Session Item

The dosimetric characteristics of the detectors in a MR-Linac are affected by the magnetic field. This work aims to study the performance of a novel plastic scintillator detector in a magnetic field by measuring the output factors and comparing them with the values obtained with other different standard detectors.

Plastic scintillators present several interesting properties, including water equivalence or linearity of the scintillation response with the incident radiation energy. A novel plastic scintillator (Bluephysics, Tampa, FL), which consists in a cylinder of 1 mm diameter and 1 mm long with 0.00078 cm³ sensitivity volume has been employed to measure the output factors in an 7 MV nominal energy Elekta Unity MR-Linac. Output factors have been obtained with the scintillator placed at the isocenter at 10 cm depth and SSD 133.5 cm, using a PTW Beamscan MR water phantom. This is possible because the scintillator is embedded in a plastic housing that mimics a semiflex 3D PTW ionisation chamber. Results have been compared with those obtained using four different PTW detectors: semiflex 3D, pinpoint and farmer ionization chambers and a diamond diode. The measurement procedure has been the same for all the detectors.

Figure2 shows the differences obtained in output factors values obtained with the ionization chambers, diode, and the scintillator. All values are similar down to a 5 x 5 cm² field size, while for smaller field sizes, the values obtained with the scintillator in relation with the ones acquired with the diamond diode were about 0.8 % higher for field sizes between 4x4 cm² and 2x2 cm², but 3.1 % lower for the smallest field sizes, 1x1 cm² and 0.5x0.5 cm².

Results obtained with the plastic scintillators were similar to the ones obtained with the diamond diode for fields sizes below 4 x 4 cm², and coincide with all detectors for bigger field sizes, what indicates that the scintillator is capable of measure correctly the output factors with all the possible field sizes in an Unity MR-Linac, and could be used in several future applications.