ESTRO 2023 - Abstract Book

S1776

Digital Posters

ESTRO 2023

2 3 4 5 6 7

-1.25 -0.46 2.69 4.35 0.01 6.54

-0.09 -3.56

-1.59 N.A. -3.88 -2.22 -2.14

-10.47 -11.44

-5.08

-29.06

-10.84

Conclusion Manual lattice contouring is a tedious activity that requires around 30 minutes and is subjected to non-optimal solutions. For that reason, using an automatic tool is advisable. LRT optimizer was validated on clinical cases and demonstrated to find the optimal lattice structure in terms of spatial arrangements and to allow dose sparing of OARs while saving target coverage.

PO-2003 Comparison of Hybrid and VMAT planning technique for breast and lymph nodes irradiation

D. Bodzak 1 , P. Weso ł owska 2 , E. Fujak 1 , A. Walewska 1 , M. Fillmann 1 , M. Kogut 3 , A. Niwi ń ska 4 , A. Zawadzka 1

1 The Maria Sk ł odowska-Curie National Research Institute of Oncology, Depatment of Medical Physics, Warsaw, Poland; 2 The Maria Sk ł odowska-Curie National Research Institute of Oncology, Depatment of Medical Phycics, Warszawa, Poland; 3 AGH University of Science and Technology, Depatment of Physics, Kraków, Poland; 4 The Maria Sk ł odowska-Curie National Research Institute of Oncology, Department of Breast Cancer and Reconstructive Surgery, Warsaw, Poland Purpose or Objective This study aimed to assess the difference between hybrid (H-VMAT) and VMAT techniques for breast and lymph nodes irradiation. Additionally, the influence of change in breast shape on dose distribution for both techniques was evaluated. Materials and Methods Fourteen patients irradiated with SIB VMAT technique to the breast and lymph nodes regions (9 left, 5 right) were selected in the study. Eight patients were irradiated using DIBH and six using the free-breathing technique. SIB fractionation schedules of 54-56/45 Gy in 20 fractions were used. The calculations were conducted using Eclipse TPS (Varian, AAA v15.6). Statistics in PTV and OARs (lungs, heart, contralateral breast and humerus head) for VMAT and hybrid (3D-CRT+VMAT) plans were compared. The artificial bolus was used in VMAT plans to ensure the beam opening greater than 1 cm above the breast surface. H-VMAT plans were prepared with 3D-CRT field openings extended above the breast surface. The effect of breast shape (PTV volume) change during the treatment on dose distribution in both techniques was evaluated. The breast volume was expanded non-isotropically by a maximum of 1 cm outside the patient's body. Dose distribution with fixed monitor units was recalculated on expanded image and PTV coverage was evaluated. The statistical difference was tested with Wilcoxon matched-pairs signed-ranks test (p< 0.05). Results PTVs coverage analysis shows on average higher doses to the elective regions for H-VMAT plans (mean dose of 45.4±0.3 Gy vs 45.0±0.2 Gy, V95% of 98.6%±0.5% vs 96.3%±0.9% and V107% of 4.3%±2.6% vs 1.4%±1.2% for H-VMAT and VMAT, respectively). All differences were statistically significant. Boost PTV region is equally covered except V95% (99.9%±0.2% for H-VMAT vs 99.6%±0.4% for VMAT). For the heart, mean dose and V5Gy were significantly lower for H-VMAT plans (1.7±0.5 Gy vs 2.5±0.6 Gy and 2.6%±2.9% vs 8.9%±6.6%). For the contralateral breast, V20Gy was equal. Other parameters were lower for H-VMAT (mean dose of 0.9±0.3 Gy vs 4.0±1.1 Gy, V10Gy of 0.1%±0.3% vs 5.6%±4.2% and V3Gy of 2.3%±2.5% vs 52.8%±19.8%). For both lungs, the most pronounced differences occurred for V20Gy and V5Gy (9.0%±2.8% vs 6.5%±1.3% and 22.5%±5.6% vs 34.6%±7.3% for H-VMAT and VMAT, respectively). For humerus head mean dose was 11.9±7.2 Gy for H-VMAT and 8.7±4.2 Gy for VMAT. For extended breast PTV, V95% was 97.4%±1.3% vs 90.0%±3.4% for H-VMAT and VMAT plans. For extended breast and lymph nodes PTV V107% was 3.8%±1.9% for H-VMAT and 0.5%±0.5% for VMAT. Conclusion In H-VMAT plans an increase in V20Gy in the lung and humerus head on the irradiated side is to be expected due to the presence of static fields. This feature also significantly reduces the volume of low isodoses (V3Gy, V5Gy) in key healthy tissues (e.g., the heart). In addition, extending static fields above the patient's surface ensures plan robustness by sufficient PTV coverage if the volume of the breast changes during treatment and the use of an artificial bolus is no longer necessary.

PO-2004 Nontarget dose accuracy and reliability of a treatment planning system during prostate radiotherapy

W. Danckaert 1,2 , P. Ost 1,3 , C. De Wagter 1,2

1 Ghent University, Department of Human Structure and Repair, Ghent, Belgium; 2 Ghent University Hospital, Department of Radiation Oncology, Ghent, Belgium; 3 Iridium Netwerk, Department of Radiation Oncology, Wilrijk, Belgium Purpose or Objective Nontarget dose should be minimized as it offers no therapeutic benefit. The currently available treatment planning systems (TPSs) are not specifically designed nor intended for accurate dose calculations in regions outside the treated volume. This work presents the rigorous evaluation of RayStation's dose calculation accuracy and reliability in nontarget low dose regions.

Materials and Methods