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A Dosimetric Comparison Between Superposition Algorithm and Monte Carlo Simulation for SBRT

W Luo

W Luo1*, A Meacham2, J Molloy3, (1) University of Kentucky, Lexington, KY, (2) University of kentucky, Lexington, Kentucky, (3) Univ Kentucky, Lexington, KY

SU-E-T-511 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To verify the superposition dose calculation algorithm for SBRT plans using the Monte Carlo simulation and investigate the related issues.
Methods: The SBRT plans with 6 MV were originally created on CMS Xio treatment planning system with superposition algorithm. The same patient CT, beam geometry and MUs were used in the Monte Carlo simulation (MC) on MCSIM. MCSIM is an EGS4-based MC dose calculation system for photon and electron beams. The MC plans were compared with the Xio plans to verify Xio superposition algorithm for SBRT. The electron disequilibrium was particularly investigated by comparing the DVHs for a 2-mm thick peel of the GTV. The 18-MV plans and one- and two-beam plans were also created for the dosimetric comparison.
Results: Five SBRT lung plans created on CMS Xio and delivered on Varian 21 EX linac were included in this study. The tumor GTV ranged from 1.4 cc to 11 cc and the dose ranged from 1950 cGy to 5400 cGy. The comparisons were made in terms of DVHs, mean doses, minimal doses, and maximal doses for GTV. The results showed the mean dose values of the real patient plans agreed with MC to within 3%. However, the differences between Xio and MC were increased to up to 10% for 1-beam and 2-beam plans, and the differences were more pronounced for the 18-MV plans than the 6-MV plans.
Conclusions: The Xio superposition algorithm may not adequately account for electron disequilibrium for one or two beam plans; however, it can perform accurate dose calculation for SBRT with large number of beams. Compared to 18-MV beams, 6-MV is preferable as the dose calculation for the 6-MV plans was more accurate based on the Monte Carlo simulation.

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