Direct calibration in megavoltage photon beams using Monte Carlo conversion factor: validation and clinical implications

Wright, T.; Lye, Jessica; Ramanathan, G.; Harty, P. D.; Oliver, C.J.; Webb, D.V.; Butler, Duncan

doi:10.1088/0031-9155/60/2/883

Cited by 11 publications

(11 citation statements)

References 35 publications

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“…This increase can largely be explained by the changes in the restricted Spencer‐Attix water/air stopping power ratio, s w,air . Following TRS‐398, the known TPR 20,10 values for ARPANSA's MV photon beams can be used to calculate a s w,air as 1.120 for 6 MV (TPR 20,10 = 0.673), 1.105 for 10 MV (TPR 20,10 = 0.734), and 1.089 for 18 MV (TPR 20,10 = 0.777) 19, 29. The expected change in response for the LAC when going from 6 to 10 and to 18 MV is +1.3% and +2.8%, and this agrees reasonably with the actual observed increase of 1.0% and 2.7%.…”

Section: Discussionmentioning

confidence: 99%

Commissioning of a PTW 34070 large‐area plane‐parallel ionization chamber for small field megavoltage photon dosimetry

Kupfer

Lehmann

Butler³

et al. 2017

J Applied Clin Med Phys

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PurposeThis study investigates a large‐area plane‐parallel ionization chamber (LAC) for measurements of dose‐area product in water (DAP w) in megavoltage (MV) photon fields.MethodsUniformity of electrode separation of the LAC (PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high‐resolution microCT. Signal dependence on angle α of beam incidence for square 6 MV fields of side length s = 20 cm and 1 cm was measured in air. Polarity and recombination effects were characterized in 6, 10, and 18 MV photons fields. To assess the lateral setup tolerance, scanned LAC profiles of a 1 × 1 cm2 field were acquired. A 6 MV calibration coefficient, ND ,w, LAC, was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAP w. Additional calibrations in 10 × 10 cm2 fields at 6, 10, and 18 MV were performed.ResultsElectrode separation is uniform and agrees with specifications. Volume‐averaging leads to a signal increase proportional to ~1/cos(α) in small fields. Correction factors for polarity and recombination range between 0.9986 to 0.9996 and 1.0007 to 1.0024, respectively. Off‐axis displacement by up to 0.5 cm did not change the measured signal in a 1 × 1 cm2 field. ND ,w, LAC was 163.7 mGy cm−2 nC −1 and differs by +3.0% from the coefficient derived in the 10 × 10 cm2 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV.ConclusionsThe LAC requires only small correction factors for DAP w measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm2 fields, but beam incidence must be kept as close to normal as possible. Calibration in 10 × 10 fields is not recommended because of the LAC's over‐response. The accuracy of relative point‐dose measurements in the field's periphery is an important limiting factor for the accuracy of DAP w measurements.

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Section: Discussionmentioning

confidence: 99%

Commissioning of a PTW 34070 large‐area plane‐parallel ionization chamber for small field megavoltage photon dosimetry

Kupfer

Lehmann

Butler³

et al. 2017

J Applied Clin Med Phys

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“…This method of MC calculation is analogous to the direct conversion method described by Wright et al. of the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) for their standard graphite calorimeter, or for that matter, the beam quality conversion factors for ionization chambers disseminated in dosimetry protocols . In this work, a 3D model of the Aerrow inside a 30 × 30 × 30 cm 3 water phantom was modeled using the EGSnrc Monte Carlo code system with the egs_chamber user‐code of Wulff et al .…”

Section: Methodsmentioning

confidence: 99%

“…46 This method of MC calculation is analogous to the direct conversion method described by Wright et al of the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) for their standard graphite calorimeter, or for that matter, the beam quality conversion factors for ionization chambers disseminated in dosimetry protocols. 47 In this work, a 3D model of the Aerrow inside a 30 9 30 9 30 cm 3 water phantom was modeled using the EGSnrc 48,49 Monte Carlo code system with the egs_chamber user-code of Wulff et al 50 Geometries were modeled with the egs++ geometry package. 51 Cobalt-60 and Mohan photon spectra (4 MV, 6 MV, 10 MV, 15 MV, and 24 MV; 58.4% < %dd(10)9 < 86.8%) were used as simulation sources, which were set as 10 9 10 cm 2 parallel beams.…”

Section: E Dose Conversionmentioning

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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“…Perturbative effects of the aerogel gaps and the impurities are implicitly included in its determination. This approach is analogous to the direct dose conversion method described by Wright et al of the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) for their standard graphite calorimeter, or similarly, the beam quality conversion factors for ICs disseminated in dosimetry protocol. The EGSnrc MC code system with the egs_chamber user‐code of Wulff et al and the egs++ geometry package was used to create a 3D model of the Aerrow inside a ( w × d × h ) 30 × 30 × 20 cm 3 water phantom and a 20 × 20 × 23.7 cm 3 solid phantom (see Section 2.E. for solid phantom details) .…”

Section: Methodsmentioning

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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Direct calibration in megavoltage photon beams using Monte Carlo conversion factor: validation and clinical implications

Cited by 11 publications

References 35 publications

Commissioning of a PTW 34070 large‐area plane‐parallel ionization chamber for small field megavoltage photon dosimetry

Commissioning of a PTW 34070 large‐area plane‐parallel ionization chamber for small field megavoltage photon dosimetry

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

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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