TH‐C‐137‐02: Robotic Radiotherapy Using Intermediate Beam Energies

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.