ESTRO 2023 - Abstract Book

S2097

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ESTRO 2023

A light green colour indicates when the value of the residual deviation was lower. For example, in patient 2, markers 1, 2 and 3 (M1, M2 and M3) had a smaller residual deviation after the IR of the p-CT with the GRE. In patient 4, all markers had a smaller residual deviation after IR of the p-CT with the bSSFP-SPAIR. In total, the residual deviation of the 72 markers after IR of the p-CT with the GRE is 19% smaller compared to the residual deviation after IR of the p-CT with the bSSFP- SPAIR. Conclusion With the bSSFP-SPAIR sequence, gold markers are depicted smaller. This leads to a more accurate determination of the marker position on MRI and thus a better image registration between MRI and p-CT. The SPAIR sequence leads to a more accurate external irradiation of prostate patients.

PO-2328 Brain Metastases SRS: Single-isocentre VMAT vs Multi-isocentre Dynamic Conformal Arc

P.C.L. Leung 1 , M.C.H. Lee 2 , P.Y. Wu 3

1 Pamela Youde Nethersole Eastern Hospital, Department of Clinical Oncology , Hong Kong, Hong Kong (SAR) China; 2 Pamela Youde Nethersole Eastern Hospital, Department of Medical Physics, Hong Kong, Hong Kong (SAR) China; 3 Pamela Youde Nethersole Eastern Hospital, Department of Clinical Oncology, Hong Kong, Hong Kong (SAR) China Purpose or Objective Linac-based stereotactic radiosurgery (SRS) of brain metastases (BM) is traditionally treated with dynamic conformal arc technique (DCAT) with the lesion positioned at the isocentre. Multiple BM would be treated with DCAT using a multiple- isocentre approach, it is very time-consuming thus limiting the treatment to patients with very few lesions only. A stereotactic volumetric modulated arc therapy technique with mono-isocentric approach for single and multiple BM (HyperArc™, Varian Medical System Inc. denoted as HA-VMAT) was recently introduced, promising faster treatment with better dose conformity. This study compared single-isocentre HA-VMAT with DCAT in terms of target coverage, brain tissue sparing, as well as treatment efficiency, and examined their correlation with the and the number of lesions per patient. Materials and Methods HA-VMAT plans were retrospectively generated for 30 consecutive patients with single or multiple BM (1–5) previously treated with DCAT planned using BrainLab iPlanTM system in our institution. Prescription dose ranged from 12Gy to 20Gy. Dosimetric parameters of the GTV (homogeneity index (HI); conformity index (CIPaddick); gradient index (GI)) and normal brain tissue dose (V4Gy;V12Gy) from the two techniques were compared. The overall treatment times (OTT) (from patient positioning until the last beam stop) of the actual DCAT treatment were also compared with the simulated ones for HA- VMAT (using data from other real HA-VMAT treatment). The difference in their dosimetric performance and treatment efficiency in relation to the and number of lesions per patient were also studied and compared. Results HA-VMAT produced significantly more homogeneous (HI: single-lesion: 0.099 vs 0.134 (p=0.002); multiple-lesion: 0.076 vs 0.111 (p<0.001)), and conformal (CIPaddick: single-lesion: 0.755 vs 0.585 (p<0.001); multiple-lesion: 0.689 vs 0.455 (p<0.001)) target dose than DCAT. The improvement in conformity with HA-VMAT was fairly constant with GTV down to around 2cc, after which the improvement increased significantly with decreasing GTV volume. HA-VMAT also produced smaller low and medium dose volumes in normal brain tissue (V4Gy : 37.3cc vs 48.3cc, p=0.003; V12Gy : 6.6cc vs 9.3cc, p= 0.002) for single-lesion cases. OTT with HA-VMAT was significantly shorter than DCAT even for single-lesion cases (23 minutes vs 33 minutes, p=0.002). For multiple BM, the reduction in OTT with HA-VMAT ranged from 36 minutes for 2-lesion cases (p=0.005) to one and a half hour for 5 lesions compared with DCAT. Conclusion Single-isocentre HA-VMAT was superior to DCAT for SRS of both single and multiple BM. Dose conformity improved even further with lesions smaller than 2cc. Better brain tissue sparing was seen with single-lesion cases. HA-VMAT also greatly improved treatment delivery efficiency, making SRS more available for patients with multiple BM.