Abstract Book

S1029

ESTRO 37

to worst case dose distributions and may inform practical robustness analysis.

Electronic Poster: Physics track: Treatment planning: applications

EP-1899 HyperArc^TM: a new mono-isocenter VMAT technique for SRS of multiple brain lesions. R. Ruggieri 1 , S. Naccarato 1 , A. Fiorentino 1 , F. Ricchetti 1 , R. Mazzola 1 , F. Alongi 1 1 Sacro Cuore Don Calabria Hospital, Radiation Oncology, Negrar, Italy Purpose or Objective Linac based VMAT-SRS of multiple brain lesions is typically performed by a multiple-isocenters approach, i.e. one isocenter per lesion, which is time-demanding for the need of independent setup verifications of each isocenter. Here we present the first worldwide experience with a new mono-isocenter technique with multiple non-coplanar arcs (HyperArc TM , Varian Inc.) in terms of a plan comparison with our previous multiple- isocenters VMAT approach. Material and Methods From August 2017, 13 patients have been treated by HyperArc TM for multiple (mean 5, range 2-21) brain lesions (PTV(cc): mean 10.8, range 0.5-42.2) in 1-3 fractions: the prescribed doses (D p ) were 18-25 Gy for single-fraction, and 21-27 Gy for three-fractions SRS, respectively. By HyperArc TM , an add-on of Eclipse TM (v.15.5, Varian Inc.), we generated mono-isocenter VMAT plans (HA) with 5 non-coplanar 180°-arcs (couch at 0°, ±45°, ±90°). Multiple-isocenters VMAT plans (RA) with 2 coplanar 360°-arcs per isocenter were also planned by Eclipse TM (v.15.5, Varian Inc.). A dose normalization of 100%D p at 98%PTV was adopted, where a 2 mm isotropic margin from the union of all lesions gave PTV, while D 2% (PTV)<150%D p was accepted. All plans respected the constraints on maximum dose, D(0.5cc), to the brainstem (18 Gy), and to chiasm, optical nerves, eyes and lens (15 Gy). By hypothesis testing, HA and RA plans were then compared in terms of dose-volume metrics, by Paddick conformity (CI) and gradient (GI) index and by V 12 and mean dose to the brain-minus-PTV, and in terms of MU (MU/fr.) and total treatment time (T) per fraction. Whereas measured for HA treatments, for RA plans T(min.) was estimated by assuming 3 min. for initial patient setup plus 5 min. per each CBCT-guided setup correction per isocenter. Results As detailed in Table 1, significant variations in favour of HA plans were computed for both target dose indexes, CI and GI. The lower GI in HA plans was the likely cause of the significant reduction in V 12 to the brain-minus-PTV. At low doses, below 2-3 Gy, the sparing of the brain-minus- PTV was instead in favour of RA plans: on the whole, no significant difference between the mean doses to the brain-minus-PTV of the two groups of plans resulted. Finally, both MU and T were significantly reduced by HyperArc TM plans. Conclusion HyperArc TM plans assured a higher CI together with a lower GI than the RA plans. This is consistent with the computed reduction in V 12 to the brain-minus-PTV, i.e. the shell of surrounding healthy-brain exposed to medium-to-high doses which is typically correlated with the incidence of side effects. Finally, for all our patients HyperArc TM treatments were completed within a typical 20 min. time slot, even for a case with 21 lesions.

EP-1900 Hippocampal sparing in Fractionated Stereotactic Brain VMAT RadioTherapy M.D. Falco 1 , S. Giancaterino 1 , M. D'Andrea 2 , R. Gimenez De Lorenzo 1 , M. Trignani 1 , L. Caravatta 1 , C. Di Carlo 1 , S. Di Biase 1 , A. Allajbej 1 , D. Genovesi 1 1 Ospedale Clinicizzato S.S. Annunziata, of Radiation Oncology “G. D’Annunzio”- University of Chieti, Chieti, Italy 2 Laboratory of Medical Physics and Expert Systems- National Cancer Institute Regina Elena, Laboratory of Medical Physics and Expert Systems- National Cancer Institute Regina Elena, Rome, Italy Purpose or Objective Volumetric Modulated Arc Therapy (VMAT) techniques for fractioned stereotactic brain radiotherapy (FSBRT) can achieve highly conformal dose distribution to intracranial lesions. However, they can potentially increase the dose to hippocampus (H) causing neurocognitive toxicity during the first four months after irradiation. The purpose of this study was to assess the feasibility of hippocampal- sparing (HS) treatment plans in 22 patients with brain metastasis treated with VMAT technique. Material and Methods Firstly, we retrospectively analyzed hippocampal doses in all 22 VMAT original (not hippocampal-sparing, NHS) plans. Plans with hippocampal dose exceeding constraints (9 out of 22) were re-planned considering dose constraints on the hippocampus (H) and on hippocampal avoidance zone (HAZ) generated using 5 mm isotropic margin to the hippocampus. Conformity (CI) and homogeneity indexes (HI) on the target and MUs, were maintained as close as possible to the original plans. Results Mean CI NHS and CI HS obtained were: 0.79±0.11 and 0.81±0.10, respectively (p= 0.75); mean HI NHS and HI HS were 1.05±0.02 and 1.04±0.01, respectively (p= 0.72). In both sets of plans, the mean MU values were similar: 1033±275 and 1022±234 for NHS and HS, respectively. In HS plans, the mean hippocampal dose was decreased by an average of 35%. After re-planning, the D max (21.3 Gy) for HAZ and H was met by 45% (4/9) and 78% (7/9) of the NHS plans, respectively. The worst results were obtained for cases with target volumes extension closer than12