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Conical Beam Geometry in Radiation Therapy

E Schueler

E Schueler*, B Loo , P Maxim , Stanford University, Stanford, CA


WE-RAM1-GePD-TT-1 (Wednesday, August 2, 2017) 9:30 AM - 10:00 AM Room: Therapy ePoster Theater

Purpose: Traditionally in radiation therapy, treatments are delivered in a coplanar geometry. We are evaluating the dosimetric performance of a novel beam arrangement in a next generation linac, that is arranged in a conical geometry. Utilizing this geometry, the linac would allow incorporation of a full ring CT that would share the treatment isocenter.

Methods: Three clinical cases were selected (prostate-, lung-, and pediatric brain cancer). Coplanar VMAT and conical IMRT plans with a 16-equidistant beam arrangement in the azimuthal angle were studied. The conical beam angles of 45 and 60 degrees (relative the patient’s longitudinal plane) were investigated. The different plans were optimized in RayStation using an in house developed script to minimize operator bias between the different techniques.

Results: For the prostate cancer case, the VMAT plan showed lower organ at risk doses compared to the conical technique with the exception of the femurs. In the lung case, a small dose reduction to the carina, bronchial tree, and lungs were observed for the conical beam arrangement. The largest reduction was found for the 45-degree geometry that resulted in a 24% reduction in the right lung dose. In the pediatric brain case, a large increase in dose for the chiasm and brain were observed for the 45-degree geometry (56 and 40%, respectively); However, a strong dose reduction for the cochlea and parotid gland were observed (50% reduction). For the 60-degree geometry, a 50% reduction in both chiasm and pituitary gland doses was found.

Conclusion: With conical beam arrangement, a comparable or better dose distribution could be achieved as compared to state-of-the-art VMAT plans. However, out-of-plane dose as well as the integral body dose was always higher with the conical beam arrangement. For conical beams, the 60-degree beam arrangement always achieved superior dose distributions compared to the 45-degree beam technique.

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