Characterization of GaN dosimetry for 6 MV photon beam in clinical conditions

• We describe the calibration and correction factors for using GaN as a dosimeter.
• Repeatability and reproducibility of GaN probe were estimated to be 0.5% and 2% respectively.
• The correction factors remain close to unity, a part from field size and temperature dependence.
• The field size dependence shows an over-compensation of the GaN response.
• For the temperature dependence, a sensitivity of (−1.4%/°C) has been estimated.

We characterized a recently proposed implantable GaN-based dosimeter in clinical conditions, for its application in external radiotherapy according to ESTRO (European Society for Radiotherapy & Oncology) practical guidelines. Our studies were carried out using a 6 MV photon beam with the dosimeter under test in a water tank or a PMMA phantom. They were focused on evaluating short term and long term reproducibility of measurements, and assessing the effects of parameters such as field size, source-skin distance, use of wedge filter, beam incidence, dose rate, accumulated dose, GaN-induced dose perturbation, air cavities and temperature. The estimated repeatability and reproducibility are better than 0.5% and 2% at 1σ respectively. There are no significant effects of the parameters under our studies, apart from field size and temperature. The field-size dependence is due to over-compensation of the GaN response method of the dosimeter, the resulting errors remain lower than 5% for field sizes up to 10 × 10 cm2. The temperature dependence mainly results from the GaN luminescence properties, and causes the GaN response to decrease steadily when increasing temperature, with a sensitivity of −1.4%/°C. The observed quasi-linear temperature dependence may facilitate the correction to improve the accuracy of measurements.