ESTRO 2024 - Abstract Book

S3431

Physics - Dose calculation algorithms

ESTRO 2024

System (TPS). After configuring a new treatment unit, a series of tests must be made in the TPS to detect possible discrepancies.

Following the recent installation of an linear accelerator (Linac) model Ethos (Varian Medical Systems, Palo Alto, CA,US) at the Catalan Institute of Oncology (ICO) in L’Hospitalet de Llobregat (Barcelona, Spain), differences were detected in some of the measurements compared to those calculated using the pre-configured Varian Acuros XB algorithm. The aim of this work is to bring these differences to light and make the scientific community aware of the possible causes of these discrepancies.

Material/Methods:

A set of measurements were performed on a Varian Ethos Linac. Dose profiles and PDDs for square fields from 2 to 28 cm with an SSD of 90 cm were compared with calculations using the Eclipse planning system with the Acuros XB v.15.6.06 calculation algorithm. The measurements in the treatment unit were conducted using the Beamscan water phantom (PTW, Freiburg, Germany) with a PTW microDiamond detector (60019) and a PTW semiflex 3D ionization chamber (31021) as the reference detector. All measurements were analyzed using PTW Mephysto software.

To quantify the discrepancies between the measurements in the treatment unit and the planning system, gamma criteria 1%/1mm was performed using an in-house software developed in Python (v.3.7.6).

Additionally, a comparison was made between the measurements taken in the Ethos and those obtained during the commissioning of two operational Halcyon that are currently in clinical operation at the ICO hospital, using the gamma criteria 1%/1mm with Mephysto.

Results:

The comparison between Ethos measurements vs. Eclipse Acuros XB calculations show discrepancies up to 64% for gamma 1%/1mm for PDDs of small fields and profiles at depths greater than 20 cm (Figure 1), with differences increasing for medium-sized fields (Table 1).