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A Framework for 4pi Radiotherapy

K Sheng

K Sheng1*, E Romeijn2, P Dong1, T Long2, D Low1, D Ruan1, (1) UCLA School of Medicine, LOS ANGELES, CA, (2) The University of Michigan, Ann Arbor, MI

SU-D-211-2 Sunday 2:15:00 PM - 3:00:00 PM Room: 211

Purpose: Inclusion of highly non-coplanar treatment angles increases radiations dose conformality and critical organ sparing. However, implementation of this treatment strategy has been hampered by inaccurate solution space modeling, limited automated beam selection methods, the lack of efficient beam sequencing program and integrated collision prevention. The aim is to develop a 4pi radiotherapy paradigm that takes full advantage of modern computer-controlled robotic C-arm linear accelerators.

Methods: The beam geometry solution space was modeled by 3D surface scanning of the couch, gantry and patient. In order to utilize the entire solution space and optimize MLC resolution, variable source-to-tumor distances were introduced. Conformal radiation doses were computed using convolution/superposition from uniformly distributed solid angles. Beam orientation optimization was performed using a column generation and pricing approach, which was also used to optimize beam fluence intensity modulation. A level set method was then employed to automatically sequence beams so the treatment time and couch motion can be minimized while avoiding collision on the path.

Results: The machine and patient surface was accurately measured and a cocoon shaped solution space was created with an integrated gap buffer of 4 cm. 14 conformal beams were typically selected to maximize target dose coverage and minimize critical organ doses. Compared with manual non-coplanar and coplanar volumetric modulated arc therapy plans, an average 20% improvement was observed in high dose spillage, defined as the 50% isodose volume divided by the target volume, in a wide range of clinical cases including brain, lung, liver and partial breast cancer.

Conclusions: We have established a framework that overcomes major technical difficulties associated with automated planning and delivery of highly non-coplanar treatment on the widely available C-arm linacs. Compared with coplanar volumetric modulated arc therapy plans, 4pi plans improve nearly all aspects of the dosimetry while remain highly deliverable.

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