Abstract
Assessment of the radiation dose delivered to a tumor and different organs is a major issue when using radiolabelled compounds for diagnostic imaging or endoradiotherapy. The present article reports on a study to correlate the mean 18F-fluorodeoxyglucose (18F-FDG) activity in different tissues measured in a mouse model by positron emission tomography (PET) imaging, with the dose assessed in vitro by Fricke dosimetry. The dose-response relationship of the Fricke dosimeter and PET data was determined at different times after adding 18F-FDG (0-80 MBq) to a Fricke solution (1 mM ferrous ammonium sulfate in 0.4 M sulfuric acid). The total dose was assessed at 24 h (~13 half-lives of 18F-FDG). The number of coincident events produced in 3 mL of Fricke solution or 3 mL of deionized water that contained 60 MBq of 18F-FDG was measured using the Triumph/LabPET8TM preclinical PET/CT scanner. The total activity concentration measured by PET was correlated with the calculated dose from the Fricke dosimeter, at any exposure activity of 18F-FDG. The radiation dose measured with the Fricke dosimeter increased rapidly during the first 4 h after adding 18F-FDG and then gradually reached a plateau. Presence of non-radioactive-FDG did not alter the Fricke dosimetry. The characteristic responses of the dosimeter and PET imaging clearly exhibit linearity with injected activity of 18F-FDG. The dose (Gy) to time-integrated activity (MBq.h) relationship was measured, yielding a conversion factor of 0.064 ± 0.06 Gy/MBq.h in the present mouse model. This correlation provides an efficient alternative method to measure, three-dimensionally, the total and regional dose absorbed from 18F-radiotracers. The Fricke dosimeter can be used to calibrate a PET scanner, thus enabling the determination of dose from the measured radioactivity emitted by 18F-FDG in tissues. The method should be applicable to radiotracers with other positron-emitting radionuclides.
Highlights
In all preclinical and clinical applications of radiotracers, a major parameter, which should be known and controlled to minimize side effects, is the energy imparted per unit mass by the energetic primary photons or fast charged particles to different tissues and organs [1,2,3]
The assessment of the radiation dose from radiotracers delivered to malignant cells and different normal tissues still remains a major issue in diagnostic, and even more so, in radiation molecular targeted procedures
To test the viability of such methods, we evaluate the dose administered by 18F-FDG using the chemical Fricke dosimeter
Summary
In all preclinical and clinical applications of radiotracers, a major parameter, which should be known and controlled to minimize side effects, is the energy imparted per unit mass (i.e., the radiation dose) by the energetic primary photons or fast charged particles to different tissues and organs [1,2,3]. Positron emission tomography (PET) scans deliver one of the highest effective radiation doses to patients (0.019 to 14.1 mSv/MBq) when compared to other nuclear medicine procedures [5, 6]. As a result, it becomes a challenging issue of radiation safety for internal radiation exposure from the radiotracers. Direct intratumoral injection of radiopharmaceuticals has shown its efficacy and initial promise in animal models and a few clinical cases [8, 9] This type of administration approach could outperform the efficacy of systemic targeted radionuclide therapy. While the use of intratumoral delivery of radiopharmaceutical is increasing, it is imperative to be able to verify the accuracy of absorbed dose in the tumor tissue as well as in nearby normal tissues
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