Field-size correction factors of a radiophotoluminescent glass dosimeter for small-field and intensity-modulated radiation therapy beams

Abstract

Purpose: We evaluated the energy responses of a radiophotoluminescent glass dosimeter (RPLD) to variations in small-field and intensity-modulated radiation therapy (IMRT) conditions using experimental measurements and Monte Carlo simulation.

Methods: Several sizes of the jaw and multileaf collimator fields and various plan-class IMRT-beam measurements were performed using the RPLD and an ionization chamber. The field-size correction factor for the RPLD was determined for 6- and 10-MV x rays. This correction factor, together with the perturbation factor, was also calculated using Monte Carlo simulation with the EGSnrc/egs_chamber user code. In addition, to evaluate the response of the RPLD to clinical-class-specific reference fields, the field-size correction factor for the clinical IMRT plan was measured.

Results: The calculated field-size correction factor ranged from 1.007 to 0.981 (for 6-MV x rays) and from 1.012 to 0.990 (for 10-MV x rays) as the jaw-field size ranged from 1 × 1 cm² to 20 × 20 cm² . The atomic composition perturbation factor for these jaw fields decreased by 3.2% and 1.9% for the 6- and 10-MV fields, respectively. The density perturbation factor was unity for field sizes ranging from 3 × 3 cm² to 20 × 20 cm² , whereas that for field sizes ranging from 3 × 3 cm² to 1 × 1 cm² decreased by 3.2% (for 6-MV x rays) and 4.3% (for 10-MV x rays). The volume-averaging factor rapidly increased for field sizes below 1.6 × 1.6 cm² . The results for the MLC fields were similar to those for the jaw fields. For plan-class IMRT beams, the field-size correction and perturbation factors were almost unity. The difference between the doses measured using the RPLD and ionization chamber was within 1.2% for the clinical IMRT plan at the planning-target volume (PTV) region.

Conclusions: For small fields of size 1.6 × 1.6 cm² or less, it was clarified that the volume averaging and density perturbation were the dominant effects responsible for the variation in the RPLD response. Moreover, perturbation correction is required when measuring a field size 1.0 × 1.0 cm² or less. Under the IMRT conditions, the difference in the responses of the RPLD between the reference conditions and the PTV region calculated by Monte Carlo simulation did not exceed 0.8%. These results indicate that it is feasible to measure IMRT dosage using an RPLD at the PTV region.

Keywords: energy response; intensity-modulated radiation therap; radiophotoluminescent glass dosimeter; small field.