Report of AAPM Therapy Physics Committee Task Group 74: In‐air output ratio, , for megavoltage photon beams

Zhu, Timothy C.; Ahnesjö, Anders; Lam, Kwok L.; Li, X. Allen; Charlie, C.‐M.; Palta, Jatinder; Sharpe, Michael B.; Thomadsen, Bruce R.; Tailor, Ramesh

doi:10.1118/1.3227367

Cited by 81 publications

(119 citation statements)

References 165 publications

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“…linac head scatter) 9 , 10 . The exchange effect is tested using the ratio of the output factors for two long, narrow fields with the length greater than five times the width.…”

Section: Methodsmentioning

confidence: 99%

Using a 2D detector array for meaningful and efficient linear accelerator beam property validations

Ritter

Gallagher

Roberson

2014

J Applied Clin Med Phys

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Following linear accelerator commissioning, the qualified medical physicist is responsible for monitoring the machine's ongoing performance, detecting and investigating any changes in beam properties, and assessing the impact of unscheduled repairs. In support of these responsibilities, the authors developed a method of using a 2D ionization chamber array to efficiently test and validate important linear accelerator photon beam properties. A team of three physicists identified critical properties of the accelerator and developed constancy tests that were sensitive to each of the properties. The result was a 14‐field test plan. The test plan includes large and small fields at varying depths, a reduced SSD field at shallow depth for sensitivity to extra focal photon and electron components, and analysis of flatness, symmetry, dose, dose profiles, and dose ratios. Constancy tests were repeated five times over a period of six weeks and used to set upper and lower investigation levels at ±.15em3 SDs. Deliberate variations in output, penumbra, and energy were tested to determine the suitability of the proposed method. Measurements were also performed on a similar, but distinct, machine to assess test sensitivity. The results demonstrated upper and lower investigation levels significantly smaller than the comparable TG‐142 annual recommendations, with the exception of the surrogate used for output calibration, which still fell within the TG‐142 monthly recommendation. Subtle changes in output, beam energy, and penumbra were swiftly identified for further investigation. The test set identified the distinct nature of the second accelerator. The beam properties of two photon energies can be validated in approximately 1.5 hrs using this method. The test suite can be used to evaluate the impact of minor repairs, detect changes in machine performance over time, and supplement other machine quality assurance testing.PACS numbers: 87.56bd, 87.56Fc

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“…linac head scatter) 9 , 10 . The exchange effect is tested using the ratio of the output factors for two long, narrow fields with the length greater than five times the width.…”

Section: Methodsmentioning

confidence: 99%

Using a 2D detector array for meaningful and efficient linear accelerator beam property validations

Ritter

Gallagher

Roberson

2014

J Applied Clin Med Phys

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“…Task Group 74 was one of the first to define this concept. Collimator exchange effect in the range of 3%–5% was reported for Elekta, Siemens, and Varian machines at three different energies (26) . However, the smallest field size TG‐74 studied was 4 cm.…”

Section: Introductionmentioning

confidence: 93%

“…The collimator exchange effect describes the difference in OFs in rectangular field, depending on which side of the rectangle delineates the jaws (26) . It results from the difference in X and Y jaw backscatter into the monitor chamber.…”

Section: Introductionmentioning

confidence: 99%

“…With the recent improvement in image quality and localization accuracy, stereotactic radiosurgery (SRS) often target much smaller tumors. At small fields, the extent of Y jaw backscatter increases exponentially, contributing to a significant decrease in OF (26) . Consequently, the collimator exchange effect should be quantified and accounted for in the treatment planning system for small fields.…”

Section: Introductionmentioning

confidence: 99%

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Deriving detector‐specific correction factors for rectangular small fields using a scintillator detector

Qin

Zhong

Snyder

et al. 2016

J Applied Clin Med Phys

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The goal of this study was to investigate small field output factors (OFs) for flattening filter‐free (FFF) beams on a dedicated stereotactic linear accelerator‐based system. From this data, the collimator exchange effect was quantified, and detector‐specific correction factors were generated. Output factors for 16 jaw‐collimated small fields (from 0.5 to 2 cm) were measured using five different detectors including an ion chamber (CC01), a stereotactic field diode (SFD), a diode detector (Edge), Gafchromic film (EBT3), and a plastic scintillator detector (PSD, W1). Chamber, diodes, and PSD measurements were performed in a Wellhofer water tank, while films were irradiated in solid water at 100 cm source‐to‐surface distance and 10 cm depth. The collimator exchange effect was quantified for rectangular fields. Monte Carlo (MC) simulations of the measured configurations were also performed using the EGSnrc/DOSXYZnrc code. Output factors measured by the PSD and verified against film and MC calculations were chosen as the benchmark measurements. Compared with plastic scintillator detector (PSD), the small volume ion chamber (CC01) underestimated output factors by an average of ‐1.0%±4.9%(max.=‐11.7% for 0.5×0.5.2emcm2 square field). The stereotactic diode (SFD) overestimated output factors by 2.5%±0.4%(max.=3.3% for 0.5×1.2emcm2 rectangular field). The other diode detector (Edge) also overestimated the OFs by an average of 4.2%±0.9%(max.=6.0% for 1×1.2emcm2 square field). Gafchromic film (EBT3) measurements and MC calculations agreed with the scintillator detector measurements within 0.6%±1.8% and 1.2%±1.5%, respectively. Across all the X and Y jaw combinations, the average collimator exchange effect was computed: 1.4%±1.1% (CC01), 5.8%±5.4% (SFD), 5.1%±4.8% (Edge diode), 3.5%±5.0% (Monte Carlo), 3.8%±4.7% (film), and 5.5%±5.1% (PSD). Small field detectors should be used with caution with a clear understanding of their behaviors, especially for FFF beams and small, elongated fields. The scintillator detector exhibited good agreement against Gafchromic film measurements and MC simulations over the range of field sizes studied. The collimator exchange effect was found to be important at these small field sizes. Detector‐specific correction factors were computed using the scintillator measurements as the benchmark.PACS number(s): 87.56.Fc

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“…It could be claimed that the medium (water or phantom) scatter contains a minor part of output. Output is the main topic discussed today for extremely small and/or severe irregularly shaped fields [5]. The air scatter data are involved directly or indirectly in obtaining the output.…”

Section: Introductionmentioning

confidence: 99%

The Impact of Small Field’s Off-Set on Output Factor in Intensity Modulated Radiation Treatment

Özdemir¹,

Tunçel²,

Kızıldaǧ³

2017

IJMPCERO

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In intensity modulated radiation treatment (IMRT) planning, the use of asymmetrically collimated fields that are placed on central axis or its off-set is mostly required. Output is the main topic discussed today for extremely small and/or severe irregularly shaped fields. The air scatter data are involved directly or indirectly in obtaining the output. Despite the fact that extensive data have been published in many studies to provide a guide on the magnitude of output factor for clinical accelerators, there are very few data reviewed about output factor in-air or phantom for off-set fields. This study was aimed to investigate the impact of these conditions for small fields. This study was conducted in Elekta Synergy linear accelerator which produces 6 MV X-ray energy. The in-air output factor (S c ) has been measured by CC04 ion chamber with brass-alloy "build-up" cap and Dose-1 electrometer, and the total output (S cp ) measurements were carried on at dose maximum depth in phantom by the same chamber and Thermoluminescence dosimeter (TLD) for 1 -10 cm 2 fields. The all measurements at center of isocenter and off-set fields at three directions (X2, Y1, Diagonal) were done. By decreasing field size from 10 to 2 cm 2 at isocenter, the Sc value using CC04 was decreased to 5.4% and S cp using CC04 and TLD to 14.5% and 11% respectively. By increasing off-set value, the S c and S cp values were increased in all directions comparing to central fields. The maximum increase was obtained in Y1 direction for S c and S cp . TLD results for S cp is slightly higher than CC04. The dosimetric properties of small fields and their off-set should be evaluated and modelled appropriately in the treatment planning system to ensure accurate dose calculation in Intensity Modulated Radiation Treatment.

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Report of AAPM Therapy Physics Committee Task Group 74: In‐air output ratio, , for megavoltage photon beams

Cited by 81 publications

References 165 publications

Using a 2D detector array for meaningful and efficient linear accelerator beam property validations

Using a 2D detector array for meaningful and efficient linear accelerator beam property validations

Deriving detector‐specific correction factors for rectangular small fields using a scintillator detector

The Impact of Small Field’s Off-Set on Output Factor in Intensity Modulated Radiation Treatment

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