An absorbed dose calorimeter for IMRT dosimetry

Abstract

A new calorimeter for dosimetry in small and complex fields has been built. The device is intended for the direct determination of absorbed dose to water in moderately small fields and in composite fields such as IMRT treatments, and as a transfer instrument calibrated against existing absorbed dose standards in conventional reference conditions. The geometry, materials and mode of operation have been chosen to minimize detector perturbations when used in a water phantom, to give a reasonably isotropic response and to minimize the effects of heat transfer when the calorimeter is used in non-reference conditions in a water phantom. The size of the core is meant to meet the needs of measurement in IMRT treatments and is comparable to the size of the air cavity in a type NE2611 ionization chamber. The calorimeter may also be used for small field dosimetry. Initial measurements in reference conditions and in an IMRT head and neck plan, collapsed to gantry angle zero, have been made to estimate the thermal characteristics of the device, and to assess its performance in use. The standard deviation (estimated repeatability) of the reference absorbed dose measurements was 0.02 Gy (0.6%).

Aerrow: A probe‐format graphite calorimeter for absolute dosimetry of high‐energy photon beams in the clinical environment

“…As a more direct alternative method to realize absorbed dose in nonstandard fields, new graphite and water calorimeters specifically designed to measure dose based on first principles are being developed . Oftentimes, these calorimetry systems are also made to be transportable to permit operation at the user's facility .…”

“…Once again, this was done to keep the Aerrow more in line with ionization chambers, but more importantly, measuring directly in water simplifies the dose conversion process and does away with the need for an additional transfer step. In contrast to all other graphite calorimeters, the Aerrow design incorporates aerogel‐based material as opposed to a vacuum to achieve thermal isolation from the surrounding environment . Air gaps have also been successfully used to provide thermal insulation in graphite calorimeter designs (e.g., NPL's portable photon/electron calorimeter), however this design feature necessitates the inclusion of mechanical supports such as expanded polystyrene beads .…”

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

“…As a more direct alternative method to realize absorbed dose in MRgRT, and more generally, in nonstandard fields, new transportable graphite and water calorimeters specifically designed to measure absorbed dose based on first principles at the user’s facilities have been developed . Even among primary standards, absorbed dose calorimetry is considered the most direct and absolute method of determining absorbed dose to water since device calibration can be achieved in terms of quantities with traceable standards (i.e.…”

“…Once again, this was done to keep the Aerrow more in‐line with ICs, but more importantly, measuring directly in water simplifies the dose conversion process and does away with the need for an additional transfer step. In contrast to all other graphite calorimeters, the Aerrow design incorporates a rigid aerogel‐based material as opposed to a vacuum to achieve thermal isolation from the surrounding environment …”

“…As a more direct alternative method to realize absorbed dose in MRgRT, and more generally, in nonstandard fields, new transportable graphite and water calorimeters specifically designed to measure absorbed dose based on first principles at the user's facilities have been developed. [20][21][22][23][24][25][26][27][28] Even among primary standards, absorbed dose calorimetry is considered the most direct and absolute method of determining absorbed dose to water since device calibration can be achieved in terms of quantities with traceable standards (i.e., electrical and temperature), entirely independent of radiation. [29][30][31][32] Thus, calorimetry-based measurements can form the basis of a direct absorbed dose calibration of an IC in the clinically relevant field, or a derivation of their correction factors (e.g., k B ) for those beams.…”