ESTRO 2024 - Abstract Book

S3178

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

the middle of a 10 cm RW3 slab phantom to reproduce a measurement inside a volume of density close to the patient and irradiated the film with a 2 Gy dose with a posterior 10 cm x 10 cm 6 MV FFF beam.

Posterior kV images were acquired in the presence of the fiber without and with an in-house 3D-printed thoracic phantom by using two acquisition protocols, namely, “Extremity” with 65 kV and 3.5 mAs and “Thorax Small” with 80 kV and 5.0 mAs. The images were assessed for the visible presence of the fiber. Finally, the CBCT image sets acquired in the presence of the fiber without a phantom using the “Head Full Fan” protocol with 100 kV and 270 mAs were assessed visually. To assess the performance of the detector, we did various measurements in the air with the fiber at the isocenter. These CTDIs were compared to the pencil chamber CTDI measurements for several acquisition protocols. We varied the tube voltage (100 or 125 kV), tube current (20, 60, or 100 mA), collimation (2, 4, 10, 15, or 17.5 cm), and images per second (IPS) (3 or 15 IPS).

The minimum time required to realize the CTDI measurement by each detector to measure the CTDI was also evaluated.

Results:

The impact of the fiber on the delivery was minimal.

We observed a maximal drop of the fluence of -1.2% for the 6 MV FFF beam but it did not correlate with the radiochromic film measurements on which no relevant differences between the film and the TPS could be seen.

The detector could be seen on kV images with the "Extremity" protocol but only on pixels that where not covered by the phantom. The fiber did not impact CBCT images.

Conclusion:

The dosimeter may be a promising device for CBCT recurrent dosimetry quality control or dose optimization. Its measurements could be correlated to a patient specific dose index for further in-vivo CBCT dosimetry.

Keywords: CBCT, scintillating dosimetry, optical fiber

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