This work, for the first time, reports the use of europium doped potassium chloride ͑KCl: Eu 2+ ͒ storage phosphor for quantitative megavoltage radiation therapy dosimetry. In principle, KCl: Eu 2+ functions using the same photostimulatated luminescence ͑PSL͒ mechanism as commercially available BaFBr 0.85 I 0.15 :Eu 2+ material that is used for computed radiography ͑CR͒ but features a significantly smaller effective atomic number-18 versus 49-making it a potentially useful material for nearly tissue-equivalent radiation dosimetry. Cylindrical KCl: Eu 2+ dosimeters, 7 mm in diameter and 1 mm thick, were fabricated in-house. Dosimetric properties, including radiation hardness, response linearity, signal fading, dose rate sensitivity, and energy dependence, were studied with a laboratory optical reader after irradiation by a linear accelerator. The overall experimental uncertainty was estimated to be within Ϯ2.5%. The findings were ͑1͒ KCl: Eu 2+ showed satisfactory radiation hardness. There was no significant change in the stimulation spectra after irradiation up to 200 Gy when compared to a fresh dosimeter, indicating that this material could be reused at least 100 times if 2 Gy per use was assumed, e.g., for patient-specific IMRT QA. ͑2͒ KCl: Eu 2+ exhibited supralinear response to dose after irradiation from 0 to 800 cGy. ͑3͒ After x ray irradiation, the PSL signal faded with time and eventually reached a fading rate of about 0.1% / h after 12 h. ͑4͒ The sensitivity of the dosimeter was independent of the dose rate ranging from 15 to 1000 cGy/ min. ͑5͒ The sensitivity showed no beam energy dependence for either open x ray or megavoltage electron fields. ͑6͒ Over-response to low-energy scattered photons was comparable to radiographic film, e.g., Kodak EDR2 film. By sandwiching dosimeters between low-energy photon filters ͑0.3 mm thick lead foils͒ during irradiation, the over-response was reduced. The authors have demonstrated that KCl: Eu 2+ dosimeters have many desirable dosimetric characteristics that make the material conducive to radiation therapy dosimetry. In the future, a large-area KCl: Eu 2+ -based CR plate with a thickness of the order of a few microns, created using modern thin film techniques, could provide a reusable, quantitative, high-resolution two-dimensional dosimeter with minimal energy dependence.

With the capability to more accurately model inhomogeneity, Monte Carlo (Type-C) algorithms are sensitive to respiration-induced local and global tissue density changes and exhibit a strong correlation between dosimetric and density differences. However, FB and AIP CTs may still be considered equivalent for dose calculation in the Monte Carlo era, due to the small magnitude of lung density differences between these two datasets.

In conclusion, the authors demonstrate that KCl : Eu(2+)-based dosimeters can be accurately modeled by a MC method and that 2D KCl : Eu2+ films of the order of 1 microm thick would have minimal energy dependence. The data support the future research and development of a KCl : Eu2+ storage phosphor-based system for quantitative, high-resolution multidimensional radiation therapy dosimetry.