Investigation of four-dimensional (4D) Monte Carlo dose calculation in real-time tumor tracking stereotatic body radiotherapy for lung cancers.

PURPOSE: To investigate the dosimetric variations and radiobiological impacts as a consequence of delivering treatment plans of 3D nature in 4D manner based on the 4D Monte Carlo treatment planning framework implemented on Cyberknife.
METHODS: Dose distributions were optimized on reference 3D images at end of exhale phase of a 4DCT dataset for 25 lung cancer patients treated with 60 Gy∕3Fx or 48 Gy∕4Fx. Deformable image registrations between individual 3DCT images to the reference 3DCT image in the 4DCT study were performed to interpolate doses calculated on multiple anatomical geometries back on to the reference geometry to compose a 4D dose distribution that included the tracking beam motion and organ deformation. The 3D and 4D dose distributions that were initially calculated with the equivalent path-length (EPL) algorithm (3D(EPL) dose and 4D(EPL) dose) were recalculated with the Monte Carlo algorithm (3D(MC) dose and 4D(MC) dose). Dosimetric variations of V(60Gy∕48Gy) and D(99) of GTV, mean doses to the lung and the heart and maximum dose (D(1)) of the spinal cord as a consequence of tracking beam motion in deforming anatomy, dose calculation algorithm, and both were quantified by the relative change from 4D(MC) to 3D(MC) doses, from 4D(MC) to 4D(EPL) doses, and from 4D(MC) to 3D(EPL) doses, respectively.
RESULTS: Comparing 4D(MC) to 3D(EPL) plans, V(60Gy ∕ 48Gy) and D(99) of GTV decreased considerably by 13 ± 22% (mean ± 1SD) and 9.2 ± 5.5 Gy but changes of normal tissue doses were not more than 0.5 Gy on average. The generalized equivalent uniform dose (gEUD) and tumor control probability (TCP) were reduced by 14.3 ± 8.8 Gy and 7.5 ± 5.2%, and normal tissue complication probability (NTCP) for myelopathy and pericarditis were close to zero and NTCP for radiation pneumonitis was reduced by 2.5% ± 4.1%. Comparing 4D(MC) to 4D(EPL) plans found decreased V(60Gy∕48Gy) and D(99) by 12.3% ± 21.6% and 7.3 ± 5.3 Gy, the normal tissues doses by 0.5 Gy on average, gEUD and TCP by 13.0 ± 8.6 Gy and 7.1% ± 5.1%. Comparing 4D(MC) to 3D(MC) doses, V(60Gy∕48Gy) and D(99) of GTV was reduced by 5.2% ± 8.8% and 2.6 ± 3.3 Gy, and normal tissues hardly changed from 4D(MC) to 3D(MC) doses. The corresponding decreases of gEUD and TCP were 2.8 ± 4.0 Gy and 1.6 ± 2.4%.
CONCLUSIONS: The large discrepancy between original 3D(EPL) plan and benchmarking 4D(MC) plan is predominately due to dose calculation algorithms as the tracking beam motion and organ deformation hardly influenced doses of normal tissues and moderately decreased V(60Gy∕48Gy) and D(99) of GTV. It is worth to make a thoughtful weight of the benefits of full 4D(MC) dose calculation and consider 3D(MC) dose calculation as a compromise of 4D(MC) dose calculation considering the multifold computation time.