Quantitative megavoltage radiation therapy dosimetry using the storage phosphor KCl: Eu²⁺

This work, for the first time, reports the use of europium doped potassium chloride (KCl: Eu²⁺) storage phosphor for quantitative megavoltage radiation therapy dosimetry. In principle, KCl: Eu²⁺ functions using the same photostimulatated luminescence (PSL) mechanism as commercially available BaF Br_0.85 I_0.15: Eu²⁺ material that is used for computed radiography (CR) but features a significantly smaller effective atomic number18 versus 49making it a potentially useful material for nearly tissue-equivalent radiation dosimetry. Cylindrical KCl: Eu²⁺ 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 ±2.5%. The findings were (1) KCl: Eu²⁺ 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. (2) KCl: Eu²⁺ exhibited supralinear response to dose after irradiation from 0 to 800 cGy. (3) After x ray irradiation, the PSL signal faded with time and eventually reached a fading rate of about 0.1%/h after 12 h. (4) The sensitivity of the dosimeter was independent of the dose rate ranging from 15 to 1000 cGy/min. (5) The sensitivity showed no beam energy dependence for either open x ray or megavoltage electron fields. (6) 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 foils) during irradiation, the over-response was reduced. The authors have demonstrated that KCl: Eu²⁺ dosimeters have many desirable dosimetric characteristics that make the material conducive to radiation therapy dosimetry. In the future, a large-area KCl: Eu²⁺ -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.