Planning: a 16×16×14 cm3 RW3 slab phantom (EasyCube, Scanditronix) enabled to support a film on its middle plane (film plane) was used. The phantom was CT scanned (CT Optima CT660, G. E.) with 1 mm-distance slices, and the images were imported into the Eclipse v. 13.7 treatment planning system (Varian Medical Systems). Several targets were outlined: spheres of 0.5, 1, 2 and 3 cm-diameter mimicking small lesions, and 3 spheres of 1 cm-diameter. Also one brain metastasis (1 cm3), and one vestibular schwannoma target (1 cm3) which were mapped from clinical cases to the phantom. All targets were placed at the EasyCube with their centers at the film plane. For each target, an SRS plan with 11-14 non-coplanar IMRT fields (sliding window) was designed, using 6 MV photon beams from a Varian Clinac 2100 CD equipped with a Millennium 120 MLC.
Measurements: each plan was delivered onto the EasyCube containing a Gafchromic EBT3 film (Ashland). Multichannel film dosimetry was done using radiocromic.com v. 3.0.15. In addition, film measurements at 10 cm-depth on a water phantom were performed for the static 1x1 cm2 and 2x2 cm2 field sizes defined by MLC. Three irradiations were performed in all cases.
Simulations: each plan was exported from Eclipse to PRIMO v. 0.3.1.1681 to be simulated. Simulation conditions: PRIMO default transport parameters for 6 MV from Clinac 2100; 850 million histories; DPM algorithm; particle splitting ×170; CT scanner calibration curve of the CT Optima CT660; assignment of RW3 density for the EasyCube; and calibration factor of 0.0174 Gy/MU g/eV.
Analysis: A MATLAB script was written to extract the film dose plane from the 3D dose distribution computed by PRIMO on the EasyCube, with the same scanning resolution of the film (0.35 mm/bin). Using radiochromic.com, the PRIMO dose plane was registered with the corresponding measured film dose plane, and the agreement between both dose planes was evaluated with the 2D gamma tool using 5% (global)/1 mm criteria and a 10% dose threshold.