ESTRO 2022 - Abstract Book

S1292

Abstract book

ESTRO 2022

Medical Physics, Misterbianco, Italy; 4 Humanitas - Istituto Clinico Catanese, Department of Radiotherapy , Misterbianco, Italy; 5 University of Catania, School of Medical Physics, Catania, Italy; 6 Humanitas - Istituto Clinico Catanese, Department of Medical Physics, Catania, Italy Purpose or Objective Conserving surgery followed by external beam radiotherapy is considered the "Gold Standard" for early stage of breast cancer. This approach may introduce an excess risk for second cancer induction due to breast exposure to therapeutic doses. The aim of this study was to estimate radiotherapy-induced lung cancer risk and to propose simplified models useful in clinical routine to reduce it in a preventive way during the treatment planning phase. Materials and Methods Using the Schneider Mechanistic Model, radiotherapy-induced lung cancer risk for breast cancer has been estimated for 288 patients (aged between 30 and 70 years) treated with Three-Dimensional Conformal Radiation Therapy and Standard Fractioned (3D-CRT SF) at Humanitas - Istituto Clinico Catanese (H-ICC) (Catania, Italy). Organ Equivalent Dose (OED), Excess Absolute Risk (EAR), Lifetime Attributable Risk (LAR) and Relative Risk (RR) values has been calculated implementing a Script (C# language) through the Varian Eclipse Scripting Application Programming Interface (ESAPI). Statistical parameters have been provided by several sources: H-ICC, Istituto Nazionale di Statistica (ISTAT) and Integrated Cancer Registry CT-ME-EN. Using a C++ code, simulations have been performed on the whole statistical sample imposing an attained patient’s age (age a ) equal to 75 Y and an age of the patient during exposure (age e ) varying between 30 and 70 years in steps of 5 years. In order to minimise the difference between LAR from Schneider model (LAR Schneider ) and LAR from simplified model a parameter optimisation process has been performed, i.e., a minimization of Mean Square Error (MSE). Results The first step was to fit linearly LAR values as a function of OED for i-th age e (i = 30, 35, …70) (1), where a i is the i-th angular coefficient for the i-th age e . he next step was to relate the growth rate of the LAR to the time range age e -age a (fig.).The analytic relationship (R 2 =0.99) found was (1). Substituting (2) to (1) we obtained the simplified model named OSM (3). OSM optimised parameters obtained after optimisation process were: A=-0.340 [(1/10000 P) Gy -1 ], B=-11.688 [Y], C=144.557 [Gy -1 ]. The percentage differences ( Δ %) between LAR Schneider and LAR OSM values were < 2%. A linear relation (R 2 =0.96) was found between OED and V4 (% lung volume absorbing a dose of 4 Gy) (4). Substituting (4) to (3) we obtained the simplified model named VSM (5). VSM optimised parameters obtained, after optimisation process were: A=-0.300 [(1/10000 P) Gy -1 ], B=-11.387 [Y], C=147.601 [Gy -1 ], M=0.830 [(1/10000 P) Gy -1 ], N =0.069 [Y]. The percentage differences ( Δ %) between LAR Schneider and LAR VSM values were < 5%.

Conclusion This study provided three different tools for risk calculation: Eclipse™ script, OSM and VSM. These, in different ways, allow the medical physicist to quickly obtain LAR values for each treatment plan, which is why they could easily be used in clinical practice.

PO-1512 Dosimetric comparison between VMAT and electron treatments for skin cancers of the head and neck

J. Ashman 1

1 The Clatterbridge Cancer Centre, Physics, Liverpool, United Kingdom

Purpose or Objective Electron radiotherapy is a frequently used treatment for skin cancers due to the high surface dose and sharp dose fall off beyond D max sparing healthy tissue beyond the target. At our centre, the electron service is resource intensive due to the limited number of trained staff and the additional quality assurance (QA) required. Brachytherapy and kV x-rays are alternative treatment methods, but come with their limitations. Low energy x-rays incident on materials of different density, such as the skull and tissue, will result in a perturbed dose. Brachytherapy is even more resource intensive than electron treatments due to the extensive patient moulds and immobilisation required. Volumetric Modulated Arc Therapy (VMAT) is widely used in our center and staff are familiar with the planning processes. If skin cancers were to be treated with VMAT then there would be reduced QA and an increase in the staff able to plan. It is hypothesised that VMAT will improve dose conformity and homogeneity of the PTV whilst being more resilient to patient contour.

Materials and Methods