Ball phantom for dosimetry of surface and build-up region in megavoltage photon beam of oblique incidence through the couch

This study investigates surface dose and build-up region dosimetry as well as beam attenuation by ball phantom for coplanar and non-coplanar oblique photon beam incidence through a treatment couch in radiation therapy. The dosimetry of the surface and build-up region and attenuating incidence of true posterior and maximum oblique incidence (150 below surface) beams on the treatment couch are measured and compared with an open field. Photon calculation kernels of the computerized treatment planning system do not include the factor of scatter doses induced by the couch in the build-up region. These scatter doses can have some implications for skin reactions. Therefore, monitor unit calculation is necessary in posterior oblique angles or non-coplanar incidence of the treatment couch.