P132] Robust proton CSI planning in proteusone

Abstract

Purpose This work studies the craniospinal irradiation (CSI) with pencil beam scanning proton therapy in an IBA Proteus® ONE gantry. Robust optimization can be used to create gradient-shaped field-junctions if robustness against setup uncertainties is considered in all dimensions. However, the number of scenarios to compute increases exponentially with the number of beams. In the case of Proteus® ONE, where the maximum field size is 20 × 24 cm2; at least four beams are typically needed to cover the whole craniospinal axis. This results in too many scenarios to be computed (7203 if 5 beams are used) and the optimization is not viable. In this work we investigated two possible solutions: (i) we limited the robust optimization to setup uncertainties in the craniocaudal direction only and (ii) we implemented the ancillary beam technique (ABT) proposed by Farace et al. (Acta Oncol 2015:54:1075–8). Methods Plans were generated with RayStation 6 on computed tomography images of two patients in supine position. Two lateral beams were used for brain irradiation, whereas three posterior beams were used to cover the spine. Approach 1 was optimized on the clinical target volume (CTV), considering robustness against setup uncertainties in the craniocaudal direction only (3 mm) and range uncertainties (3%). These settings reduced the number of scenarios to be computed to 729. For Approach 2, a planning target volume (PTV) was created as a 3-mm isotropic expansion of the CTV, and the ABT was implemented. The robustness of all plans was evaluated by simulating setup and range errors. Results Both approaches yielded an optimal nominal plan. For approach 1 a dose gradient was obtained through the junction, but the dose distribution for each individual beam was highly inhomogeneous in the anterior-posterior and left–right directions. Therefore, the plan was not robust to setup uncertainties in those directions. On the other hand, the ancillary beam method, together with the PTV, offered a homogeneous and robust dose distribution throughout the whole craniospinal axis. Conclusions The ancillary beam technique offered a superior plan quality and it is therefore the recommended solution to plan CSI irradiations in a Proteus® ONE proton therapy gantry.