Two‐dimensional measurement of photon beam attenuation by the treatment couch and immobilization devices using an electronic portal imaging device

Abstract

In our institution, an individualized dosimetric quality assurance protocol for intensity modulated radiotherapy (IMRT) is being implemented. This protocol includes dosimetric measurements with a fluoroscopic electronic portal imaging device (EPID) for all IMRT fields while the patient is being irradiated. For some of the first patients enrolled in this protocol, significant beam attenuation by (carbon fiber) components of the treatment couch was observed. To study this beam attenuation in two-dimensional, EPID images were also acquired in absence of the patient, both with and without treatment couch and immobilization devices, as positioned during treatment. For treatments of head and neck cancer patients with a 6 MV photon beam, attenuation of up to 15% was detected. These findings led to the development of new tools and procedures for planning and treatment delivery to avoid underdosages in the tumor.

Effect of a carbon fiber tabletop on the surface dose and attenuation for high-energy photon beams

“…Second, the dose delivered to the patient's skin due to the loss of the skin-sparing effect in the buildup region may lead to clinically important skin reactions. Although the attenuation and loss of buildup caused by the carbon fi ber tabletop has already been demonstrated by several groups, [12][13][14][15] there are few studies with respect to its effects on the surface dose. On the other hand, these studies have not refl ected the data concerning all types of tabletop.…”

Dose perturbations by two carbon fiber treatment couches and the ability of a commercial treatment planning system to predict these effects

“…4 When the couch is between the radiation source and the patient, the dose distribution is altered by both attenuation and the loss of the skin sparing in the dose buildup region. [5][6][7] This work examines these two effects for two different solid top carbon fiber couches as well as the ability of a commercial treatment planning system ͑CMS XiO version 4.33.02͒ to model these perturbations. It is important to document the magnitude of these dose effects and how well they are modeled by a modern TPS, as this information is required to determine if the clinical problem warrants resource allocation.…”

The Effect of a Carbon-Fiber Couch on the Depth-Dose Curves and Transmission Properties for Megavoltage Photon Beams

“…However, attention must be paid when irradiating through the couch with oblique gantry angles. In their study on head-and-neck cancer irradiation with a 6-MV photon beam, Vieira et al [19] measured 15% beam attenuation for a posterior oblique beam. Yorke [23] and Muthuswamy & Lam [14] calculated a range of forbidden gantry angles for specific patient setups.…”