A portable calorimeter for measuring absorbed dose in the radiotherapy clinic

Abstract

This paper describes the development of a robust and portable calorimeter for use in clinical electron and photon beams. Although intended for therapy-level dosimetry, the new calorimeter can also be used for high-dose measurements at industrial facilities. The system consists of a front end (the calorimeter itself), means for thermal isolation and temperature control, and a measurement system based on thermistors in a dc Wheatstone bridge. It was found from investigation that the heat transfer between components was significant. The restrictions on the design placed by the requirement for portability led to higher heat transfer than was desirable. Much effort was put into thermodynamic modelling of the system and determining the heat transfer coefficients. Effort was also focused on the development of a temperature control system sensitive enough to allow measurements of temperature rises of the order of 1 mK. The control system responds to the calorimeter, phantom and air temperatures and maintains the temperature of the calorimeter to within +/-0.2 mK over several hours. Initial operation at the National Physical Laboratory (NPL) in x-ray and electron beams from the NPL linear accelerator shows that the system is capable of measurements of 1 Gy at 2 Gy min(-1) with a random uncertainty of +/-0.3% (1 standard deviation). Operation in 60Co at a doserate of 1 Gy min(-1) has also been achieved with a similar uncertainty. It is intended to test the calorimeter 'in the field' during 2000.

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 . Thus, calorimetry‐based dose measurements can form the basis of a direct dose calibration of an ionization chamber in the clinically relevant field, or a derivation of their correction factors for those beams.…”

“…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 . In this work, aerogel was opted to simplify the assembly process, to maximize the compactness, and to improve the structural robustness of the device.…”

“…Although calorimeters have been used as transfer standards by primary dosimetry calibration laboratories (e.g., Hofmeester and McEwen and Duane), this work is the first that has the overarching goal to develop a calorimeter suitable for widespread use by clinicians. Given the state of the calorimetry at the level of the standards labs, it is not inconceivable that such a device could be refined to the point of measuring absorbed dose to water more accurately than currently achievable with ionization chambers, currently the ‘gold standard’.…”

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.…”

“…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.…”