Measuring midline dose without build-up cap for patients with brain tumor undergoing 15MV external radiotherapy by using EBT3 Gafchromic film

Introduction: The Purpose of this study has been measuring midline dose for the patients undergoing brain radiotherapy without any need to build-up cap. Materials and Methods: The study was performed on 28 patients with brain tumor undergoing 15 MV radiotherapy by a Siemens LINAC. To estimate the midline dose, the entrance and exit doses were calculated by a conversion ratio. The conversion ratio was determined as the ratio of the measured dose by EBT3 Gafchromic film at the phantom surface to that measured by an ionization chamber at the depth of 3 and 12 cm. Then, the midline and exit transmission curves were obtained in a solid water phantom for various irradiation conditions with and without a 30-degree wedge for various field sizes of 5×5, 10×10 and 15×15 cm2 and at various SSDs including 80, 90 and 100 cm at various depths ranged from 6 to 22 cm by a 0.6 CC ionization chamber. Eventually, to measure the midline dose, we used both of the midline transmission and arithmetic mean algorithms. Results: The mean and standard deviation of the whole sample for midline transmission algorithm were -3% and 4.52% and for that of arithmetic mean algorithm -2.97% and 4.3%, respectively. However, for 9 patients, more than 5% error was noted in estimated midline doses. Conclusion: A new method was developed for measuring the midline dose without any need to use the build-up cap. The main advantage of the developed method is that there is no disruption in the dose reached to the treatment volume.