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Implementation of 4Pi Non-Coplanar Converging (4PiNC) Beams Using a Conical Delivery Geometry

J Chang

J Chang*, S Wang, D Cho, A Sabbas, K Chao, New York Weill Cornell Medical Ctr, New York, AA

MO-F-108-10 Monday 4:30PM - 6:00PM Room: 108

Purpose: To study the feasibility of a proposed conical delivery system for implementing 4PiNC beams.

Methods: In the proposed novel 4PiNC beam delivery system, the x-ray source aims at the isocenter but rotates around a point on the superior-inferior axis in a conical fashion. Mathematically, each rotation forms a cone where the vertex is the isocenter and the base is the area bounded by the circular trajectory of the x-ray source. By changing the cone angle (90 - vertex angle), the proposed 4PiNC system allows hundreds of non-coplanar beams to be delivered without couch rotation. We developed a simple treatment planning system to investigate the dosimetry for an elliptic cylinder phantom. The planning target volume was a sphere inside the phantom. The three-dimensional dose calculation was calculated using the TMR table, off-axis ratios and output factors of a 6 x-ray beam commissioned for radiosurgery cones. Planning parameters (depth, off-axis distance...) were calculated analytically. The dose grid contained 100x100x100 voxels with a voxel size of 2x2x2 mm^3.

Results: Dose distributions were calculated for (A) 263 beams using cone angles 0, ±20, ±40, ±60, ±80 degree, (B) 153 beam using cone angles 0, 20, 40, 60, 80 degree and (C) 179 beams using cone angles 0, ±20, ±40 degree. Highly conformed dose distributions were achieved with conformity indexes 1.12, 1.11 and 1.19 for case (A), (B) and (C), respectively. The dose falloff (80%-20% of the central dose profile) was 8 mm, 8 mm and 11 mm, respectively.

Conclusion: We proved in principle that the proposed 4PiNC beam delivery system can provide excellent dose distribution (low conformity index and fast dose falloff) without couch rotation. The proposed system can significantly improve the treatment plan quality and avoid couch-gantry collisions, a common patient safety concern of c-arm linear accelerators, when delivering non-coplanar beams.

Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by a DOD grant DOD W81XWH1010862.

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