SU‐E‐T‐612: Integral Dose Study of Non‐Coplanar 4pi Radiotherapy

Purpose: To quantify the integral doses of 4π highly non‐coplanar treatment plans and compare them to traditional coplanar IMRT plans. Methods: A systematic study was conducted using an elliptical cylinder, with a major axis length of 35.4 cm and minor axis length of 23.6cm. A 4.5cm diameter spherical tumor was embedded at 4 equally spaced radial locations on the long axis from the center to the periphery. Twenty 6MV conformal non‐coplanar beams were selected in a fashion so that their trajectories would form a circular cone 24° with respect to the cone axis that converged on the tumor. A coplanar plan using twenty evenly spaced conformal 6MV coplanar beams was conducted for comparison. The dose distribution was calculated using the convolution/superposition algorithm. A patient study was also conducted for 10 liver and 12 lung SBRT cases. A column generation algorithm was used to optimize both beam orientations and the fluence map. The dosimetry was compared to clinical plans using 2‐arc volumetric modulated arc therapy (VMAT). Results: For the centrally‐located tumor, non‐coplanar plans delivered up to 37% greater integral dose but the ratio decreased rapidly with the tumor locating more superficially. Approximately the same integral dose resulted from both planning methods at a tumor depth of 10 cm. For the tumor at 6cm depth, the non‐coplanar plan integral dose was approximately 10% lower. Compared to VMAT, the integral dose to the liver and lung SBRT patients were reduced by 17% and 42% using 4π, respectively. Conclusion: Non‐coplanar plans can reduce integral doses as exemplified by both the phantom and patient studies. The effect is most significant with shallower tumors, e.g. the tumor center within 10 cm of a given surface in the simplified phantom model. Actual patient treatments benefit more due to the optimization of the non‐coplanar beam angles.