Investigation of the bolusing effect of the Varian Exact™ IGRT couch on flattened and flattening filter-free (FFF) photon beams of a Varian TrueBeam linac

The bolusing effect of radiotherapy treatment couches has been studied under various conditions for conventional flattened beams (cFF) but data for flattening filter-free (FFF) beams are still limited. The multiple commercial implementations of FFF technology in addition to the variation in couch-top types used in clinics lead to potential variation in build-up characteristics and therefore investigations should be vendor-specific. This study explores the relative changes in the build-up region caused by the Varian Exact (TM) IGRT couch insert (medium) for both cFF and FFF beams of nominal energies of 6- and 10 MV in a Varian TrueBeam v.2.5 linear accelerator. A small volume parallel plate chamber was used to measure surface dose data with and without the presence of the couch for a range of field sizes. Surface dose increases significantly with respect to dmax when the couch is present. Relative skin/surface dose (RSD) - defined as the ratio of the ionisation at 0.5 mm depth to the maximum ionisation along the reference axis-for static beams increases almost linearly with increasing field size for all beams investigated but the variation of the RSD with field size is larger for flattened beams as compared with FFF beams and is attributed to the fact that this effect is dominated by electron contamination. As beam quality becomes softer, the variation of RSD with field size becomes smaller and it seems that the presence of the couch has a smaller effect. TPS investigation of skin doses with and without the presence of the couch has shown that in rotational beams the presence of the couch produces much less exaggerated changes to skin dose due to the smearing effect in VMAT dose deposition however direct translation of relative results to actual clinical implications is challenging.