Field output correction factors using a scintillation detector

As small radiation fields play an ever-increasing role in radiation therapy, accurate dosimetry of these fields becomes critical to ensure high quality dose calculation and treatment optimization. Despite the availability of several small volume dose detectors, small field dosimetry remains challenging. The PRB-0002, a new plastic scintillation detector part of the Hyperscint RP-200 dosimetric platform from Medscint, that requires only minimal corrections can potentially facilitate small field measurements.

The main objective of this work is to adapt small field formalism to plastic scintillation dosimetry using both Monte Carlo (MC) simulations and measurements. The secondary objective is to use the fully characterized PRB-0002 for accurate and precise measurements of field output correction factors and compare those measurements with that from other small field detectors.

Our work is based on IAEA TRS-483 report. EGSnrc MC simulations of the PRB-0002 were conducted to determine the impact of detector composition, surrounding materials, dose averaging within the sensitive volume as well as ionization quenching. From these simulations, the field output correction factors of PRB-0002 were determined. Then, by experimental comparisons, field output correction factors for 2 solid state detectors and 3 small volume ion chambers have been determined.

With PRB-0002, the material composition factor is well balanced with the ionization quenching making the field output correction factor near unity. For fields between 0.6  $\times$  0.6 and 30  $\times$  30  ${\mathrm{cm}}^2$ , the field output correction factors of the PRB-0002 were between 1.002 and 0.999 with a total uncertainty of 0.5%. Analysis of the uncertainty budget showed that, using PRB-0002 for measuring output factors an overall uncertainty of 0.59% can be achieved for a 1  $\times$  1  ${\mathrm{cm}}^2$ field size.

With field output correction factors close to unity for a wide range of field sizes, the PRB-0002 is a near-ideal detector for small field dosimetry. Furthermore, it can be used to experimentally determine the field output correction factors of other dosimeters with great accuracy.