Abstract Book

S1047

ESTRO 37

three actual OAR/CTVs representing the actual position of the structures. Using Monte-Carlo simulation with 100 random uncertainties for each type of plan and uncertainty, we estimated the probability of constraint violation. Results All OAR- and PRV-plans met the dose constraints. Adequate PTV coverage was achieved in all plans (average, OAR-plan: 94.6%; PRV-plan: 94.8%). Coverage of actual CTVs was adequate in OAR plans and PRV plans (97.8±2.1% vs. 97.3±1.6%). Conformity index (0.978 vs. 0.972, p<.001) and homogeneity index (0.150 vs. 0.200, p<.001) were better in OAR-plan; however, the maximal dose to OAR was greater in OAR-plan (SC: 10.89Gy vs. 9.32Gy, p=.001; CE: 13.53Gy vs. 12.32Gy, p=.04). Dose constraints were violated for at least one of the three actual OARs in 5 out of 22 OAR-plans while all actual OARs met the constraints in all PRV-plans. In Monte-Carlo simulation, violation probability of SC and CE was >1% in 73% (8/11) and 29% (4/14) of OAR-plans, respectively; however, all PRV-plans resulted in violation probability ≤1% with any type of uncertainty. Conclusion Spinal SBRT without PRV margin may risk violating constraints for neural OARs due to geometric uncertainties. Introduction of 1mm PRV margin seems to guarantee safe treatment as well as adequate CTV coverage. Therefore, 1mm PRV margin should be routinely applied in spinal SBRT planning. EP-1926 Planning approaches and impact of breathing motion for proton radiotherapy in the mediastinum C. Cases 1 , A.M. Flejmer 2 , A. Dasu 3 1 Hospital de la Santa Creu i Sant Pau, Radiofisica i Radioprotecció, Barcelona, Spain 2 Linköping University, Department of Clinical and Experimental Medicine, Linköping, Sweden 3 The Skandion Clinic, Medical Physics, Uppsala, Sweden Purpose or Objective Proton Pencil Beam Scanning (PBS) has the potential to improve mediastinum treatments by reducing doses to organs at risk (OAR) compared to photons. This study investigates different planning approaches with PBS to maximize target coverage, robustness and OAR sparing accounting for breathing motion. Material and Methods The study was based on CT datasets of twelve thoracic patients acquired in free-breathing (FB), breath-hold-at- inhalation (BHI) and breath-hold-at-exhalation (BHE). CTV and OAR were delineated by an experienced radiation oncologist on each dataset. The prescribed dose was 57.5 Gy (RBE) in 25 fractions of 2.3 Gy (RBE), assuming a Relative Biological Effectiveness (RBE) for protons of 1.1. Two planning approaches were used: a PTV approach with the PTV generated by expanding the CTV 7 mm in Lateral (L) and Craniocaudal (CC) directions and 1 cm in Superio-Inferior (SI) direction and a robust CTV optimization considering 4.5% range uncertainty and 4/6 mm position uncertainty in L, CC/ SI directions respectively, as suggested in the literature. Several field arrangements were compared: 1 Field (1F) technique, with the field at a gantry angle of 0° was compared with 2 Field (2F) techniques (using both single field uniform dose - SFUD and intensity modulated proton therapy - IMPT) with symmetrical gantry positions between 30° and 330° (depending on patient anatomy). PBS plans were designed for the FB dataset and then recalculated to BHI and BHE to assess robustness against breathing motion. Comparisons were carried out for the following DVH parameters: • CTV coverage: D 98% , D 2% , D mean • CTV robustness: D 98%,w , D 2%,w , where w indicates worst-case-scenario.

• • • • •

Heart: D mean(RBE) Lung: D mean(RBE)

, V 25Gy(RBE) , V 20Gy(RBE)

Esophagus: D mean(RBE)

, V 35Gy(RBE)

LAD: D mean(RBE)

V iso90%

Results All dosimetric indices are summarized in Tables 1 and 2. The robust CTV approach improves coverage, robustness, OAR and healthy tissue irradiation compared to the PTV approach, but shows poor coverage for the BHI and BHO datasets. In contrast, the PTV approach leads to better coverage for BHI and BHO, but CTV D 98% <95% in some cases. 2F SFUD approaches lead to poor homogeneity, worse coverage and robustness compared with 1F or IMPT techniques. 1F plans have similar coverage, slightly better robustness and better homogeneity compared to 2F IMPT approaches, but increased OAR and healthy tissues dose.

Conclusion Robust CTV optimization leads to better coverage and robustness while allowing OAR dose sparing. The PTV approach leads to worst overall results, but provides better target coverage when breathing motion is accounted for. These results suggest that gating or breath-hold techniques are desirable for proton PBS, thus allowing low doses to OAR while assuring good coverage to the CTV. Alternatively, breathing motion needs to be accounted for, either by using an ITV or by adjusting uncertainty parameters to the patient-specific breathing motion. Further work will be carried out to efficiently account for breathing movement. EP-1927 VMAT versus CyberKnife MLC non-isocentric planning for brain metastases stereotactic radiotherapy T. Felefly 1 , F. Martinetti 2 , P. Maroun 2 , F. Marsolet 2 , A. Huertas 2 , C.H. Canova 2 , C. Chargari 2 , A. Escande 2 , F. Dhermain 2 , D. Lefkopoulos 2 , E. Deutsch 2 , A. Guemnie Tafo 2 , G. Louvel 2 1 Mount Lebanon Hospital, Radiation Oncology, Beirut, Lebanon 2 Gustave Roussy - Université Paris-Saclay, Radiation Oncology, Villejuif, France Purpose or Objective This study aims to compare dose metrics using VMAT versus CyberKnife multi-leaf collimator (MLC) non- isocentric targeting for stereotactic radiotherapy (SRT) of solitary brain metastases. Material and Methods Ten patients treated for single brain metastases with the CyberKnife M6 system using MLC and allowing partial beam segments (MultiPlan) were retrospectively included in this monocentric study. Median lesion size was 5.7cm 3 (IQR: 3.8 - 16.9 cm 3 ), with a prescribed dose ranging from 25 to 30 Gy in 3 to 5 fractions. Patients were replanned for the NovalisTX system using four arcs VMAT technique (Eclipse). Plans were normalized to achieve same target