Effect of a carbon fiber tabletop on the surface dose and attenuation for high-energy photon beams

Purpose  The dose changes in the buildup region and beam attenuation by a carbon fiber tabletop were investigated for 6-and 18-MV photon beams. Materials and methods  Measurements were performed for 2 × 2 cm to 40 × 40 cm field sizes. The surface dose and percentage depth doses (PDD) were measured by a Markus parallel plate chamber. Attenuation measurements were made at the cylindrical phantom for 180° rotation of the beam. Results  A carbon fiber tabletop increases the surface dose from 7.5% to 63.0% and from 4% to 43% for small fields at 6 and 18 MV, respectively. The increase was nearly fivefold for the 10 × 10 cm field and nearly twofold for the 40 × 40 cm field. Beam attenuation of the tabletop varies from 3.0% to 5.6% for 180° and 120° gantry angles for 6 MV. Conclusion  The carbon fiber tabletop significantly decreases the skin-sparing effect. The dosimetric effect of the tabletop may be higher, especially for the intensity-modulated radiation therapy depending on the beam orientation. Attenuation should be considered and corrected such as any material under the patient at the treatment planning stage.