The extraction of true profiles for TPS commissioning and its impact on IMRT patient-specific QA

Yan, G; Fox, Christopher; Liu, Chihray; Li, Jonathan G.

doi:10.1118/1.2952643

Cited by 37 publications

(93 citation statements)

References 41 publications

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“…The method of profile deconvolution used in this work has been previously discussed 12 and is based on the work by Garcia-Vicente et al, 9 and will be discussed briefly here. The measured profile data can be described using the convolution of the detector response function and the true profile from…”

Section: Iib Profile Deconvolution and Beam Profile Fittingmentioning

confidence: 99%

“…This was considered a good approximation to the true penumbra. 9,12 It is known that diode detectors over respond to the low energy part of the spectrum. Consequently, obtaining the response function using large fields would include the effects of the over response.…”

Section: ͑3͒mentioning

confidence: 99%

“…However, the detector response has to be known in advance. 9,11 It has been shown by Yan et al 12 that for a particular choice of ionization chamber, a single detector response function can be used in the deconvolution process over a range of field sizes. Additionally, the benefits of profile deconvolution in TPS commissioning models applied to IMRT were investigated using the MAPCHECK ͑Sun Nuclear Inc. Melbourne, FL͒ 2D array for patient QA.…”

Section: Introductionmentioning

confidence: 99%

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Assessment of the setup dependence of detector response functions for mega‐voltage linear accelerators

Fox

Simon

et al. 2010

Medical Physics

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Purpose: Accurate modeling of beam profiles is important for precise treatment planning dosimetry. Calculated beam profiles need to precisely replicate profiles measured during machine commissioning. Finite detector size introduces perturbations into the measured profiles, which, in turn, impact the resulting modeled profiles. The authors investigate a method for extracting the unperturbed beam profiles from those measured during linear accelerator commissioning. Methods: In-plane and cross-plane data were collected for an Elekta Synergy linac at 6 MV using ionization chambers of volume 0.01, 0.04, 0.13, and 0.65 cm 3 and a diode of surface area 0.64 mm 2 . The detectors were orientated with the stem perpendicular to the beam and pointing away from the gantry. Profiles were measured for a 10ϫ 10 cm 2 field at depths ranging from 0.8 to 25.0 cm and SSDs from 90 to 110 cm. Shaping parameters of a Gaussian response function were obtained relative to the Edge detector. The Gaussian function was deconvolved from the measured ionization chamber data. The Edge detector profile was taken as an approximation to the true profile, to which deconvolved data were compared. Data were also collected with CC13 and Edge detectors for additional fields and energies on an Elekta Synergy, Varian Trilogy, and Siemens Oncor linear accelerator and response functions obtained. Response functions were compared as a function of depth, SSD, and detector scan direction. Variations in the shaping parameter were introduced and the effect on the resulting deconvolution profiles assessed. Results: Up to 10% setup dependence in the Gaussian shaping parameter occurred, for each detector for a particular plane. This translated to less than a Ϯ0.7 mm variation in the 80%-20% penumbral width. For large volume ionization chambers such as the FC65 Farmer type, where the cavity length to diameter ratio is far from 1, the scan direction produced up to a 40% difference in the shaping parameter between in-plane and cross-plane measurements. This is primarily due to the directional difference in penumbral width measured by the FC65 chamber, which can more than double in profiles obtained with the detector stem parallel compared to perpendicular to the scan direction. For the more symmetric CC13 chamber the variation was only 3% between in-plane and cross-plane measurements. Conclusions:The authors have shown that the detector response varies with detector type, depth, SSD, and detector scan direction. In-plane vs cross-plane scanning can require calculation of a direction dependent response function. The effect of a 10% overall variation in the response function, for an ionization chamber, translates to a small deviation in the penumbra from that of the Edge detector measured profile when deconvolved. Due to the uncertainties introduced by deconvolution the Edge detector would be preferable in obtaining an approximation of the true profile, particularly for field sizes where the energy dependence of the diode can be neglected. However, an averaged respons...

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Section: Iib Profile Deconvolution and Beam Profile Fittingmentioning

confidence: 99%

Section: ͑3͒mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Assessment of the setup dependence of detector response functions for mega‐voltage linear accelerators

Fox

Simon

et al. 2010

Medical Physics

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“…Previous studies concerning this specific case clearly show that a broadening of a beam's penumbra can be substantial when comparing measurements using different detectors 4 , 5 , 6 , 7 , 8 , 9 . This effect is also clinically demonstrated by Yan and colleagues (10) . With emphasis placed on the penumbra, methods for reconstructing beam profiles to account for the overestimation of the penumbra are available for ion chambers 10 , 11 , 12 .…”

Section: Introductionmentioning

confidence: 78%

The clinical impact of detector choice for beam scanning

Gersh

Best

Watts³

2014

J Applied Clin Med Phys

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Recently, the developers of Eclipse have recommended the use of ionization chambers for all profile scanning, including for the modeling of VMAT and stereotactic applications. The purpose of this study is to show the clinical impact caused by the choice of detector with respect to its ability to accurately measure dose in the penumbra and tail regions of a scanned profile. Using scan data acquired with several detectors, including an IBA CC13, a PTW 60012, and a Sun Nuclear EDGE Detector, three complete beam models are created, one for each respective detector. Next, using each beam model, dose volumes are retrospectively recalculated from actual anonymous patient plans. These plans include three full‐arc VMAT prostate plans, three left chest wall plans delivered using irregular compensators, two half‐arc VMAT lung plans, three MLC‐collimated static‐field pairs, and two SBRT liver plans. Finally, plans are reweighted to deliver the same number of monitor units, and mean dose‐to‐target volumes and organs at risk are calculated and compared. Penumbra width did not play a role. Dose in the tail region of the profile made the largest difference. By overresponding in the tail region of the profile, the 60012 diode detector scan data affected the beam model in such a way that target doses were reduced by as much as 0.4% (in comparison to CC13 and EDGE data). This overresponse also resulted in an overestimation of dose to peripheral critical structure, whose dose consisted mainly of scatter. This study shows that, for modeling the 6 MV beam of Acuros XB in Eclipse Version 11, the choice to use a CC13 scanning ion chamber or an EDGE Detector was an unimportant choice, providing nearly identical models in the treatment planning system.PACS number: 87.55.kh

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“…Yan et al [17] measured the effect of the detector volume on patient specific IMRT (intensity-modulated radiotherapy) QA and found that the passing rate between calculation and measurement with a Gamma index function [18] increased for a beam model measured with a smaller detector size. The main source of discrepancy between calculated and measured dose distribution was the detector size effect that produces enlarged penumbral widths in the measured data used for RTPS beam modeling [17].…”

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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The extraction of true profiles for TPS commissioning and its impact on IMRT patient-specific QA

Cited by 37 publications

References 41 publications

Assessment of the setup dependence of detector response functions for mega‐voltage linear accelerators

Assessment of the setup dependence of detector response functions for mega‐voltage linear accelerators

The clinical impact of detector choice for beam scanning

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

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