Influence of Dose Calculation Algorithm According to Dose Prescription Methods in Stereotactic Body Radiotherapy for Lung Cancer

Purpose: Prescribed doses for stereotactic body radiotherapy (SBRT) of lung tumors are defined as the minimum dose received by 95% volume of the planning target volumes (D95 dose prescription method) or the dose at isocenter (isocentric dose prescription method). At present, both dose prescription methods and various calculation algorithm are used for SBRT. With regard to SBRT for lung tumors, impact of dose calculation algorithm according to dose prescription methods was evaluated. Material and Methods: For eight patients with lung cancer, SBRT plans with 6-10 fixed beams of 6MV photon were prepared. In the isocentric dose prescription method, 12 Gy was delivered to the isocenter. In the D95 dose prescription method, 10 Gy or more was delivered to 95% volume of the planning target volumes. Monitor units (MUs) determined by the D95 dose prescription method and the isocentric dose prescription method were calculated by both pencil beam convolution algorithm with Batho Power Law (BPL) and heterogeneity corrected anisotropic analytical algorithm (AAA) in each patient. Results: The differences between MUs calculated by BPL and AAA were 0.6-7.5% (mean, 2.7%) for the isocentric dose prescription method, and 2.7- 32.1% (mean, 12.2%) for the D95 dose prescription method (p=0.0176, paired t-test). Conclusion: The differences of MUs according to calculation algorithm were larger in the D95 dose prescription method compared to the isocentric dose prescription method. In SBRT for lung cancer, differences of dose calculation algorithm should be noticed when the D95 dose prescription method is used.