ObjectiveTo compare the probe detection method with the image quantification method when estimating 131I biokinetics and radiation doses to the red marrow and whole body in the treatment of thyroid cancer patients.Materials and MethodsFourteen patients with metastatic thyroid cancer, without metastatic bone involvement, were submitted to therapy planning in order to tailor the therapeutic amount of 131I to each individual. Whole-body scans and probe measurements were performed at 4, 24, 48, 72, and 96 h after 131I administration in order to estimate the effective half-life (Teff) and residence time of 131I in the body.ResultsThe mean values for Teff and residence time, respectively, were 19 ± 9 h and 28 ± 12 h for probe detection, compared with 20 ± 13 h and 29 ± 18 h for image quantification. The average dose to the red marrow and whole body, respectively, was 0.061 ± 0.041 mGy/MBq and 0.073 ± 0.040 mGy/MBq for probe detection, compared with 0.066 ± 0.055 mGy/MBq and 0.078 ± 0.056 mGy/MBq for image quantification. Statistical analysis proved that there were no significant differences between the two methods for estimating the Teff (p = 0.801), residence time (p = 0.801), dose to the red marrow (p = 0.708), and dose to the whole body (p = 0.811), even when we considered an optimized approach for calculating doses only at 4 h and 96 h after 131I administration (p > 0.914).ConclusionThere is full agreement as to the feasibility of using probe detection and image quantification when estimating 131I biokinetics and red-marrow/whole-body doses. However, because the probe detection method is inefficacious in identifying tumor sites and critical organs during radionuclide therapy and therefore liable to skew adjustment of the amount of 131I to be administered to patients under such therapy, it should be used with caution.
I biokinetics and radiation doses to rm and wb in therapy procedures are well predicted by diagnostic activities when average values of a group of patients are compared. Nonetheless, when patients are analyzed individually, significant differences may be encountered, thus implying that nuclear medicine therapy-planning requires due consideration of changes in individual patient-body status from initial tracer to final therapy procedures to thus provide appropriate adjustments in therapeutic activities.
In this study, we evaluated the whole-body clearance of I-MIBG in 23 pediatric patients, and the radiation doses received by family caregivers and medical staff during these therapy procedures, thus facilitating the establishment of radiation safety measures to be applied in pediatric therapy.
Objective: To measure the potential radiation dose emitted by patients who have recently undergone diagnostic nuclear medicine procedures, in order to establish optimal radiation safety measures for such procedures. Materials and Methods: We evaluated the radiation doses emitted by 175 adult patients in whom technetium-99m, iodine-131, and fluorine-18 radionuclides were administered for bone, kidney, heart, brain, and whole-body scans, as measured with a radiation detector. Those values served as the basis for evaluating whole-body radiopharmaceutical clearance, as well as the risk for the exposure of others to radiation, depending on the time elapsed since administration of the radiopharmaceutical. Results: The mean time to clearance of the radiopharmaceuticals administered, expressed as the effective half-life, ranged from 1.18 ± 0.30 h to 11.41 ± 0.02 h, and the mean maximum cumulative radiation dose at 1.0 m from the patients was 149.74 ± 56.72 µSv. Even at a distance of 0.5 m, the cumulative dose was found to be only half and one tenth of the limits established for exposure of the general public and family members/caregivers (1.0 mSv and 5.0 mSv per episode, respectively). Conclusion: Cumulative radiation doses emitted by patients immediately after diagnostic nuclear medicine procedures are considerably lower than the limits established by the International Commission on Radiological Protection and the International Atomic Energy Agency, and precautionary measures to avoid radiation exposure are therefore not required after such procedures.
Introduction: Radioiodine therapy with iodine-131 (131I) is the most commonly performed radionuclide therapy in Nuclear Medicine[1]. When used in the treatment of differentiated thyroid cancer (DTC), it consists in the administration of a quantity of radioactive iodine to the patient in order to reduce the risk of relapse and mortality of the disease. In the presence of metastases, a higher activity of 131I may be required, thus recommending a previous dosimetric evaluation, in order to define the maximum activity to be administered to the patient. This activity should be sufficient for a higher exposure of neoplastic tissues and, at the same time, reducing exposure to healthy or critical internal organs in the dosimetry aspect[2]. Although, internal dosimetry is a method capable of giving information about the dose of radiation to be absorbed by internal organs, making possible an individualized and safe protocol for the treatment of each patient, this dosimetry is not routinely used in the nuclear medicine clinics and the patients indicated to the radioiodine therapy are treated with fixed or semifixed activities of 131I, independently of the individual biokinetic characteristics presented by them[3]. The exception to this scenario is usually in cases of patients with metastatic disease, when it is necessary to evaluate the dose to be received by the internal organs. In this context, the bone marrow is one of the most important organs for the dosimetric calculation, since it is highly radiosensitive and should receive a maximum of 2 to 3 Gy[4,5]. As the current dosimetric protocol can be performed by delimiting a region of interest (ROI) in whole body images to evaluate the biokinetics and absorbed dose by the patient, the objective of this study was to analyze the possibility that this measurement was restricted to only one thigh region. That way, it would be possible to optimize the time of adjustment of the ROIs and the processing of information for the calculations of internal dosimetry.Methods: For the retrospective study, were selected 13 patients with metastatic thyroid cancer who had already done therapeutic treatment at the Nuclear Medicine Service of the Cancer Institute of the State of São Paulo (ICESP). The selected patients were submmited scintigraphic imaging procedures at 4, 24, 48, 72 and 96 h after the administration of 131I tracer activity (~74 MBq) in a gamma camera to establish the dose to be administered in the therapy. The scanning images were obtained on a single proton emission computed tomography equipment (SPECT) (Symbus T16 - Siemens Healthcare, Illinois, USA) using a high energy collimator, with the purpose of estimating a radioactive activity present in the body of the patient as a function of time and analyzed using the ImageJ® software. By delimiting the different ROIs (whole-body and thigh) it was possible to obtain the number of counts per pixel in each of the ROIs drawn. As the study objective was analyze the possibility of restricting ROI only as a region of the patient's thigh, the images were analyzed in two steps: in the first, a ROI was designed around the patient's whole-body; in the second, a ROI was drawn in a patient's thigh region. Later, an internal dosimetry was performed by the software OLINDA/EXM and a dose to be received by the bone marrow and whole-body were exposed in the results of this study.Results: The dosimetry based on whole-body ROI, indicated the average absorbed dose by the bone marrow and whole-body of 0.0519 ± 0.0250 mGy/MBq and 0.0634 ± 0.0229 mGy/MBq, respectively. Based on ROI dosimetry in the thigh region, the average absorbed dose by bone marrow and whole-body was 0.0450 ± 0.0239 mGy/MBq and 0.0548 ± 0.0226 mGy/MBq, respectively. It has been observed that the absorbed dose values provided by thigh region dosimetry represent 87% of the absorbed dose value provided by whole-body dosimetry. Supposing the percentage difference between the values obtained by both dosimetric methods, it was possible to find a average correction factor that can be applied to the dosimetry data based on ROI in the thigh region, making dose values absorbed to become similar in the two dosimetric methods. Thus, by adding the correction factor of 13%, which represents the existing difference, in the values provided by thigh region dosimetry, the average absorbed dose by bone marrow and whole-body was 0.0509 ± 0.0270 mGy/MBq and 0.0620 ± 0.0256 mGy/MBq, respectively.Conclusions: Analyzing the dosimetry data obtained through internal dosimetry with whole-body ROI and ROI in the thigh region, it was possible to identify that the internal dosimetry by ROI in the thigh provides a dose estimate (mGy/MBq) similar to that estimated with the delimitation of full body ROI when a correction factor is applied to the data obtained from the thigh region, which is within the uncertainties associated with internal dosimetry in Nuclear Medicine. Even presenting correlation between the data obtained through the two ROIs, we suggest further studies with a larger group of patients, which could increase the level of reliability of the dosimetric method using ROI in the thigh region.
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