Abstract
(calculated vs. measured) within ±2%. For that tuning task, isocenter dose measurements in a polystyrene phantom were compared to the calculated ones for five IMRS stereotactic plans. Three correction values to the factory DLG value were analyzed: 0.0, -0.25 and -0.5 mm. Accuracy of the M3D software to reproduce the penumbra of stereotactic fields was investigated by comparing the profiles measured in water with the calculated ones for a 1x1 cm MLC-collimated field size. Twelve cranial IMRS plans calculated using the Eclipse were retrospectively recalculated using the Mobius3D software (version 1.3). The same monitor units and calculation voxel sizes (1 mm) were used for both systems. The aperture (complete irradiation area outline) of the modulated beams ranged from 0.9 to 4.4 cm . Differences between both algorithms were evaluated using the 3D gamma tool available in the M3D system. Gamma passing rates for the target and organs at risks (OARs: brainstem, chiasm, optic nerves and normal brain tissue) were compared for 3%/1 mm, 3%/2 mm and 5%/1 mm criteria. Results: 1) Differences (M3D vs. measured) within 1 mm were found for the penumbras of the 1x1 cm field. 2) Dose differences of 2.7% (SD: 1.6%), 1.5% (SD: 1.9%) and 0.4% (SD: 2.0%) were found for the DLG correction values of 0.0, -0.25 and -0.5 mm, respectively. 3) Using the optimal DLG correction (-0.5 mm), the target 3D gamma passing rates were: 94% (73-94%), 97% (80-100%) and 100% (97-100%) for the 3%/1 mm, 3%/2 mm and 5%/1 mm criteria, respectively. 100% rates were obtained for all OARs regardless of the gamma criterium. Conclusions: Great agreement was obtained (within 5% and 1 mm) between IMRS plans calculated by the Eclipse and by the independent dose calculation software M3D. Our findings are restricted to small field sizes down to 1x1 cm . The M3D software may be proposed as an alternative to patientspecific QA based on measurements for IMRS plans.
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