Purpose: Ion chambers are prone to saturation effects at the dose rates delivered by modern flattening filter free (FFF) beams. The purpose of this work is to characterize the dose linearity and dose rate constancy of an Exradrin W2 scintillator in a FFF beam as a potential alternative to ion chambers for FFF beam dosimetry.
Methods: An Exradin W2 scintillator detector was placed at reference conditions (100cm SSD, 10cmx10cm field, depth of dmax). All measurements were taken in a 10MV FFF beam calibrated to deliver 1cGy/MU at reference conditions according to AAPM-TG51 protocol. Cerenkov light ratio corrections were computed according to the manufacturer’s instructions. The W2 output was calibrated by defining the reading from 100MU at the reference point as 100cGy. To test the linearity of the response with delivered dose, 200, 400, 600, and 800cGy were delivered at dose rates of 800 and 2400MU/min. To test the detector’s constancy with dose rate, 100MU was delivered at nominal dose rates of 400, 800, 1200, 1600, 2000, and 2400MU/min. The measurements were repeated at 90cm and 80cm SSD, where the inverse-square adjusted dose rates are higher than the nominal rates, reaching a maximum of 3750cGy/min. The dose rate constancy was quantified by evaluating the standard error (std/expected value) of the measurements at each dose rate.
Results: At 800 and 2400 MU/min, the difference between measured and delivered dose was <0.15%; and the slope of the linear fit of the data was 0.99 and 1.00, respectively. The standard error across dose rates were 0.04%, 0.04%, and 0.06% for measurements at 100cm, 90cm, and 80cm SSD, respectively.
Conclusion: The W2 produces consistent readings up to 3750cGy/min without showing any saturation effects, and therefore may be a better choice than an ion chamber for dosimetry of FFF beams.
Funding Support, Disclosures, and Conflict of Interest: The authors would like to thank Standard Imaging for their guidance and for providing the W2 detector so that we could perform this study.