Dosimetric characterization of the iBEAM evo carbon fiber couch for radiotherapy.

PURPOSE This study characterizes the dosimetric properties of the iBEAM evo carbon fiber couch manufactured by Medical Intelligence and examines the accuracy of the CMS XiO and Nucletron Oncentra Masterplan (OMP) treatment planning systems for calculating beam attenuation due to the presence of the couch. METHODS To assess the homogeneity of the couch, it was CT scanned at isocentric height and a number of signal intensity profiles were generated and analyzed. To simplify experimental procedures, surface dose and central axis depth dose measurements were performed in a solid water slab phantom using Gafchromic film for 6 and 10 MV photon beams at gantry angles of 0 degree (normal incidence), 30 degrees, and 60 degrees with an inverted iBEAM couch placed on top of the phantom. Attenuation measurements were performed in a cylindrical solid water phantom with an ionization chamber positioned at the isocenter. Measurements were taken for gantry angles from 0 degree to 90 degrees in 10 degrees increments for both 6 and 10 MV photon beams. This setup was replicated in the XiO and OMP treatment planning systems. Dose was calculated using the pencil beam, collapsed cone, convolution, and superposition algorithms. RESULTS The CT scan of the couch showed that it was uniformly constructed. Surface dose increased by (510 +/-0)% for a 6 MV beam and (600 +/- 20)% for a 10 MV beam passing through the couch at normal incidence. Obliquely incident beams resulted in a higher surface dose compared to normally incident beams for both open fields and fields with the couch present. Depth dose curves showed that the presence of the couch resulted in an increase in dose in the build up region. For 6 and 10 MV beams incident at 60 degrees, nearly all skin sparing was lost. Attenuation measurements derived using the ionization chamber varied from 2.7% (0 degree) to a maximum of 4.6% (50 degrees) for a 6 MV beam and from 1.9% (0 degree) to a maximum of 4.0% (50 degrees) for a 10 MV beam. The pencil beam and convolution algorithms failed to accurately calculate couch attenuation. The collapsed cone and superposition algorithms calculated attenuation within an absolute error of +/- 1.2% for 6 MV and +/- 0.8% for 10 MV for gantry angles from 0 degree to 40 degrees. Some differences in attenuation were observed dependent on how the couch was contoured. CONCLUSIONS These results demonstrate that the presence of the iBEAM evo carbon fiber couch increases the surface dose and dose in the build up region. The inclusion of the couch in the planning scan is limited by the field of view employed and the couch height at the time of CT scanning.