Why diamond is a good choice for small beam dosimeter?

Abstract

Introduction In small beam dosimetry, the detector has to be as close as possible to tissue equivalence and exhibit a small detection volume compared to the size of the irradiation field, because of the lack of lateral electronic equilibrium. Characteristics of single crystal diamond (tissue equivalent material Z = 6, high density) make it an ideal candidate to fulfil most of small beam dosimetry requirements. Material and methods We developed a single crystal diamond dosimeter (SCDDo) with a small detection volume of 0.165 mm3. Basic SCDDo behaviours in large field were checked successfully: repeatability, dose linearity, dose rate dependence, energy dependence and long term stability. SCDDo lateral dose profile, depth dose curve and output factor (OF) measurements up to 6 × 6mm2 were performed for small photon beams with a micro-MLC m3 (BrainLab) attached to a Varian Clinac 2100C linear accelerator. These measurements were compared to different commercial detectors: a shielded silicon diode (Sun Nuclear EDGE), an unshielded silicon diode (PTW 60017), a PinPoint ionisation chamber (PTW 31014) and two natural diamond detectors (PTW 60003). Results In small field dosimetry, SCDDo presents an excellent spatial resolution. Low energy dependence (1.2% between 6 MV and 18 MV photon beam) and dose rate dependence less than 1% from 0.53 to 2.64 Gy min−1 explain the good agreement between the SCDDo and the other detectors in depth dose curve measurements. OF values obtained with the SCDDo from 2 × 2 cm2 to 10 × 10 cm2 field sizes, show no significant differences as expected for these file sizes compared to the other devices. For the critical field size of 0.6 × 0.6 cm2, SCDDo exhibits the best result compared to the PinPoint ionisation chamber and to the EDGE diode that are known to respectively underestimate and overestimate OF values in small beam. Conclusion To our knowledge, this is the first study demonstrating the feasibility of diamond dosimeter for field size less than 1 × 1 cm2.