ESTRO 2020 Abstract book

S845 ESTRO 2020

Conclusion Prone position and supine position present similar frequency of skin toxicity. The patients treated in prone position have presented higher grades of toxicity, but no one has presented significant adverse effects. A higher degree of toxicity found in prone position can be related to the inmobilization system. Patients positioned in prone have presented strongly reduced toxicity in organs at risk, with the same local tumor control. PO-1481 A recommendation for density overrides in lung SBRT through model theory S. Velázquez Miranda 1 , M. Toscano Jiménez 2 1 Hospital Universitario Virgen del Rocio, Medical Physics, Sevilla, Spain ; 2 ESCUELA TÉCNICA SUPERIOR DE INGENIERÍA Universidad de Sevilla, Fisica Aplicada III, Seville, Spain Purpose or Objective The external margin of the pulmonary planning target volume (PTV) is of lower density, hence it needs a greater fluence so that the dose is homogeneous. But really if we do so, we provide a totally unnecessary fluence because it is precisely there where the tumor will be less time. This would produce excessive dose to healthy tissue in stereotactic body radiotherapy (SBRT), also produce excessive modulation, which is likely to cause problems for the verification of volumetric modulated arc therapy (VMAT) plans, and a risk of interplay effect. Density overrides is a way to deal with this problem. We propose a solution through Model Theory with radial The core of more than seven tens experiments is the design of a virtual patient that meets the conditions of Model Theory. We call it VILMA (VIrtual Lung Movement Approach) and it is defined to contain the most relevant details of the patient-accelerator system with different diameters and positions of PTVs and tumors, and VMAT plans .VILMA is designed to be a cylinder concentrically divided into an outer ring of water density that represents the body around the lungs, and a central cylinder of 0.25 relative lung density (Fig. 1). dependence for do so. Material and Methods

Survival in breast cancer has been increasing and therefore it is mandatory to decrease secondary toxicity of breast cancer treatment. Changes in position of the patient from supine to prone have allowed to separate glandular tissue from chest wall, moving away the organs at risk (OAR) and reducing the side effects while maintaining local tumor control accordingly. The objective of this study is to analyze the differences in toxicity and local tumor control observed between patients with breast cancer treated in supine position and in prone position. Material and Methods All patients with early stage breast cancer have been treated with adjuvant radiotherapy after conservative surgery. The treatments were performed hypofractionated, 2,67Gy/fraction in breast and 0,3Gy/fraction in the concomitant boost. 72 patients have been analyzed, 75% have been treated in prone position with a Bionix prone breast system 6025-03 (Bionix, Toledo, OH, USA) and 25% have been treated in supine position. Dose parameters in lung, V4, V8 and V16 have also been evaluated. A retrospective analysis has been performed in terms of side effects, dosimetry and local tumor control. Results Acute skin toxicity has been the unique adverse effect observed. No Grade 4 radiodermatitis was observed. Grade 1 was found in 52% patients, grade 2 in 19 % patients and grade 3 in 11% patients. Around 80% of the patients treated in supine position presented radiodermatitis Grade 1. No patient presented anymore acute skin toxicity in supine position (figure1). Skin toxicity was observed in 85% patients situated in prone position. Grade 1 was observed in 44% patients, grade 2 in 26% patients and grade 3 was observed in 15% prone patients (figure1). Bionix prone breast system multi- layered variable density foam pad set has allowed patients to be comfortable, but also it is not completely rigid in the opening breast bridge and a small bolus effect has been observed. Doses in lung have decreased significantly in patients situated in prone position compared to supine. Volume of lung which has received more than 4 Gy (V4) was 2,95% in prone position and 20,22 % in supine position (table1). Both positions have presented local tumor control for all patients.

There should be a radius where density override and non- density override of PTV gives the same result, the same real dose to tumor, under VMAT irradiation. To be more precise, this will occur when the dose difference, under modulated fluence optimize at PTV, between the displaced at the edge of PTV and non-displaced tumor plans is equal to the differences in the same positions but under non-modulated fluence. This radio marks the frontier of convenience or not of density override.