ESTRO 2020 Abstract Book

S796 ESTRO 2020

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. The plans of the different VILMA configurations with collapsed cone algorithm are represented graphically to know which are the radii between which the solution of the density override is better. Results When plotting the dose difference (Fig.2), under modulated fluence, between the displaced and non- displaced tumor, and the dose difference in the same positions but under non-modulated fluence curve, we found these two curves intersect in two PTV diameters, approximately 2.5cm and 10cm.

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

Conclusion From the results between 2.5cm to 10cm PTV diameter we can plan without modulating fluence due to the density inhomogeneity of the PTV margin. Thus, we avoid overdosing and ensure tumor coverage. The modulation is low so Interplay Effects should not appear. For larger diameters, where the margin of PTV is small compared to the tumor (the larger the tumors tend to move less), scattered radiation from the tumor provides sufficient doses to the areas of the PTV with lung density as to be, without override density, we do not need a strong modulation of fluence at the edges. For diameters smaller than 2.5cm when the tumor radii is far from containing the electronic equilibrium, the override density must follow other strategies. PO-1482 Feasibility of 3D-printed electron-modulated bolus for post lumpectomy left breast cancer patients W.Y.V. Lee 1 , K.W.K. Cheng 1 , C.H.A. Liu 1 , T.C. Lam 2 , H.F.V. Lee 2 1 Tuen Mun Hospital, Department of Clinical Oncology, New Territories, Hong Kong SAR China ; 2 The University of Hong Kong, Clinical Oncology, Hong Kong, Hong Kong SAR China Purpose or Objective Electron radiotherapy is usually used for boosting the tumor bed after tangential photon therapy as an adjuvant treatment in post lumpectomy breast cancer patients. Due to the availability of Monte Carlo dose calculation in treatment planning system (RTP), electron dose can be reviewed in CT images prior to treatment. However, the electron dose cannot conform to the tumor bed as no optimization is available in the RTP. 3D-printed electron- modulated bolus allows optimization of electron beams so as to minimize the dose to the organs-at-risk whilst keeping the same coverage on tumor bed when comparing with conventional electron therapy. Material and Methods Thirteen patients with left breast cancers who underwent lumpectomy were recruited in the study. For each patient, a tangential photon plan in Raystation RTP (v8.0b, Sweden) with prescription of 2.17Gy x 21 fractions was created as the base plan. Different electron plans were