Background and purpose: In-vivo dosimetry is used to ensure accurate delivery of dose to tumors during radiotherapy. This study aimed at measuring the entrance doses without the build-up cap for patients undergoing 15MV radiotherapy and determining the percentage errors between doses measured by films and those calculated with a treatment planning system (TPS).

Materials and methods: The study was performed on 30 patients with brain tumor undergoing radiotherapy by 15MV photons of a Siemens linac. A novel method was developed for in-vivo dosimetry, without the need to use build-up caps and was used to estimate conversion coeficients in a solid water phantom for various irradiation conditions with and without a 30º wedge for 5×5, 10×10, and 15×15cm² field sizes and 80, 90 and 100cm SSDs. The conversion coeficients provided ratios of surface doses measured by gafchromic films to those of an ioniztion chamber for various conditions.

Results: For 4 patients, more than 5% error was noted in in-vivo doses. The mean and standard deviation of errors between the doses measured and those calculated with TPS for the patients’ skulls were -2.23% and 3.36%, respectively. The conversion coefficients increased by increase in field size and decrease of SSD. The conversion coefficients for all irradiation conditions were greater for the fields with the wedge than those without it.

Conclusion:  The novel dosimetry method developed for measuring the entrance dose without any need to use build-up caps usual in conventional dosimetry methods, not only makes no disruption in the dose reached to patient’s treatment volume, but also provides an accurate estimation of entrance dose in clinical situations.