Comparison of 3D and 4D Monte Carlo optimization in robotic tracking stereotactic body radiotherapy of lung cancer

PURPOSE: To investigate the adequacy of three-dimensional (3D) Monte Carlo (MC) optimization (3DMCO) and the potential of four-dimensional (4D) dose renormalization (4DMC(renorm)) and optimization (4DMCO) for CyberKnife (Accuray Inc., Sunnyvale, CA) radiotherapy planning in lung cancer. MATERIALS AND METHODS: For 20 lung tumors, 3DMCO and 4DMCO plans were generated with planning target volume (PTV(5 mm)) = gross tumor volume (GTV) plus 5 mm, assuming 3 mm for tracking errors (PTV(3 mm)) and 2 mm for residual organ deformations. Three fractions of 60 Gy were prescribed to ≥ 95 % of the PTV(5 mm). Each 3DMCO plan was recalculated by 4D MC dose calculation (4DMC(recal)) to assess the dosimetric impact of organ deformations. The 4DMC(recal) plans were renormalized (4DMC(renorm)) to 95 % dose coverage of the PTV(5 mm) for comparisons with the 4DMCO plans. A 3DMCO plan was considered adequate if the 4DMC(recal) plan showed ≥ 95 % of the PTV(3 mm) receiving 60 Gy and doses to other organs at risk (OARs) were below the limits. RESULTS: In seven lesions, 3DMCO was inadequate, providing < 95 % dose coverage to the PTV(3 mm). Comparison of 4DMC(recal) and 3DMCO plans showed that organ deformations resulted in lower OAR doses. Renormalizing the 4DMC(recal) plans could produce OAR doses higher than the tolerances in some 4DMC(renorm) plans. Dose conformity of the 4DMC(renorm) plans was inferior to that of the 3DMCO and 4DMCO plans. The 4DMCO plans did not always achieve OAR dose reductions compared to 3DMCO and 4DMC(renorm) plans. CONCLUSION: This study indicates that 3DMCO with 2 mm margins for organ deformations may be inadequate for Cyberknife-based lung stereotactic body radiotherapy (SBRT). Renormalizing the 4DMC(recal) plans could produce degraded dose conformity and increased OAR doses; 4DMCO can resolve this problem. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi: 10.1007/s00066-014-0747-5) contains supplementary material, which is available to authorized users.