Feasibility of Treating Brain Lesions with a Horizontal Beam Line and a Couch with Adjustable Backrest Angle

Abstract

<h3>Purpose/Objective(s)</h3> A gantry enabling beam delivery from any angle is technically complex and expensive. This is especially true for particle beams. An alternative is a horizontal beam line and upright positioning of the patient. This requires a vertical CT scanner and dedicated positioning tools for accurate delivery. We propose a couch with an adjustable backrest in order to mimic a large range of gantry and table rotation combinations. In this work, we investigated which dose distributions are achievable with such a unit and compared it with those achieved with a gantry and a conventional couch. <h3>Materials/Methods</h3> Ten stereotactic treatment plans of cerebral metastases (1.8 cm³ to 26.5 cm³) comprising 8-10 beams were investigated. The treatments were re-planned by mimicking backrest angles of 0°, 30° and 60° by gantry angles of 90°, 120° and 150° respectively. Ten beams with meaningful isocentric table angles were set. The beam shapes and weights were optimized. The calculations were performed with a treatment planning system, simulating a 6MV FFF photon beam. Dose-volume parameters of the clinically used plan were compared to the one mimicking the horizontal beam line. <h3>Results</h3> For the target and a 2 mm shell around the target, approximately identical dose distributions were achieved. The mean dose to the shell decreased by 0.5% on average. The coverage index enhanced from a value of 1.41 ± 0.2 to 1.38 ± 0.2 on average. Only the gradient index got worse with an average of 3.8 ± 0.7 compared to 3.0 ± 0.3. <h3>Conclusion</h3> Despite the restriction to only 3 backrest angles, for photon beams, comparable dose distributions were achieved with the proposed couch unit. For protons we expect at least similar results, as the range of possible angles of incidence is even larger. For the head, displacement of anatomical structures is not expected. Therefore, saving a gantry and a huge amount of radiation shielding, the proposed device is a cost-effective treatment option for brain lesions. Conventional CT and supportive image data could be used.