ESTRO 2021 Abstract Book

S1609

ESTRO 2021

PO-1889 A TPS-based approach for single isocenter open-arc Total Body Irradiation in a standard size bunker E. Villaggi 1 , A. Terulla 1 1 AUSL Piacenza, Medical Physics, Piacenza, Italy Purpose or Objective Arc Total Body Irradiation (Arc-TBI) delivered in a single isocenter is a convenient technique to deliver homogeneous dose of radiation at extended source-to-surface distance (SSD) in a standard-size treatment room, with the patient setup supine and prone next to the floor at moderately extended SSD near to 190 cm. The aim of the study was to develop a planning methodology based on the use of a standard beam model in the treatment planning system (TPS) and a semi-automated weighing forward computation. Materials and Methods A plan meterset weights solver was developed in R language ( https://www.R-project.org/ ) to compute forward weighing for open sub-arc fields of 40x40 cm x cm aperture and 5° amplitude with variable total gantry angle width according to the length of the patient. The methodology was applied to an analytical anisotropic algorithm (AAA) standard beam model commissioned at conventional SSD (100 cm) in Eclipse v 15.6 (Varian Medical Systems, Palo Alto, CA) TPS and tested at moderately extended SSD through AAPM Medical Physics Practice Guideline (MPPG) 5.a for static fields. Accuracy of the planning approach and dose homogeneity were assessed for arc fields in a RW3 homogeneous slab phantom of various thickness (18, 22, 26 cm) and in the ATOM anthropomorphic phantom model 701 ( CIRS, Inc., Norfolk, USA ) . Measurements were performed at various depths and along longitudinal arc direction with (I) a Farmer chamber and MOSFET detectors for point dose measurements and (II) gafchromic films for 2D dose measurements, assessing dose calculation accuracy in anatomical sites. Results The investigated TPS standard beam model showed accuracy up to 3% for single point measurements and 1D gamma indices were greater than 95% at 2%/2mm gamma passing rate for static fields. The proposed weighing computation method allowed full control of dose homogeneity within 1%for lengths up to 200 cm in homogenous phantom at the midplane thickness. Unweighted and weighted arc fields measurements were in agreement with TPS calculations within 1.5%. The TBI planning delivered to the anthropomorphic phantom provided minor deviations between calculations and point measurements (median 2.2%, max 8.4%). Film dosimetry confirmed 2D dose TPS calculation accuracy along transversal profiles and antero-posterior percentage depth doses curves (median 0.3%, max 10.9%). Major deviations were mainly due to inhomogeneity tissues management combined with phantom rotational positioning issues. Conclusion Previous commissioning of the TPS at extended SSDs, the forward weighing computation allows accurate planning of the arc-TBI technique with the existing equipment, unlocking the route towards accurate dosimetric assessment in the patient anatomy and fully integration of the open-arc TBI treatment in the record & verify system. The proposed TPS-based approach provided high control of dose homogeneity for arc-TBI and accurate 3D dose calculation. PO-1890 Dosimetric comparison of modern auto-planning solutions for SRS of multiple brain metastases M. Grohmann 1 , M. Todorovic 1 , C. Petersen 1 1 University Medical Center Hamburg- Eppendorf, Department of Radiotherapy and Radiation Oncology, Hamburg, Germany Purpose or Objective In recent publications several popular treatment options for stereotactic radiosurgery (SRS) of multiple brain metastases were compared. Given the high degree of complexity of these cases, the inter-planner variability can vary to a considerable extent and affect the results more than the technique itself. In addition, parameters such as radiation device, dose grid resolution and evaluation platform can play an important role. The purpose of this study is to compare the plan quality of two modern auto-planning approaches while minimizing possible misleading differences as much as possible. Materials and Methods The employed treatment planning solutions were Brainlab Elements - Multiple Brain Mets SRS - version 3 (MBM) and Varian - HyperArc - Eclipse 16.1 (HA). The main difference between both systems is the treatment technique. HA utilizes RapidArc-VMAT and MBM depends on multi-aperture dynamic conformal arcs. Similarities include jaw tracking and using a single isocenter at the centroid of all targets. Furthermore, beam data is the same and origins from a Varian TrueBeam STx LINAC. Both systems can operate highly automated but were influenced by the same planner to ensure that prescription coverage was tolerable (98.5% ± 1%) and no SRS dose constraints (QUANTEC) were violated. Ten cases with a total of 70 metastases (mean volume 1.1cc ± 0.9cc) were compared . The prescription of 20Gy was the same for all metastases. Results The following table summarizes some of the most important SRS plan quality parameters. These include the Paddick conformity index (CI), the Paddick gradient index (GI), and volume dose metrics for the normal brain (whole brain minus targets). V5Gy is used to quantify the low dose spread and V12Gy is a well-established predictor for radio-necrosis.

CI_MM CI_HA GI_MM GI_HA V12Gy[cc]_MM V12Gy[cc]_MM V12Gy[cc]_HA V5Gy[cc]_MM

Mean

0.78 0.86 4.19 5.54 8.86

9.81

90.13

59.42

Standard deviation 0.02 0.04 0.49 1.1 2.61

2.7

51.42

20.17

To get a better overview of all cases, a DVH-comparison for both techniques is plotted.