ESTRO 2021 Abstract Book

S1602

ESTRO 2021

technique improved dose coverage of lymph nodes, without increased dose to contralateral organs nor heart. Furthermore, the number of breath-holds during treatment was reduced by a factor of two for the VMAT treatments, thereby increasing patient comfort and reducing risk of patient displacement. 1 shorturl.at/dsAGR

PO-1882 A preliminary dosimetric study of multiple brain metastases using HyperArc and RapidArc techniques Y. Wang 1 , Z. Peng 1 , P. Yang 1 , Y. Liu 1 1 Peking University Shenzhen Hospital, Radiation Oncology, Shenzhen, China Purpose or Objective To evaluate dosimetric differences for patients undergoing cranial stereotactic radiosurgery (SRS) of multiple brain metastases using RapidArc (RA) and HyperArc (HA) techniques. Materials and Methods A total of 6 patients with total , with 27 brain metastases were enrolledretrospectively selected at Peking University Shenzhen Hospital , Shenzhen, China. All lesions were prescribed with a total dose of 36 Gy delivered in 3 fractions, where >95% of PTV receiving 100% prescription dose. RapidArc (RA) and HyperArc (RA) plans were created using Eclipse treatment planning system v15.5 (Varian Medical Systems) on the same TrueBeam 2.7 machine model. For RA, two co-planar full arcs were used for the SRS planning, while HA planning was created with the virtual Encompass couch (QFix, USA) using 3-5 non co-planar arcs in 0, 45, 90, 315 degrees . Homogeneity index ( HI ) and conformity index ( CI ) were calculated according to their definitions in ICRU-83 and Paddick’s reference, respectively . Also the total MUs delivered and OAR doses were compared between RA and HA techniques. Results The volume of all 27 PTVs was 7.9±9.7 cc. HI of RA =0 .13±0.05, while HI of RA=0.288±0.068. The homogeneity of RA was significantly better than that of HA (p<0.05). However, CI of RA =0.83±0.08, while HI of RA=0.94±0.06, which showed the conformity of HA was significantly better than that of RA (p<0.05). The MU required for RA and HA planning techniques was 3654±679 and 3702±699, respectively, which had no significant difference. For all OARs, no significant differences were observed in terms of Dmax in brainstem. However, there was significant lower dose in (brain-GTV) for HA, which showed better protection of brain using the non-coplanar HA technique.