ESTRO 2021 Abstract Book

S1575

ESTRO 2021

Results For conformity index (CI), Co-CRIM performed comparably to IMRT/VMAT, but better than CRT. For gradient index (GI), Co- CRIM were similar to other techniques. For heterogeneity index (HI), Co-CRIM was comparable to IMRT/VMAT. CRT showed lower HI, while hotspots outside of GTV were found in 20% CRT plans. The dosimetric parameters of spinal cord and total lung with Co-CRIM were better than CRT, comparable to IMRT, but inferior to VMAT. The MU resulted from Co-CRIM was lower than IMRT/VMAT, but higher than CRT. For plan verification pass-rate γ, the value with Co-CRIM was higher than IMRT/VMAT, comparable to CRT. For planning time, Co-CRIM was shorter than CRT or VMAT, but not different from IMRT. Conclusion The proposed novel Co-CRIM planning technique on Pinnacle TPS is an effective planning technique for peripheral lung SBRT. PO-1848 HyperArc and RapidArc dosimetric comparison for resected brain metastases stereotactic radiotherapy R. MUNI 1 , P. Colleoni 2 , M. Fortunato 2 , M.I. Benfetima 3 , C. Bianchi 4 , M. Motta 5 , S. Andreoli 2 , F. Cretti 2 , F.R. Filippone 1 , L. Maffioletti 1 , F. Piccoli 1 , E. Vitali 1 , S. Vukcaj 6 , G. Rinaldi 6 , L.F. Cazzaniga 1 1 Ospedale Papa Giovanni XXIII, Radiation Oncology, Bergamo, Italy; 2 Ospedale Papa Giovanni XXIII, Medical Physics, Bergamo, Italy; 3 International Centre for Theoretical Physics, Medical Physics, Trieste, Italy; 4 Ospedale Papa Giovanni XXIII, Medical Physics , Bergamo, Italy; 5 Ospedale Papa Giovanni XXIII, Radiation Oncology, ergamo, Italy; 6 Ospedale Papa Giovanni XXIII, Radiation Oncology , Bergamo, Italy Purpose or Objective HyperArc (HA) is a non-coplanar volumetric modulated arc therapy (VMAT) treatment approach with single isocenter for intracranial stereotactic radiotherapy. HA provide the advantage of delivering high dose to target volume while minimizing dose to normal tissue for single or multiple targets compared to the conventional VMAT technique. Stereotactic radiotherapy for brain metastases resection cavities represents a challenge because of large volume and irregular shape with related high toxicity. HA plans were calculated for 10 consecutive patients who received hypofractionated stereotactic radiotherapy on resection cavity with VMAT Varian RapidArc (RA). Materials and Methods HA plans were generated with 6MV FFF for a TrueBeam LINAC equipped with 2.5 mm central leaves. RA plans previously delivered on a TRILOGY LINAC equipped with 5 mm leaves MLC with 6MV. All plans were calculated with Acuros XB algorithm on Eclipse TPS. Homogeneity index (HI), RTOG and Paddick conformity index (CI), gradient index (GI) and V2Gy-25Gy for normal brain tissue were compared as indicator of PTV coverage and brain tissue spare. Monitor units (MU) per fraction were evaluated as an indicator of irradiation efficiency. Dose was prescribed to the minimum of the target (100% of the prescription dose to 100% of PTV) for both HA and RA plans. For HA plans Dmax was limited to 130% of the prescription dose. Total dose was 27-35 Gy in 3-5 fractions. Mean PTV was 31.37 cc (range 7.20-81.31). Results HA and RA provided comparable RTOG CI (mean±SD 1.14±0.03 vs.1.16 ±0.04, p=0.28) and Paddick CI (0.88±0.02 vs 0.87±0.30, p=0.29). HI (1.22±0.06 vs 1.15±0.05, p<0.01) and GI (2.31±0.22 vs 2.59±0.34, p<0.01) values were more favorable for HA than RA. V2Gy of normal brain tissue values were similar for HA and RA plans (539.8±249.4 vs 568.1±257.9 cc, p=0.11) Low-to-moderate dose spreads (V4Gy-V18Gy) were significantly reduced (p<0.01) in the HA plans over that of RA. Mean V18Gy was 24.6±14.2 cc vs 26.4±17.5 cc (p<0.01) and V25Gy was 11.7±7.9 cc vs 11.8±8.7 cc (p=0.35) for HA and RA respectively. Mean MU per fraction were 2910±1129 vs 3740±1061 for HA and RA plans respectively.