Quantifying the ionization quenching effect in organic plastic scintillators used in MV photon dosimetry

Abstract

Miniature organic plastic scintillators attached to optical fiber cables have found important uses in medical dosimetry, in particular for small-field dosimetry and field output factor measurements in megavoltage linear accelerator beams. Plastic scintillators are well suited for these applications due to their high degree of water equivalence and their small size. Plastic scintillators, however, are known to suffer from ionization quenching, meaning that the light production per absorbed dose decreases if the dose deposition is caused by low energy electrons. As MV beams also lead to dose deposition by low energy electrons, ionization quenching will also affect, for example, field output factor measurements in MV beams if the fraction of dose delivered by low energy electrons changes with field size or beam quality. While the influence of ionization quenching on scintillator dosimetry in beams of heavy charge particles or kV x-ray has been subject to much research, the effect in MV beams is less well studied, and it is often implicitly assumed that there is no effect. The purpose of this work therefore was to quantify the influence of ionization quenching on plastic scintillators, specifically BCF-60, for the two application in MV photon dosimetry: (i) field output factor measurements for field sizes from 0.5×0.5 cm2 to 10×10 cm2 for a 6 MV beam and (ii) ionization chambers beam quality correction factor (kQ-factors) measurements for beams between 4 MV and 15 MV at a fixed 10×10 cm2 field size. The quenching was quantified using Monte Carlo modelling of the MV beams, a variation of Birks formalism that accounted for the detailed dose deposition by secondary electrons, and quenching parameters established previously using experiments in kV x-ray beams. This enabled the computation of the light yield in a “quenching free” scintillator and therefore a computation of a correction factor due to ionization quenching. Ionization quenching was found to have a small but statistically significant influence on both of the studied applications: (0.6±0.2)% for the field output factor measurements between 0.5×0.5 cm2 and 10×10 cm2 and about (2±0.4)% for the kQ-factor application for beams between 4 MV and 15 MV. The modelling results were in agreement with experimental measurements. The results support that the ionization quenching effect has a small effect on field-output factor measurements and it can probably be neglected during clinical measurements. For direct scintillator-based measurements of beam quality correction factors for ionization chambers, ionization quenching is larger and this effect needs to be accounted for if anyone wants to measure or verify ionization chamber kQ-factors using organic plastic scintillators.