A standard for absorbed dose rate to water in a 60Co field using a graphite calorimeter at the National Metrology Institute of Japan

Morishita, Y.; Kato, Masahiro; Takata, N.; Kurosawa, Tadahiro; Tanaka, Takahiro; Saitô, Norio

doi:10.1093/rpd/ncs235

Cited by 17 publications

(14 citation statements)

References 4 publications

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“…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 .…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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Purpose: In this work, the design, operation, initial experimental evaluation, and characterization of a small-scale graphite calorimeter probe -herein referred to as the Aerrow -developed for routine use in the clinical environment, are described. Similar in size and shape to a Farmer type cylindrical ionization chamber, the Aerrow represents the first translation of calorimetry intended for direct use by clinical physicists in the radiotherapy clinic. Methods: Based on a numerically optimized design obtained in previous work, a functioning Aerrow prototype capable of two independent modes of operation (quasi-adiabatic and isothermal) was constructed in-house. Reference dose measurements were performed using both Aerrow operation modes in a 6 MV photon beam and were directly compared to results obtained with a calibrated reference-class ionization chamber. The Aerrow was then used to quantify the absolute output of five clinical linac-based photon beams (6 MV, 6 MV FFF, 10 MV, 10 MV FFF, and 15 MV; 63.2% < %dd(10)9 < 76.3%). Linearity, dose rate, and orientation dependences were also investigated. Results: Compared to an ion chamber-derived dose to water of 76.3 AE 0.7 cGy, the average doses measured using the Aerrow were 75.6 AE 0.7 and 74.7 AE 0.7 cGy/MU for the quasi-adiabatic and isothermal modes, respectively. All photon beam output measurements using the Aerrow in waterequivalent phantom agreed with chamber-based clinical reference dosimetry data within combined standard uncertainties. The linearity of the Aerrow's response was characterized by an adjusted R 2 value of 0.9998 in the dose range of 80 cGy to 470 cGy. For the dose-rate dependence, no statistically significant effects were observed in the range of 0.5 Gy/min to 5.4 Gy/min. A relative photon beam quality dependence of 1.7% was calculated in the range of 60 Co to 24 MV (58.4% < %dd(10)9 < 86.8%) using Monte Carlo. Finally, the angular dependence (gantry stationary and detector rotated) of the Aerrow's response was found to be insignificant to within AE0.5%. Conclusions: This work demonstrates the feasibility of using an ion chamber-sized calorimeter as a practical means of measuring absolute dose to water in the radiotherapy clinic. The potential introduction of calorimetry as a mainstream device into the clinical setting is powerful, as this fundamental technique has formed the basis of absorbed dose standards in many countries for decades and could one day form the basis of a new local absorbed dose standard for clinics.

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Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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“…This study focused on the point dose measurements. The ion chamber used in study was calibrated relative to the absorbed dose to water for reference beam quality, using 60 Co gamma-rays according to Japanese dose protocol [13]. The types of chamber used were PTW model 30013 (the ‘farmer’ chamber, active volume 0.6 cm 3 ) and PTW model 31016 (the ‘pin-point’ chamber, active volume 0.016 cm 3 ).…”

Section: Methodsmentioning

confidence: 99%

A management method for the statistical results of patient-specific quality assurance for intensity-modulated radiation therapy

Nakamura¹,

Okamoto²,

Wakita³

et al. 2016

Journal of Radiation Research

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There are many reports concerning patient-specific quality assurance (QA) for intensity-modulated radiation therapy (IMRT). However, reports about the statistical results of QA are lacking. Management methods for the results of the QA are needed, even though we have the ESTRO group recommendation that a tolerance limit of 1.96 standard deviation (SD) be established in each institution. The purpose of this study was to establish a management method for determining the tolerance limit and to report the statistical results of patient-specific QA. From April 2006 to March 2015, five linacs in the National Cancer Center, Tokyo, Japan, were used to treat 1185 patients with IMRT. Patient-specific QA was performed using an ion chamber, films, and some detectors. To establish a management method for the results, differences between the measured and calculated doses in the ion chamber were analyzed for each linac, each phantom, and each treatment site. The overall mean dose difference was 0.5 ± 1.3%, and the mean dose difference in each linac was 0.6 ± 1.2%, 0.9 ± 1.3%, −0.4 ± 1.4%, −0.1 ± 1.2% and −0.1 ± 0.9%. The difference between linacs and between treatment sites was significant (P < 0.001 and 0.01, respectively). The proportion of the dose difference within ±3% was 97.7%, and that was improved from 2006 to 2014. The results of the patient-specific QA should be managed for each linac and each treatment site in order to decide the suitable tolerance limit. Reports of statistical results will be helped if a new tolerance limit and action level will be considered.

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“…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 …”

Section: Introductionmentioning

confidence: 99%

“…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. [20][21][22][37][38][39][40][41][42][43][44][45] The purpose of this study is to build a prototype Aerrow and to evaluate its use as an absolute clinical dosimeter of high-energy photon beams while in the presence of a B = 1.5 T magnetic field. In this paper, the design and operating principles of the Aerrow system are presented, and a direct comparison of the detector against two reference-class IC types in a high-energy MRgRT photon beam is described.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2020

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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.

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A standard for absorbed dose rate to water in a 60Co field using a graphite calorimeter at the National Metrology Institute of Japan

Cited by 17 publications

References 4 publications

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

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

A management method for the statistical results of patient-specific quality assurance for intensity-modulated radiation therapy

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

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