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Robotic Radiotherapy Using Intermediate Beam Energies

P Dong

P Dong1*, D Nguyen1, T Long2, D Ruan1, E Romeijn2, D Low1, K Sheng1, (1) Department of Radiation Oncology, UCLA, Los Angeles, CA, (2) Industrial & Operations Engineering, The University of Michigan, Ann Arbor, MI

TH-C-137-2 Thursday 10:30AM - 12:30PM Room: 137

Purpose: Intermediate energy (1-2MV) x-rays have steeper depth dose drop-off and sharper penumbra than commonly used 6MV x-rays. Dosimetry benefits of these characteristics are studied on a robotic non-coplanar planning and delivery platform.
Methods: Dose of 1MV and 6MV x-rays was calculated using the convolution/superposition algorithm with heterogeneity correction and Monte Carlo calculated dose kernels. The X-ray spectrum was adjusted to match depth dose curves of published data. Thirty noncoplanar beams were selected by a pricing approach from a candidate beam pool, which consisted of 1162 uniformly distributed non-coplanar beams minus beams leading to collision. The collision model was fit to individual treatment sites. Fluence optimization based on 5 mm MLC was performed after adding each beam. Identical objective functions for PTV and organs-at-risk (OARs) were employed in the 3 planning scenarios: 1 MV alone, 6 MV alone and the combination of 1 MV and 6 MV beams (1&6 MV) with the prescription dose covering 95% of the PTV. Four representative cases from the following anatomical sites were included in the study: head and neck, partial breast, lung and liver.
Results: 1 MV and 1&6 MV plans provided superior OAR sparing for head, liver, partial breast and lung cases while maintaining the same PTV coverage. Compared with 6 MV plan, 1 MV plans reduced the integral dose by 25%, 23%, 19% and 9% for lung, breast, head and liver cases respectively. The plan quality of 1&6 MV plans, which primarily was slightly superior to that of the 1MV only plans.
Conclusion: The dosimetric drawbacks of intermediate energy x-rays are higher skin doses and shallower penetration when few of them are used on a coplanar platform but these drawbacks were effectively overcome on a highly non-coplanar treatment planning platform, where its advantages of normal tissue sparing and sharp penumbra are manifested.

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