ESTRO 37 Abstract book

S1049

ESTRO 37

(Eclipse). Plans were normalized to achieve same target coverage for both techniques, while maintaining comparable maximal dose values within the target. Paddick new conformity index (nCI), gradient index (pGI), and V15% were compared for both techniques. Wagner conformity gradient index (CGI), gradient index (CGIg), and conformity index (CGIc) were also used since they were correlated with brain normal tissue complication probability. Results For the same PTV coverage, VMAT plans achieved significantly better dose conformity (p<0.01) and lesser low dose spread, V15% (p<0.01) as compared with the CyberKnife plans ( table1 ). Although pGI was similar for both techniques (p=0.55), CGIg was higher using VMAT (p=0.013). CGIc and the combined CGI were both significantly higher for VMAT plans (p<0.01). Table 1: Plan metrics (median values) Metrics VMAT CyberKnife MLC p-value CGIc 101.87 87.29 0.002 CGIg 74.46 71.07 0.013 CGI 87.77 78.68 0.002 nCI 1.03 1.17 0.002 pGI 2.81 2.83 0.556 PTV coverage 98.18 98.18 n/a Conclusion For solitary intracranial targets, Novalis VMAT seems to offer higher conformity and steeper dose gradients than CyberKnife MLC non-isocentric plans, even when beam modulation using partial segments is allowed. EP-1928 VMAT-SIB WBRT hippocampal sparing in patients with large single metastasis O. Cudic 1 , S. Cilla 2 , I. Djan 1,3 , M. Baucal 1 , B. Petrovic 1,4 , B. Djuran 1 , M. Novakovic 1 , A. Morganti 5 1 Institute of Oncology Vojvodina, Radiotherapy Department, Sremska Kamenica, Serbia 2 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Medical Physics Unit, Campobasso, Italy 3 University of Novi Sad, Faculty of Medicine, Novi Sad, Serbia 4 University of Novi Sad, Faculty of Sciences, Novi Sad, Serbia 5 Università di Bologna- Radiation Oncology Center, Department of Experimental- Diagnostic and Specialty Medicine - DIMES, Bologna, Italy Purpose or Objective Whole brain radiotherapy (WBRT) has long been considered the treatment of choice given its effectiveness in providing palliation for patients with brain metastases (BMs). Available literature data evidenced a potential detrimental effect of WBRT in neurocognitive functions (NCFs); in particular, high dose irradiation to hippocampus has shown to be critical in determining NCFs outcomes. Technological improvement, such as volumetric modulated radiotherapy (VMAT), allowed the delivery of simultaneously WBRT and a boost dose to brain metastases. These technical advancements are now able to obtain dose painting in strategic areas in brain. We evaluated the feasibility of VMAT-SIB technique for simultaneous irradiation of whole brain and single large metastases to generate hippocampal avoidance. Material and Methods Five patients underwent WBRT with a boost to a large single metastases (4 cm or more) using the VMAT-SIB technique for the purpose of hippocampal avoidance. Bilateral hippocampi were delineated on T1w MRI co- registered with the planning CT. Two plans were generated for hippocampal sparing: standard WBRT-only

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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