SU-E-T-236: Monte Carlo Calculations for Radiosurgery of the Clivus




We sought to determine if dosimetric differences exist between Ray-Trace (RT) and Monte-Carlo (MC) calculation algorithms for radiosurgery for tumors of the clivus, given their proximity to air-tissue inhomogeneity.


We retrospectively identified 10 patients with a tumor of the clivus where the target was located near an air-tissue interface. CyberKnife treatment was delivered in 1 to 5 fractions. Plans originally calculated with the RT algorithm were re-calculated with the same monitor units (MU) with the MC algorithm. Similarly, plans originally calculated with MC were recalculated with RT. MC calculations were performed using High Resolution with 2% uncertainty on the MultiPlan 4.6 planning system. If the RT plans were originally calculated using water-air density model, they were re-calculated with Body standard density model, and then compared with MC plans computed with Body standard density model. The maximum dose (Dmax) to the planning target volumes (PTV) and critical structures, minimum dose (Dmin) and coverage percentage for the PTV were calculated.


For 8 cases, MC PTV Dmin was a median 5% (range 0–10%) and Dmax a median 6% (range 3–9%) higher than for RT. For 2 cases, MC PTV Dmin was 6 and 2% and Dmax was 1 and 3% lower than for RT. MC PTV Dmin and Dmax maybe higher or lower than for RT even though the target coverage for MC was less than for RT (median 4%, range 0–13%) with the same MU used. The degree of differences depends on the location of the target relative to the air-tissue inhomogeneity. Examples are shown that the targets are underdosed near the air-tissue inhomogenity.


Monte Carlo dose calculations are recommended for targets near tissue homogeneity such as the clivus. Assuming that Monte Carlo more closely approximates the true dosimetry, the use of Ray Trace could overestimate target coverage.