Evaluating and modeling of photon beam attenuation by a standard treatment couch

Hu, Zhihui; Dai, Jianrong; Li, Liang; Cao, Y; Fu, Guangbo

doi:10.1120/jacmp.v12i4.3561

Cited by 6 publications

(1 citation statement)

References 24 publications

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“…[8][9][10] Other treatment planning systems have also begun to offer methods to directly include the treatment couch top in the planning CT while others can accept modified CT datasets. [11][12][13][14] These methods will be discussed in more detail later in the report as well as guidance on the creation of plans in which beams largely miss external structures and on methods that predict when beams will pass through these structures.…”

Section: Introductionmentioning

confidence: 99%

Dosimetric effects caused by couch tops and immobilization devices: Report of AAPM Task Group 176

et al. 2014

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The dosimetric impact from devices external to the patient is a complex combination of increased skin dose, reduced tumor dose, and altered dose distribution. Although small monitor unit or dose corrections are routinely made for blocking trays, ion chamber correction factors, e.g., accounting for temperature and pressure, or tissue inhomogeneities, the dose perturbation of the treatment couch top or immobilization devices is often overlooked. These devices also increase skin dose, an effect which is also often ignored or underestimated. These concerns have grown recently due to the increased use of monolithic carbon fiber couch tops which are optimal for imaging for patient position verification but cause attenuation and increased skin dose compared to the "tennis racket" style couch top they often replace. Also, arc delivery techniques have replaced stationary gantry techniques which cause a greater fraction of the dose to be delivered from posterior angles. A host of immobilization devices are available and used to increase patient positioning reproducibility, and these also have attenuation and skin dose implications which are often ignored. This report of Task Group 176 serves to present a survey of published data that illustrates the magnitude of the dosimetric effects of a wide range of devices external to the patient. The report also provides methods for modeling couch tops in treatment planning systems so the physicist can accurately compute the dosimetric effects for indexed patient treatments. Both photon and proton beams are considered. A discussion on avoidance of high density structures during beam planning is also provided. An important aspect of this report are the recommendations the authors make to clinical physicists, treatment planning system vendors, and device vendors on how to make measurements of surface dose and attenuation and how to report these values. For the vendors, an appeal is made to work together to provide accurate couch top models in planning systems.

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Section: Introductionmentioning

confidence: 99%

Dosimetric effects caused by couch tops and immobilization devices: Report of AAPM Task Group 176

et al. 2014

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VMAT delivery through couch tops: an illustration of loss of dose coverage for prostate plans

et al. 2015

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Depth dose comparison of measured and calculated dose for the Eclipse virtual carbon couch top models with air gap variation

Foo

Stensmyr

2013

Australas Phys Eng Sci Med

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This study assessed the accuracy of Eclipse™ (Varian Medical Systems, Palo Alto, CA, USA) treatment planning system (TPS) dose calculations when using virtual couch top models to account for couch presence in patient treatments. The Flat panel and Unipanel couch tops for the Varian Exact Couch were used in this study. Assigned Hounsfield unit (HU) for the virtual couch tops were varied and TPS calculated dose was compared to measured data to determine an optimal assigned HU. Air gaps of up to 10 cm were introduced between couch and phantom to assess the ability of the models to replicate dose in this situation, commonly seen clinically. Dose was measured at a range of depths, for each air gap thickness, in order to assess the model both near surface and at various depths beyond the dose maximum. Optimal HU was taken to be that which had the best agreement between measured and calculated dose over the range of gaps and depths tested. For the Flat panel couch top this was found to be -500 HU and for the Unipanel couch top, -200 HU. Default HU parameters originally set in the models was found to be not optimal for the whole range of depths studied. With optimal HU parameters set, there was good agreement between calculated and measured dose for depths greater than 0.5 cm, but discrepancies were still observed near surface. When implementing virtual couch top models, users could improve dose calculation accuracy by determining the optimal HU from comparisons over several clinical depths rather than a single depth.

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Evaluating and modeling of photon beam attenuation by a standard treatment couch

Cited by 6 publications

References 24 publications

Dosimetric effects caused by couch tops and immobilization devices: Report of AAPM Task Group 176

Dosimetric effects caused by couch tops and immobilization devices: Report of AAPM Task Group 176

VMAT delivery through couch tops: an illustration of loss of dose coverage for prostate plans

Depth dose comparison of measured and calculated dose for the Eclipse virtual carbon couch top models with air gap variation

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