ESTRO 2024 - Abstract Book

S4798

Physics - Quality assurance and auditing

ESTRO 2024

1100

Digital Poster

A simple method to optimise the radiation isocentre and Winston-Lutz results in linear accelerators

Josep Puxeu Vaqué, Íñigo Lara-Aristimuño, Victor HERNANDEZ

Hospital Universitari Sant Joan de Reus, Medical Physics, Reus, Spain

Purpose/Objective:

The Winston-Lutz (WL) test is commonly used to estimate the difference between the mechanical and radiation isocentres. In our case, we use the kV imaging system to position the WL-pointer emulating what is used at our centre to setup the patient. The accepted tolerance for this test is 1mm, as recommended by TG-198 report for SRS treatments.

Treatment fields are typically conformed using a MLC. Consequently, the correct calibration of the MLC will affect the definition of the radiation beam, as well as the results of the WL test.

The purpose of the present work is to propose a simple method to determine the centerline of an MLC with an accuracy of 0.05mm, which minimizes the radiation isocentre.

The method is valid for any LINAC model. For Varian TrueBeams, the parameter used is known as center offset (CO) and can be modified by the user in Service mode.

Material/Methods:

The method was applied in a TrueBeam (Varian Medical Systems, Palo Alto, CA) equipped with a Millennium 120 MLC and an EPID aS1200. A nominal energy of 6 MV was used for the calibration.

The BrainLAB Winston-Lutz pointer phantom (BrainLAB AG, Munich, Germany) was used to perform the WL test.

The Portal Dosimetry software from Aria v16 was used to collect and analyse the results. Python and the Winston Lutz and Star shot modules from Pylinac were used for the quantitative analysis of the WL and starshot tests.

The MLC calibration in the TrueBeam can be adjusted by the user in service mode using intermediate rights, where two parameters can be modified: the center offset (CO) and the Gap . The recommended value of Gap=0.5mm was used. CO represents the shift of the gap centre with respect to the installation and it can be adjusted with a resolution of 0.01mm.

The proposed half-slit method consists in the following steps (see figure 1):

1. Irradiate a half-beam (Y1=10, Y2=0) with the MLC closed at the centre with two collimator angles of 90º and 270º over the EPID. 2. Create the composite image.