ESTRO 2024 - Abstract Book

S3362

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Purpose/Objective:

The complexity of modern radiotherapy techniques can compromise the dosimetric and geometric accuracy of the dose delivery. The dose distribution must be verified using a dosimetry system that considers variations in human tissues to better represent the patients. Commercial phantoms often lack the capability to measure the dose in 3D volume and are typically made of homogenous water-equivalent materials. Level III dosimetry audit requires verification of the dose distribution in an anthropomorphic phantom. Advanced radiotherapy delivery delivers steep dose gradients that can be potentially captured by 3D dosimetry.

Material/Methods:

We developed an end-to-end 3D radiotherapy dose audit system based on MAX-HD anthropomorphic head and neck phantom (IMT, Troy, New York) with bespoke intracranial insert for PRESAGE dosimeter. The phantom has a removable four-part radiochromic film insert that forms a cubic shape in the intracranial region. The insert was modified to adapt a PRESAGE dosimeter. Intensity-modulated radiotherapy (IMRT) and volumetric modulated arch radiotherapy (VMAT) treatment were planned for a 20 mm diameter of a spherical target in the brain region and delivered to the phantom. The first irradiation was with a PRESAGE insert and the second irradiation was with the radiochromic films in the coronal and axial plane. The dosimeters were scanned using a benchtop in-house optical computed tomography (OCT) imaging system to measure the dose distribution. The OCT system consists of a CMOS Active Pixel Sensor coupled to a lens to capture projection images of the PRESAGE dosimeter being rotated on a platform. The images were reconstructed into a 3D volumetric image using a filtered back-projection algorithm with Hann filter in MATLAB (MathWorks, Natick, MA, USA).

Results:

The measured dose distribution of the PRESAGE shows good agreement with the measured dose distribution of radiochromic film and treatment plan with a maximum difference of less than 3% for both IMRT and VMAT treatment. Most discrepancies in the dose distribution may be due to the dosimetry system. OCT is prone to ring artefacts which can be due to light scattering and a mismatch in refractive index between the matching liquid and the PRESAGE®. However, having 3D volumetric dose points can compensate for these errors. The gamma analysis results of PRESAGE® in comparison to EBT film and Monaco TPS show pass rates of more than 95% for the criteria of 3% dose difference and 3 mm distance-to-agreement.

Conclusion:

This study demonstrated the capability of PRESAGE® and bespoke MAX-HD phantom in conjunction with the OCT system to measure 3D dose distribution for level III dosimetry audit.

Keywords: PRESAGE Dosimeter; Anthropomorphic Phantom; Audit

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