ESTRO 2023 - Abstract Book

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ESTRO 2023

results. In conclusion, dMLC technique with specific parameters in Monaco is an optimal technique for whole breast irradiation.

PO-2055 Impact of MLC leaf width on plan quality for single-isocenter multi-target conformal-arc SRS

D. Chen 1 , Y. Yang 1 , V. Lockamy 1 , W. Gu 1 , H. Liu 2 , W. Shi 3 , M. Alonso-Basanta 1 , L. Dong 1 , T. Li 1

1 University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA; 2 Thomas Jefferson University, Department of Radiation Oncology, Philadelphia, USA; 3 Thomas Jefferson Univerisity, Department of Radiation Oncology, Philadelphia, USA Purpose or Objective Single isocenter multi-target (SIMT) stereotactic radiosurgery (SRS) for brain metastases has been increasingly popular due to its high efficiency and low upfront & maintenance cost. Elements Multiple Brain Mets (MBM) (Brainlab AG) utilizes dynamic conformal arc (DCA) technique SIMT SRS. In this study, we evaluated the impact of MLC width on the performance of Elements MBM SRS planning system by comparing paired plans generated using two different MLC widths for various target sizes. Materials and Methods 8 patients (46 targets) were retrospectively planned using Elements MBM v3.0. The mean target diameter is 1.1 ± 0.6 (0.6 to 2.6) in cm. For each patient, 2 DCA plans using the same 5 couch angles were created using a 120-leaf high-definition MLC (HDMLC) (2.5-mm leaf width within central 8-cm region) and Millennium 120 MLC (5-mm MLC) (5-mm leaf width within central 10-cm region) respectively. Prescription doses ranged from 15 Gy to 24 Gy depending on the target; plans were normalized such that all targets have D99% > Rx. Table 1 showed the number of targets and plan characteristics. Plan quality was compared using RTOG conformity index (CI), V12Gy/V6Gy/V3Gy, and mean brain dose. Statistical significance was determined using Wilcoxon signed rank tests with p < 0.05.

Table 1 Patient

Number of targets

Total

MU

Total MU number (5-mm)

Total arc angle (HD) (º)

Total arc angle (5-mm) (º)

Target volume median (range) (cc)

Rx dose median (range) (Gy)

number (HD)

1 2 3 4 5 6 7 8

6 7 8 4 5 6 5 5

11518 10918 16980

8715

1160 1080 1410 1060

1260 1080 1380 1060

4.47 (0.37-7.12) 0.15 (0.12-7.98) 0.32 (0.10-2.17) 2.79 (0.37-9.63) 0.30 (0.20-2.73) 0.36 (0.15-3.54) 0.52 (0.25-1.41) 0.33 (0.17-0.65)

19.5 (15-24) 21 (15-21) 21 (21-21) 19.5 (15-21) 21 (15-21) 18 (15-18) 21 (21-21) 24 (21-24)

10348 17024

5837 7311 6803

5532 7114 6283 8809 9635

800

920

1140 1120

1140 1120

10670

8781

610

820

Results Figure 1a showed boxplots of the CI for the HDMLC and the 5-mm MLC, respectively, grouped into five target diameters. For targets with diameters ≤ 0.8 cm, the HDMLC resulted in a significant improved (p < 0.05) CI: median (range) of 1.48 (1.28-7.71) compared to 2.00 (1.50-9.79) using 5-mm MLC. Figure 1b showed CI difference between HD and 5-mm MLCs vs. target diameter, with positive difference corresponding to better CI using HDMLC. Stratified based on targets, it is evident that HDMLC achieved better CI compared to 5-mm MLC for targets with diameters ≤ 0.8 cm. Figure 2 showed the impact of MLC leaf width on low-dose spillage V12Gy/V6Gy/V3Gy and the mean brain dose. No statistical difference was found in all the evaluated low-dose spillage volumes or mean brain dose between paired plans using the HDMLC and the 5-mm MLC. Similarly, MLC leaf width had no significant impact on the total MU number. Fig. 1