Background: The accuracy and the reproducibility of radiotherapy can be provided by using immobilization devices such as thermoplastic masks. In head and neck cancer radiotherapy, the patients are mostly immobilized by using thermoplastic masks. In this study, the effect of the thermoplastic mask to the surface and buildup region doses was investigated by using Markus parallel plate ion chamber, Gafchromic EBT3 film and MOSFET detector for the same field size and different beam angles. Materials and Methods: The measurements were made in a water equivalent solid phantom at the surface and in the buildup region of the 6 MV photon beams at 100 cm source-detector distance for 10 × 10 cm² field size and beam angles of 0°, 30°, 60° and 80° with and without thermoplastic mask. Results: The surface doses in 0.07 mm depth using 6MV photon beams without a thermoplastic mask for 10 × 10 cm² field size were found 20.3%, 18.8%, and 41.5% for Markus chamber, EBT3 film, and MOSFET detector, respectively. The surface doses using 6 MV photon beams with a thermoplastic mask for 10 × 10 cm² field size were found 38.4%, 51.7% and 50.2% for Markus chamber, EBT3 film, and MOSFET detector, respectively. Conclusion: The thermoplastic masks placed on the surface of the medium affect the surface and buildup region doses. The effect of thermoplastic masks to surface and buildup region doses should be determined before use clinically.