A dual resolution measurement based Monte Carlo simulation technique for detailed dose analysis of small volume organs in the skull base region

The purpose of this study was to examine dose distribution of a skull base tumor and surrounding critical structures in response to high dose intensity-modulated radiosurgery (IMRS) with Monte Carlo (MC) simulation using a dual resolution sandwich phantom. The measurement-based Monte Carlo (MBMC) method (Lin et al., 2009) was adopted for the study. The major components of the MBMC technique involve (1) the BEAMnrc code for beam transport through the treatment head of a Varian 21EX linear accelerator, (2) the DOSXYZnrc code for patient dose simulation and (3) an EPID-measured efficiency map which describes non-uniform fluence distribution of the IMRS treatment beam. For the simulated case, five isocentric 6MV photon beams were designed to deliver a total dose of 1200cGy in two fractions to the skull base tumor. A sandwich phantom for the MBMC simulation was created based on the patient's CT scan of a skull base tumor [gross tumor volume (GTV)=8.4cm³] near the right 8th cranial nerve. The phantom, consisted of a 1.2-cm thick skull base region, had a voxel resolution of 0.05×0.05×0.1cm³ and was sandwiched in between 0.05×0.05×0.3cm³ slices of a head phantom. A coarser 0.2×0.2×0.3cm³ single resolution (SR) phantom was also created for comparison with the sandwich phantom. A particle history of 3×10⁸ for each beam was used for simulations of both the SR and the sandwich phantoms to achieve a statistical uncertainty of <2%. Our study showed that the planning target volume (PTV) receiving at least 95% of the prescribed dose (VPTV95) was 96.9%, 96.7% and 99.9% for the TPS, SR, and sandwich phantom, respectively. The maximum and mean doses to large organs such as the PTV, brain stem, and parotid gland for the TPS, SR and sandwich MC simulations did not show any significant difference; however, significant dose differences were observed for very small structures like the right 8th cranial nerve, right cochlea, right malleus and right semicircular canal. Dose volume histogram (DVH) analyses revealed much smoother DVH curves for the dual resolution sandwich phantom when compared to the SR phantom. In conclusion, MBMC simulations using a dual resolution sandwich phantom improved simulation spatial resolution for skull base IMRS therapy. More detailed dose analyses for small critical structures can be made available to help in clinical judgment.