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Dose Calculation Accuracy of Various Commercially Available Algorithms for Treatment of Targets in Heterogeneous Media

J Saini*, B Mathews, E Grant, CARTI, Little Rock, Ar

SU-E-T-377 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: Lack of lateral electron equilibrium is a concern when treating lung lesions through IMRT. A comparison is made for the accuracy of dose calculation algorithms for IMRT and 3D conformal arc (CA) plans calculated by three algorithms; Iplan Monte Carlo (MC), Iplan Pencil Beam (PB), and Pinnacle Superposition-Convolution (SC).

Methods: An in-house heterogeneous phantom was constructed to mimic the electron disequilibrium conditions that arise for lung tumors. The phantom has a cylindrical piece of polystyrene, used as a tumor surrogate, placed in balsa wood, whose density and CT# resemble that of lung tissue. A number of real patient cases with lesions of varying sizes were selected for this study. For each lesion, an IMRT and a CA plans were generated. These plans were then calculated with MC, PB, SC algorithms and mapped onto phantom for ion chamber and film measurement in three orthogonal plains.

Results: Preliminary results for three lesions are given. The ion chamber/ plan measurements are, (i) Tumor1 (10cc): IMRT (MC 0.99, PB 0.94, SC 1.00) CA (MC 0.98, PB 0.90, SC 1.03); (ii) Tumor2 (20cc): IMRT (MC 1.01, PB 0.92, SC 0.97) CA (MC 0.98, PB 0.89, SC 0.97); (iii) Tumor3 (80cc): IMRT (MC 0.98, PB 0.93, SC 1.01) CA (MC 0.99, PB 0.91, SC 0.98). The percentage of points passing the 3%/3mm gamma index criteria are; (i) Tumor1: IMRT (MC 99.9%, PB 96.9%, SC 99.5%) CA (MC 97.7%, PB 97.2%); (ii) Tumor2: IMRT (MC 92.1%, PB 85.7%, SC 91.5%) CA (MC 97.7%, PB 89.5%); (iii) Tumor3: (MC 92.3%, PB 85.7%, SC 92.5%) CA (MC 97.8%, PB 91.6%).

Conclusions: MC and SC algorithms accurately predict the dose while PB algorithm can under dose the target by up to 10%. IMRT is a viable option for lung tumors if dose is calculated by MC or SC algorithms.

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