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

Abstract

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.

Radiation hardness of the storage phosphor europium doped potassium chloride for radiation therapy dosimetry

“…Furthermore, the observed level of radiation hardness of KCl:Eu 2þ allows the physicist to establish and monitor dosimeter performance characteristics and represents a factor of 25 increase in reusability compared to previous data. 3 The difference between the results in Fig. 1 and the previously reported sensitivity loss after even 50 Gy (Ref.…”

“…5,6 In theory, the number of PSL active centers is proportional to the locally deposited dose. While previous data showed a supralinear response up to 800 cGy, 3 history, becoming more linear with higher cumulated doses. It is hypothesized that the supralinear response of fresh or nearly fresh dosimeters is due to deep, nonphotostimulable traps present in the material, perhaps as a result of the crude manufacturing process.…”

“…The data up to 200 Gy are consistent with previous findings, in which the slight red shift at higher dose histories was attributed to the creation of larger trap centers. 3 Importantly, however, Fig. 5 indicates that the detrapping process was not significantly altered by cumulated dose up to 5000 Gy.…”

“…3 Functionally, KCl:Eu 2þ dosimeters rely on a mechanism of photostimulated luminescence (PSL), 4-10 similar to BaFBr 0.85 I 0. 15 :Eu 2þ , a phosphor material used in diagnostic computed radiography (CR).…”

Evaluation of the absorbed dose to the breast using radiochromic film in a dedicated CT mammotomography system employing a quasi-monochromatic x-ray beam

“…Storage phosphors, or computed radiography (CR) plates, could also potentially provide a 2D dose distribution, but they have been generally used in high energy radiation therapy situations, unlike the diagnostic energy range detailed in this study. 19,20 In addition, although storage phosphor plates come in a variety of sizes, they cannot be submerged in any type of liquid since the liquid would render the plate unreadable in the CR plate readout device.…”