en Radiation Biology Radiation Biology تحقيق بديع Original Research <div style="text-align: justify;">Background: Multiple beams are generally used with an increased possibility that the beam axis intersects the treatment table. Treatment tabletops are commonly made of carbon fiber due to its high mechanical strength and rigidity, low specific density, extremely light and low radiation beam attenuation properties. Purpose of this paper is investigated the dose changes in the buildup region and beam attenuation by a carbon fiber tabletop for high energy 6- and 18-MV photon beams.&nbsp; Materials and Methods: Measurements were performed for 10 cm &times; 10 cm and 20 cm &times; 20 cm field sizes. The surface dose and percentage depth doses (%DD) were measured by a Markus parallel plate chamber at a source-surface distance (SSD) of 100 cm for 6 MV and 18 MV photon beams. Attenuation measurements were made at the solid-water phantom for gantry angles of 0^oand 180^o rotation of the beam. Results: A carbon fiber tabletop increases the surface dose from 12.87% to 86.65% for 10 cm x 10 cm and from 8.72% to 71.16% for 20 cm &times; 20 cm field at 6 and 18 MV, respectively. The surface dose with the carbon fiber tabletop in an open field (0^o) increases with field size. Conclusion: The carbon fiber tabletop causes a substantially increased surface dose, and also significantly decreases the skin-sparing effect, which is clinically important. The dosimetric effect of the tabletop may be higher, especially for the intensity-modulated radiation therapy depending on the beam orientation.<br> &nbsp;</div> Radiation, radiotherapy, carbon fiber tabletop, surface dose, buildup region. 235 241 http://ijrr.com/browse.php?a_code=A-10-1-721&slc_lang=en&sid=1 G. Gursoy 7900319475328460011457 7900319475328460011457 No Department of Medical Services and Technical, Vocational School of Health Services, Ahi Evran University, Kırşehir, Turkey E. Eser eserphy@gmail.com 7900319475328460011458 7900319475328460011458 Yes Department of Physics, Polatlı Faculty of Arts and Sciences, Gazi University, Ankara,Turkey I. Yigitoglu 7900319475328460011459 7900319475328460011459 No Department of Physics, Faculty of Arts and Sciences, Gaziosmanpasa University, Tokat,Turkey H. Koç 7900319475328460011460 7900319475328460011460 No Department of Electrical and Electronics Engineering, Faculty of Engineering, Muş Alparslan University, Muş, Turkey F.C. Kahraman 7900319475328460011461 7900319475328460011461 No Provincial Health Directorate, Isparta, Turkey S. Yamcıcier 7900319475328460011462 7900319475328460011462 No Department of Medical Services and Technical, Vocational School of Health Services, Ahi Evran University, Kırşehir, Turkey