Monte Carlo Validation of Output Factors Measurements in Stereotactic Radiosurgery with Cones

The purpose of this work is to assess 8 detectors variation performance for output factor (OF) determination, in a clinical 6 MV Varian Clinac 2100C stereotactic radiosurgery mode for cone irradiation using Monte Carlo simulation as reference. We study 10 cones with diameters between 30 and 4 mm. The evaluated detectors were ionization chambers: pinpoint and pinpoint 3D (PTW), diodes: SRS, P and E (PTW), Edge (Sun Nuclear), microdiamond (PTW) and EBT3 radiochromic films (Ashland). The OF were normalized with the 30 mm cone and compared to Monte Carlo (MC). For the 8 detectors, the OF measurements reproducibility was very high: r = 0.99 and p < 0.0001. The uncertainty of the MC calculation was lower than 0.8% (type A). The results show: pinpoints (axial position) underestimate OF until −2.3% for cone diameter ≥ 10 mm and down to −12% for smaller cones. Non-shielded (SRS and E) and shielded (P and Edge) diodes overestimate OF respectively up to 3.3% and 5.2% for cone diameter ≥ 10 mm but in both case >7% for smaller cones. Microdiamond slightly overestimates OF, 3.7% for all the cones and EBT3 film is the closest to MC with maximum difference ±1% whatever the cone size. Film is the more accurate detector for OF determination of stereotactic cones but it is restrictive to use. Pinpoints and diodes, respectively due to inappropriate size of sensitive volume and composition do not seem appropriate without corrective factors below 10 mm diameter cone. Microdiamond appears the best detector for all cones despite its sensitive volume size.