Room: Exhibit Hall | Forum 4
Purpose: In CyberKnife system, using Monte Carlo (MC) method is recommended for an accurate dose calculation, while RayTracing (RT) algorithm provides fast and acceptable result in homogeneous material. However, the accuracy of both calculation methods at out-of-field region was not fully investigated under heterogeneous condition. In this study, in-field and out-of-field dose was measured for a single beam and treatment plan for a heterogeneous phantom.
Methods: A heterogeneous phantom with three layers of lung slabs with 1.4 cm thickness each, made out of cedar wood, was used in this study. The scanned phantom images were used for in-field and out-of-field single beam dose calculation from Accuray MultiPlan 5.3 with 60 mm fixed cone at various depths. The phantom consists of a 4-cm plastic water (PW), a slab of lung phantom, a 3-cm PW(mid-slab), two slabs of lung phantoms, and a 6-cm PW(bottom-slab), from top to bottom in order. The ion chamber (IC) measurements were performed at mid- and bottom-slabs, at depth of 1 and 2 cm in each slab (total four measurements). Also, a clinical fiducial tracking plan was created on the same phantom where the target was located at 7 cm sway from points 1 and 2.
Results: The calculated isodose line shows the difference in RT and MC method in lung phantom. In-field dose calculations in RT showed higher trend than IC measurement by up to 4.3%, where MC results showed lower trend by up to 3.2%. At out-of-field, both RT and MC results were lower up to 13.8% and 7.2%, respectively. For a clinical fiducial plan, the differences were 24% and 5% for RT and MC, respectively.
Conclusion: Both RT and MC calculations showed good result at in-field, where MC was more accurate at out-of-field calculations. However, MC showed significantly better accuracy for a clinical treatment plan.