Penumbral measurements in water for high‐energy x rays

Dawson, D. J.; Schroeder, Norman Jed; Hoya, J.

doi:10.1118/1.595963

Cited by 28 publications

(23 citation statements)

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“…As shown by Dawson et al (24) , the 80% – 20% penumbra width measured with a 6‐mm diameter chamber would be 3 mm larger than the width measured with an infinitely narrow detector for 6‐MV photons. This observation is generally supported by the data in Fig.…”

Section: Resultsmentioning

confidence: 79%

Experimental validation of the Eclipse AAA algorithm

Breitman

Rathee

Newcomb

et al. 2007

J Applied Clin Med Phys

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The present study evaluates the performance of a newly released photon‐beam dose calculation algorithm that is incorporated into an established treatment planning system (TPS). We compared the analytical anisotropic algorithm (AAA) factory‐commissioned with “golden beam data” for Varian linear accelerators with measurements performed at two institutions using 6‐MV and 15‐MV beams. The TG‐53 evaluation regions and criteria were used to evaluate profiles measured in a water phantom for a wide variety of clinically relevant beam geometries. The total scatter factor (TSF) for each of these geometries was also measured and compared against the results from the AAA.At one institute, TLD measurements were performed at several points in the neck and thoracic regions of a Rando phantom; at the other institution, ion chamber measurements were performed in a CIRS inhomogeneous phantom. The phantoms were both imaged using computed tomography (CT), and the dose was calculated using the AAA at corresponding detector locations. Evaluation of measured relative dose profiles revealed that 97%, 99%, 97%, and 100% of points at one institute and 96%, 88%, 89%, and 100% of points at the other institution passed TG‐53 evaluation criteria in the outer beam, penumbra, inner beam, and buildup regions respectively. Poorer results in the inner beam regions at one institute are attributed to the mismatch of the measured profiles at shallow depths with the “golden beam data.”For validation of monitor unit (MU) calculations, the mean difference between measured and calculated TSFs was less than 0.5%; test cases involving physical wedges had, in general, differences of more than 1%. The mean difference between point measurements performed in inhomogeneous phantoms and Eclipse was 2.1% (5.3% maximum) and all differences were within TG‐53 guidelines of 7%. By intent, the methods and evaluation techniques were similar to those in a previous investigation involving another convolution–superposition photon‐beam dose calculation algorithm in another TPS, so that the current work permitted an independent comparison between the two algorithms for which results have been provided.PACS number: 87.53.Dq

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Section: Resultsmentioning

confidence: 79%

Experimental validation of the Eclipse AAA algorithm

Breitman

Rathee

Newcomb

et al. 2007

J Applied Clin Med Phys

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“…The reduction becomes more pronounced when profile data are collected in edge‐on orientation. Assuming a linear relationship with slope = 1 between detector size‐induced deviations in penumbra width and axial detector radius, r, 30 , 31 deviations over the profile penumbra are expected to be

\sim 0.5 r

or 0.3 mm for the diode detector in axial orientation. In edge‐on orientation, the effective detector cross‐section becomes rectangular in shape and over an order of magnitude smaller in width (i.e., 20 μm).…”

Section: Discussionmentioning

confidence: 99%

Evaluation of the dosimetric properties of a diode detector for small field proton radiosurgery

McAuley

Teran

Slater

et al. 2015

J Applied Clin Med Phys

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The small fields and sharp gradients typically encountered in proton radiosurgery require high spatial resolution dosimetric measurements, especially below 1–2 cm diameters. Radiochromic film provides high resolution, but requires postprocessing and special handling. Promising alternatives are diode detectors with small sensitive volumes (SV) that are capable of high resolution and real‐time dose acquisition. In this study we evaluated the PTW PR60020 proton dosimetry diode using radiation fields and beam energies relevant to radiosurgery applications. Energies of 127 and 157 MeV (9.7 to 15 cm range) and initial diameters of 8, 10, 12, and 20 mm were delivered using single‐stage scattering and four modulations (0, 15, 30, and 60 mm) to a water tank in our treatment room. Depth dose and beam profile data were compared with PTW Markus N23343 ionization chamber, EBT2 Gafchromic film, and Monte Carlo simulations. Transverse dose profiles were measured using the diode in "edge‐on" orientation or EBT2 film. Diode response was linear with respect to dose, uniform with dose rate, and showed an orientation‐dependent (i.e., beam parallel to, or perpendicular to, detector axis) response of less than 1%. Diode vs. Markus depth‐dose profiles, as well as Markus relative dose ratio vs. simulated dose‐weighted average lineal energy plots, suggest that any LET‐dependent diode response is negligible from particle entrance up to the very distal portion of the SOBP for the energies tested. Finally, while not possible with the ionization chamber due to partial volume effects, accurate diode depth‐dose measurements of 8, 10, and 12 mm diameter beams were obtained compared to Monte Carlo simulations. Because of the small SV that allows measurements without partial volume effects and the capability of submillimeter resolution (in edge‐on orientation) that is crucial for small fields and high‐dose gradients (e.g., penumbra, distal edge), as well as negligible LET dependence over nearly the full the SOBP, the PTW proton diode proved to be a useful high‐resolution, real‐time metrology device for small proton field radiation measurements such as would be encountered in radiosurgery applications.PACS numbers: 87.56.‐v, 87.56.jf, 87.56.Fc

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“…The use of microchambers (active volume = 10 -3 cm 3 ) is recommended [5,12] to minimize the broadening of the penumbra. Dawson et al reported that the dependence of the penumbral width on the detector size is independent of beam energy and depth in water [4] but is linear with detector size at a single depth. This linear relationship was used to correct penumbral width values with detector size [2].…”

Section: Introductionmentioning

confidence: 97%

“…The effect of the detector volume on measured penumbral width has been widely published in the literature [2][3][4][5][6][7][8][9] in terms of convoluting the real profile, due to volume averaging and electron transport alteration by the detector [10]. Recently, American Association of Physicists in Medicine [11] released a report recommending the use of small volume chambers for small fields B4 9 4 cm 2 [12].…”

Section: Introductionmentioning

confidence: 99%

A study into the relationship between the measured penumbra and effective source size in the modeling of the Pinnacle RTPS

Yuen

Hardcastle

Metcalfe

2011

Australas Phys Eng Sci Med

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The effect of detector size in the broadening of the penumbra on the model in the Pinnacle RTPS is investigated. A second order polynomial was devised to correlate the source size parameter with the RTPS-calculated penumbra. The optimal source size parameter was calculated for penumbra measurements based on the diamond detector and a standard ionization chamber (IC). This work was done for Jaw fields, MLC fields with a leaf end radius of 8 cm, and MLC fields with a leaf end radius of 12 cm. The optimum source size of the 8 cm MLC fields matched the jaw fields, and an average (based on field sizes studied) of 1.1 mm for the diamond detector data and 2.4 mm for the ionization chamber was established. The effect of this overestimation of the source size parameter based on detector-induced penumbra broadening was considered for a clinical IMRT prostate plan by using two models (diamond and IC). There were differences in the DVH of the PTV and of OARs but these effects were of negligible clinical significance. Dose difference distributions showed dose difference areas to be in penumbra regions of the segments, with larger dose differences where penumbras intersected and/or there was a significant weighting on the segment. Gamma analysis was also performed between the two plans, and was found to increase the amount of fail rates significantly for both 2%/2 mm and 3%/3 mm criteria. This decreases the sensitivity of IMRT QA in the detection of systematic errors.

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Penumbral measurements in water for high‐energy x rays

Cited by 28 publications

References 0 publications

Experimental validation of the Eclipse AAA algorithm

Experimental validation of the Eclipse AAA algorithm

Evaluation of the dosimetric properties of a diode detector for small field proton radiosurgery

A study into the relationship between the measured penumbra and effective source size in the modeling of the Pinnacle RTPS

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