Monte Carlo evaluation of tissue heterogeneities corrections in the treatment of head and neck cancer patients using stereotactic radiotherapy

The purpose of this study was to generate Monte Carlo computed dose distributions with the X-ray voxel Monte Carlo (XVMC) algorithm in the treatment of head and neck cancer patients using stereotactic radiotherapy (SRT) and compare to heterogeneity corrected pencil-beam (PB-hete) algorithm. This study includes 10 head and neck cancer patients who underwent SRT re-irradiation using heterogeneity corrected pencil-beam (PB-hete) algorithm for dose calculation. Prescription dose was 24-40 Gy in 3-5 fractions (treated 3-5 fractions per week) with at least 95% of the PTV volume receiving 100% of the prescription dose. A stereotactic head and neck localization box was attached to the base of the thermoplastic mask fixation for target localization. The gross tumor volume (GTV) and organs-at-risk (OARs) were contoured on the 3D CT images. The planning target volume (PTV) was generated from the GTV with 0 to 5 mm uniform expansion; PTV ranged from 10.2 to 64.3 cc (average = 35.0±17.5 cc). OARs were contoured on the 3D planning CT and consisted of spinal cord, brainstem, optic structures, parotids, and skin. In the BrainLab treatment planning system (TPS), clinically optimal SRT plans were generated using hybrid planning technique (combination of 3D conformal nonco-planar arcs and nonopposing static beams) for the Novalis-Tx linear accelerator consisting of high-definition multileaf collimators (HD-MLCs: 2.5 mm leaf width at isocenter) and 6 MV-SRS (1000 MU/min) beam. For the purposes of this study, treatment plans were recomputed using XVMC algorithm utilizing identical beam geometry, multileaf positions, and monitor units and compared to the corresponding clinical PB-hete plans. The Monte Carlo calculated dose distributions show small decreases (< 1.5%) in calculated dose for D99, Dmean, and Dmax of the PTV coverage between the two algorithms. However, the average target volume encompassed by the prescribed percent dose (Vp) was about 2.5% less with XVMC vs. PB-hete and ranged between -0.1 and 7.8%. The averages for D100 and D10 of the GTV were lower by about 2% and ranged between -0.8 and 3.1%. For the spinal cord, both the maximal dose difference and the dose to 0.35 cc of the structure were higher by an average of 4.2% (ranged 1.2 to -13.6%) and 1.4% (ranged 7.5 to -11.3%), respectively, with XVMC calculation. For the brainstem, the maximal dose dif-ferences and the dose to 0.5 cc of the structure were, on average, higher by 2.4% (ranged 6.4 to -8.0%) and 3.6% (ranged 6.4 to -9.0%), respectively. For the parotids, both the mean dose and the dose to 20 cc of parotids were higher by an average of 3% (ranged -0.2 to -5.9%) and 4% (ranged -0.2 to -8%), respectively, with XVMC calculation. For the optic apparatus, results from both algorithms were similar. However, the mean dose to skin was 3% higher (ranged 0 to -6%), on average, with XVMC compared to PB-hete, although the maximum dose to skin was 2% lower (ranged -5% to 15.5%). The results from our XVMC dose calculations for head and neck SRT patients indicate small to moderate underdosing of the tumor volume when compared to PB-hete calculation. However, Vp was up to 7.8% less for the lower-neck patient with XVMC. Critical structures, such as spinal cord, brainstem, or parotids, could potentially receive higher doses when using XVMC algorithm. Given the proximity to critical structures and the smaller volumes treated with SRT in the region of the head and neck, the differences between XVMC and PB-hete calculation methods may be of clinical interest.