field was present or not. Conclusions This work describes the successful use of Aerrow as a straightforward means of measuring absolute dose to water for large high‐energy photon fields in the presence of a 1.5 T B‐field to a greater accuracy than currently achievable with ICs. The detector‐phantom air gap does not appear to significantly influence the response of Aerrow in absolute terms, nor does it contribute to its rotational dependence. This work suggests that the accurate use of solid phantoms for absolute point dose measurement is possible with Aerrow.

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

“…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 . Renaud et al have also developed a portable graphite calorimeter and used it to conduct preliminary measurements in an MR‐integrated linac 24 …”

“…22,23 Renaud et al have also developed a portable graphite calorimeter and used it to conduct preliminary measurements in an MR-integrated linac. 24 In this work, we use our in-house designed and built MRcompatible, stagnant 4°C cooled portable water calorimeter to measure absorbed dose to water in an MR-integrated linac in order to measure k mag Q for an A1SL ionization chamber. This value is then compared against current Monte Carlo simulations for this chamber.…”

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

“…11,[19][20][21] 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][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 …”