ESTRO 2024 - Abstract Book

S3187

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

[1] Ma, C.M. et al, 2001. Med Phys, 28, (6), 868-893.

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Digital Poster

Gel dosimetry with an anthropomorphic phantom in an Elekta Unity MR-linac

Carlos Ferrer 1 , Concepción Huertas 2 , Inma Navarro 2 , Beatriz Deben 2 , Rosa Morera 2 , Moisés Sáez 1

1 HU La Paz, Medical Physics, Madrid, Spain. 2 HU La Paz, Radiation Oncology, Madrid, Spain

Purpose/Objective:

The Elekta Unity is a system that combines a 7 MV FFF linear accelerator with a magnetic resonance imaging (MRI) system with a magnetic field strength of 1.5T. The presence of the magnetic field influences the behavior of ionization chambers and diodes commonly used in dosimetry, so it is necessary to develop new protocols and measurement systems that are not affected by the presence of the magnetic field. Dosimetry with polymer gels is not significantly disturbed by the presence of a magnetic field. Moreover, they act as detector and tissue equivalent material and do not disturb the photon fluence of the beam, avoiding small air cavities that would alter the measurement. A simple dosimetric study is performed using gel in order to develop in the future a complete dosimetric evaluation protocol for the Unity system.

Material/Methods:

An anthropomorphic RTsafe head phantom has been used with bone equivalent material inside. The remaining empty space allows it to be filled with water (soft tissue equivalent) and in addition to accommodate any dosimetric system including the gel produced by RTsafe, which also acts as a soft tissue equivalent material. The phantom with the gel inside is subjected to the workflow of the Unity system. Planning of a spherical volume inside the gel was performed with Monaco (v.5.51.11), prescribed at 10 Gy in 2 sessions such that the prescribed dose covers at least 95% of the volume. The gel was read about 5 hours post-irradiation, by acquiring an MR image using a T2-weighted sequence. The acquired images were exported in DICOM format and analyzed together with the manufacturer of the RTsafe phantom. Comparison of the measured and calculated dose distributions was performed following the gamma index (GI) criterion shown in the table.

Results:

The 3D measurements of the adapted plan agreed well with the calculations, with GI pass rates above 97%, even with the most restrictive criteria (2 mm/3%). No significant discrepancies were detected between measured and calculated plans.