Determination of the photon beam attenuation by the BrainLAB imaging couch: Angular and field size dependence

Abstract

Highly attenuating radiation treatment couches are no longer useful in the present era of IMRT-and IGRT-utilizing radiotherapy. High tensile strength and low density carbon fiber couch tops present a useful alternative. The objective of this study is to quantify the attenuation of megavoltage photons through BrainLAB imaging couch top and headrest at various angles and field sizes. At normal incidence, the couch attenuated 6 MV photons by 4.9% and 3.4% for 5 × 5 cm2 and 10 × 10 cm2 field sizes, respectively. The headrest, alone, attenuated 6 MV photons by 2.5% and 1.6% for the same field sizes. There was no significant attenuation of the 18 MV beam by either the couch or the headrest. We further found attenuation to be gantry angle dependent, with the highest attenuation recorded at 120° - at which angle the couch attenuated the 6 MV photon beam by 10% for the 5 × 5 cm2 and 8.3% for the 10 × 10 cm2 field sizes. Similarly, an 18 MV photon beam was attenuated by 3.6% and 3.4% for the 5 × 5 cm2 and 10 × 10 cm2 field sizes, respectively. The highest attenuation for the headrest occurred at 110° gantry angle: for the 6 MV photon beam, the headrest attenuation was 6.3% and 5.6% (for the 5 × 5 cm2 and 10 × 10cm2 field sizes, respectively); for the 18 MV, attenuation was 2.3% (for the 5 × 5 cm2 field size) and 2.1% (for the 10 × 10 cm2 field size). It would appear, therefore, that BrainLAB imaging couch and headrest in IMRT with posterior beams results in significant decreases in the dose delivered to the target.