ESTRO 2023 - Abstract Book

S1756

Digital Posters

ESTRO 2023

of the target); b) NoBlocking: spots could be placed freely, each beam could have contribution in the whole volume. Both sets of plans were comparable in terms of LEM dose distribution, having similar coverage and OARs sparing. Results Results were consistent for all examined plans as exemplified in fig1. As expected, plans recomputed in MKM resulted generally in lower doses comparing to LEM optimized plans. Detailed evaluation showed that strategy b) achieved MKM maximum doses to OAR (rectum in the case shown) similar to LEM (72.5vs73.5 Gy RBE, respectively), while scenario a) resulted in higher doses to OAR after recomputation (92vs79 Gy RBE), exceeding the OARs constraints. Areas of increased MKM doses was also consistent with higher LETd values (reflected in the LETd histogram, rectum V10% increased from 80keV/ µ m to almost 120keV/ µ m).

Conclusion Spot distribution and therefore beam contribution has a major influence on the LEM/MKM translations and they should be included in the planning strategy, especially when escalation of the OARs is considered. Blocking allows reducing mid and low dose in the blocked OAR, but may result in worse MKM and LETd distribution. It still might be still required in certain situations, but the planner needs to be aware of the limitations: fulfilling LEM constraints does not mean that MKM constraints will automatically be respected. Special care is required, especially when OARs are in proximity of the blocked regions. Moreover, the study highlights the need for implementation of additional tools in commercial systems (i.e. RBE dose visualization per beam, SFO, LETd optimization,etc) to allow safer and optimal CIRT treatment. J.M. PEREZ MORENO 1 , E. Engwall 2 , V. Wase 2 , J. Castro Novais 1 , J.A. Vera Sánchez 1 , F. Cerrón Campoo 1 , I. Lorenzo Villanueva 1 , E. Canals de las Casas 1 , O. Martin 2 , J. Sundström 2 , M. Montero Feijoo 1 , M. Sallabanda Hajro 1 , A. de Pablo González 1 , R. Matute Martín 1 , R. Miralbell 1 , A. Mazal 1 1 Quirónsalud Protontherapy Center, Radiation Oncology, Pozuelo de Alarcón (Madrid), Spain; 2 RaySearch Laboratories, Research and Development, Stockholm, Sweden Purpose or Objective To perform initial evaluation of dynamic proton arc therapy plan robustness through a set of perturbed scenarios and intra- fractional robustness. Materials and Methods Two head and neck cases treated with IMPT were selected for a retrospective evaluation with a novel proton therapy delivery technique based on dynamic arcs (pArc) (1). The actual delivered IMPT were calculated in Raystation 10B for a Proteus One using a total of 7 proton beams irradiating different target volumes according to patient geometry. pArc plans were designed in Raystation 12B using a 360º rotation and 2 revolutions arc. In the Proteus One compact gantry the full rotation is achieved by combining two 180º arcs and two couch positions. A small couch kick was used to reduce uncertainties in shoulders positioning. Blocking contours were used to control spot selection to avoid directions and energies PO-1986 Assesment of scenarios based and intrafractional robustness of dynamic proton arc plans