Investigating treatment dose error due to beam attenuation by a carbon fiber tabletop

Abstract

Carbon fiber is a material commonly used in radiation therapy for treatment tabletops and various immobilization and support devices, partially because it is generally perceived to be almost radio-transparent to high-energy photons. Conformal avoidance of normal tissues during modern radiation therapy requires the delivery of radiation from all gantry angles. The geometry is such that many of these beams transit the couch proximal to the patient. In the planning process the effect of the radiation beam transiting the support structure of the couch is often neglected. In this study we investigate the attenuation of 6 and 18 MV photon beams by a Medtec (Orange City, IA, USA) carbon fiber couch. We have determined that neglecting the attenuation of oblique treatment fields by the carbon fiber couch can result in localized dose reduction from 4% to 16% depending on energy, field size and geometry. Further, we have investigated the ability of a commercial treatment planning system (Theraplan Plus version 3.8) to account for the attenuation by the treatment couch. Our results show that incorporating the carbon fiber couch in the patient model reduces the dose error to less than 2%. The variation in the dose reduction as a function of longitudinal position along the couch was also measured. In the triangular strut region of the couch, the attenuation fluctuated ± 0.5% following the periodic nature of the support structure. Based on our findings we propose the routine incorporation of the treatment tabletop into patient treatment planning dose calculations.