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Validation of Monte Carlo Dose Algorithm in Heterogeneous Medium

A Sethi

A Sethi1*, P Leo2, C Kabat3, P Cecilio4, (1) Loyola Univ Medical Center, Maywood, IL, (2) Yale University, New Haven, CT, (3) Loyola University Chicago, Chicago, IL, (4) Ciudade Universitaria - Campinas/sp, Sao-paulo, Sao-Paulo

SU-E-T-538 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Evaluate validity of Monte Carlo(MC) based treatment planning system(TPS) used for lung SBRT.

Methods: A commercial TPS(XVMC, iPlan4.1, BrainLAB, Germany) was validated using benchmark dose measurements in water and heterogeneous-slab phantoms. Five phantoms were built using four different density materials: tissue-equivalent plastic-water, lung-equivalent(low- & high-density) cork, and bone. EDR-films at various depths in phantoms were irradiated with 6MV photons. MC plans were calculated dose to medium using 2mm grid-size and 1% dose-variance. Ion chamber and film measurements of depth-dose, dose-profiles and output-factors for a range of field-sizes(12x12 - 60x60mm) were performed. Measured versus calculated dose profile differences were quantified using two dose-indices: Ddiff for the central 80% of field, and Dspill for dose outside field-edge(50%-10% dose).

Results: For homogeneous and hetero-bone phantoms, MC and PB calculations agreed well with measurements. For hetero-lung phantoms, there was excellent agreement between MC-calculations and measured doses (Ddiff <3%) for all field-sizes and depths. The agreement improved with increasing field-size(2.6% for 12x12mm vs. 1.1% for 60x60mm @4cm depth in lung). However, at the same depth and field-size, PB calculations significantly over-predicted measured dose (34% and 6.7% respectively). PB and measured dose differences increased with decreasing field-size, decreasing density and increasing depth within heterogeneity, but converged to <1% beyond heterogeneity. In contrast, significant under-prediction of dose by PB model(up to 50%) was seen in the penumbra region outside field. Dspill differences between calculated and measured dose increased with field size but were lower for MC than PB.

Conclusion: Our TPS validation measurements highlight need for MC planning in lung SBRT. PB calculations result in significant under-dosing of target and over-dosing of surrounding tissue. Over-prediction of target dose is inversely proportional to field-size. Knowing the magnitude of Ddiff for PB for a given field-size, one can easily calculate this index at any other field-size.

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