Bernard Aubert

Improvement of internal dose calculations using mathematical models of different adult heights.

In internal dosimetry for both nuclear medicine and radiation protection, the adult morphology is represented by a limited number of anthropomorphic models that may not be suitable for all patients. To develop more patient-specific dosimetry, we derived six mathematical models for adults of different height. Three male models (160 cm, 170 cm and 180 cm) and three female models (150 cm, 160 cm and 170 cm), based on the MIRD model design, were developed from the statistical analysis of anthropometric data gathered from autopsies. Monte Carlo calculations were used to provide an example of estimations of S value for these new models for iodine 131 uniformly distributed successively in the stomach or in the urinary bladder. On average, for both male and female models, an increase in the model height of 10 cm leads to a mean reduction in the S value for iodine-131 by 20% and 29% when the stomach and the urinary bladder respectively are selected as source regions. Similarly, when the model height increases by 20 cm, the S values decrease on average by 35% and 48%. This study presents the use of anthropometric data to develop new mathematical models for adults of different height, and shows the significant influence of the morphology on dosimetric parameters.

Physical and biological dosimetry in patients undergoing radiosynoviorthesis with erbium-169 and rhenium-186.

Physical and biological dosimetry were investigated in 45 rheumatoid arthritis patients treated by radiosynoviorthesis (RSO) with 186Re-sulphide (medium-sized joints) and 169Er-citrate (digital joints). Biological dosimetry involved scoring dicentrics in lymphocytes, cultured from blood samples withdrawn just before and 6 h, 24 h and 7 days after treatment. Physical methods included repeated blood sample counts and scintigraphy data. For erbium-169 (pure beta emitter), only bremsstrahlung could be measured and solely in the injection area. For rhenium-186 (both beta and gamma emitter), whole body scans and static images of joints and locoregional lymph nodes were performed. Dosimetry calculations were in accordance with the MIRDOSE 3 software and tables. For erbium-169 (21 patients), either metacarpophalangeal (30 MBq) or proximal interphalangeal (20 MBq) joints of the hands were treated (one joint per patient); 18 patients (out of 21) were interpretable for biological dosimetry, 10 (out of 11) for physical dosimetry and six (out of 10) for both. For rhenium-186, 23 wrists, nine elbows, three shoulders and two ankles were injected in 24 patients, with a maximum of three joints per patient (70 MBq per joint); 20 patients (out of 24) and 10 (out of 10) were interpretable for biological and physical dosimetry, respectively, and eight (out of 10) for both methods. Erbium-169 biological dosimetry was negative in all interpretable patients, and physical dosimetry gave a blood dose of 15±29 μGy and an effective dose lower than 1 mSv/30 MBq. For rhenium-186, biological results were negative in 16 patients (out of 20), but showed a blood irradiation around 200 mGy in the last four. A significant cumulative increase of dicentrics 7 days after injection (16/10,000 instead of 5/10,000 prior to treatment; p<0.04) was also noted. Gamma counts gave a blood dose of 23.9±19.8 mGy/70 MBq and the effective dose was found to be 26.7±5.1 mGy/70 MBq, i.e. about 380 μGy·MBq−1. Erbium-169 RSO is very safe from both physical and biological dosimetry standpoints. Rhenium-186 leak is greater, as demonstrated by the higher blood activity and the measurable, although limited, dicentrics induction in blood lymphocytes. However, the effective dose remains moderate, i.e. 30 times lower than in 131I therapy in benign thyroid diseases.

Application of X-ray fluorescence to the study of iodine distribution and content in the thyroid

We have been developing an X-ray fluorescence system designed to determine iodine distribution and content in the thyroid. The X-ray fluorescence unit is composed of a 80 kV X-ray excitation beam and a Si(Li) semi-conductor detector. The angle between the X-ray beam and the axis of the detector is 24°. Two single channel analyzers (SCA) are used, the first one corresponding to the Kα peak and the other to the background. The SCA signals are digitised so that the two corresponding images may be processed using a computer.After subtracting the background, an image is obtained that represents the iodine distribution alone, and the iodine content of the whole thyroid or a part of it can be determined with a reproducibility of ±5%. A good correlation (r=0.98) between iodine chemical analysis and iodine fluorescence analysis was obtained for 13 patients.

Retrospective evaluation of the dose received by the ovary after radioactive iodine therapy for thyroid cancer

In an earlier study, we evaluated the frequency of clinical manifestations of ovarian insufficiency after radioiodine therapy for thyroid cancer. We were thus led to consider the dose received by the ovary (DRO) during these treatments. In the literature, this dose is expressed as a function of the activity administered. However, in our study, the disorders were not correlated with the activity administered. Faced with this discrepancy, we have attempted to establish a dosimetric model using the parameters available for each patient. The results obtained show that besides the activity administered, which plays a role, morphological and kinetic factors specific to each individual have an importance that cannot be ignored when addressing this problem.

Radioiodinated metaiodobenzylguanidine in neuroblastoma: influence of high dose on tumour site detection.

For more than a decade radioiodinated meta-iodobenzylguanidine (mIBG) has been commonly used for neuroblastoma imaging. The accuracy of this scintigraphic method in detecting both primary and secondary tumour sites is crucial when evaluating the extent of disease. The aim of our study was to assess the impact of high-activity mIBG scintigraphy on neuroblastoma staging. Eighteen scans (TS) were obtained in 15 children after a therapeutic dose of iodine-131 mIBG and compared to diagnostic mIBG scans (DS) (in eight cases with131I-mIBG and in ten cases with123I-mIBG). The superiority of TS over DS was confirmed by the overall results: a total of 220 lesions were disclosed with TS and 171 with DS. However, in only one case did the TS findings, namely skeletal involvement not evidenced on corresponding DS, have an impact on clinical staging. In contrast, neither TS nor DS detected proven bone involvement in four patients. The dose-related sensitivity of mIBG scintigraphy in detecting neuroblastoma tumour sites was confirmed. The ultimate impact of high-dose scans on neuroblastoma management, however, seems limited.

Usefulness of technetium-99m hydroxymethylene diphosphonate scans in localizing bone metastases of differentiated thyroid carcinoma

Iodine-131 is uniquely able to demonstrate iodine uptake of differentiated thyroid carcinoma (DTC), but precise localization may be difficult, especially in the thorax, due to the quality of image resolution with 1311 and the lack of anatomical landmarks. When bone metastases do not show radioiodine uptake, bone scintigraphy can be used to detect them. We studied two groups of patients. In group 1, 15 patients with known bone metastases of DTC were treated with 3.7 GBq 131I. After 4 or 5 days, technetium-99m hydroxymethylene diphosphonate (HMDP; 740 MBq) was injected and a whole-body scan with simultaneous acquisition of 131I and 99mTc-HMDP images was carried out using a large field of view gamma camera fitted with a high-energy collimator. Technetium uptake was abnormal in 47 of 63 localizations, being increased in 29 foci, decreased in 7 and heterogeneous in 11. The superimposition of 131I and 99mTc-HMDP scans permitted an accurate localization in 80% of spine metastases and in 46% of osseous thoracic localizations, even in the presence of lung metastases. In group 2, 9 patients, who had bone pain, neurological signs or elevated serum thyroglobulin, had DTC bone metastases without iodine uptake. They received a diagnostic dose of 99mTc-HMDP 3h prior to scintigraphy with a large field of view gamma camera fitted with a low-energy collimator. Technetium uptake was abnormal in 37 of 38 localizations, being increased in 34 foci and decreased in 3. One false-negative was found in a skull metastasis. In both groups of patients, 99mTc-HMDP scans were useful. The procedure allows accurate localization of bone metastases and can be used as a guide for subsequent radiological investigations.

Comparative study of 111In and 59Fe bone marrow scanning

The aim of this work is to examine the relative merits of 59Fe and 111In scanning in estimating the functional value of the various erythropoietic areas and in appraising bone marrow distribution and extension. At first we studied a group of patients with heterogeneous distribution of bone marrow secondary to irradiation, ferrokinetics serving as reference for the comparison of 111In and 59Fe scanning. The functional value of 59Fe scanning was thus proved, in contrast to 111In scanning which was erroneous for 13 of the 29 bone marrow areas studied. A further study of patients presenting polycythemia vera or anemia with 111In demonstrated in some cases false distribution and bone marrow extension. Furthermore, 59Fe scanning permitted evaluation of the efficiency of erythropoiesis and the existence of extramedullary erythropoiesis. Review of the literature revealed that with the activity used there was no dosimetric advantage of 111In over 59Fe.

Validation of the EGS usercode DosE3D for internal beta dose calculation at the cellular and tissue levels

Internal radiotherapy is currently focusing on beta emitters such as 90Y or 131I because of their high-energy emissions. However, conventional dosimetric methods (MIRD) are known to be limited for such applications. They are unable to take into account microscopic radionuclide distribution because standardized anthropomorphic phantoms are used, and absorbed dose is calculated at the organ level. New tools are therefore required for dose assessment at cellular and tissue level (10-100 microm). The purpose of this study was to validate, at this scale, a Monte Carlo usercode (DOSE3D), based on the MORSE combinatorial geometry package and the EGS code system. Dose point-kernel calculations in water were compared to those published by Cross et al and Simpkin and Mackie. They confirm that DOSE3D is a reliable tool for cellular dosimetry in various geometric configurations.

Optimizing the use of LiF:Mg,Cu,P (GR-200 P) to measure low dose irradiation in nuclear medicine.

Abstract— We undertook a study in order to determine the optimal conditions under which thermoluminescent powder LiF:Mg,Cu,P (GR-200 P) could be used for applications in nuclear medicine and for dosimetric needs. First, we defined the conditions chiefly related to the readout chain, namely the optimal mass of GR-200 material used for each readout, which is between 15 and 30 mg, and the optimal glow curve readout, which corresponds to heating in two phases: a rise in temperature at the rate of 10°C s−1 up to 150°C followed by a plateau lasting 10 s and then another rise in temperature at the rate of 25°C s−1 up to 260°C followed by a second plateau lasting 6 s. The optimum conditions for using TL material were also studied: using GR-200 P without annealing and performing two successive readouts of the same sample only led to a 0.1% loss in sensitivity per re-use cycle, and the good reproducibility of measurements was preserved with stable and weak residual TL signal. The response of the sample irradiated at three different dose levels with a 60Co gamma photon beam remained constant over 40 d. Sensitivity was preserved to within ±4% between a few tenths of a &mgr;Gy and 1 Gy. Our observations concur with the results of the literature and show how to optimize the conditions for using the GR-200 thermoluminescent dosimeter to measure low doses with a simple procedure that preserves the sensitivity of material.

Feasibility of a 59Fe ferrokinetic study based on bone-marrow scans

The purpose of this study was to evaluate the feasibility of a method that would use 59Fe quantitative sequential scintigrams together with a few blood samples in place of conventional ferrokinetic studies. To quantitate the efficiency of erythropoiesis or iron deposition sites, the radio-iron blood activity was subtracted by means of a whole body scanning with 99mTc-labelled erythrocytes. The study was performed on 31 patients (36 examinations) and regions of interest were drawn by three physicians on the sacral bone, the liver, the spleen and the femoral bone. To assess the physical feasibility of the scintigraphic method, correlation coefficients were computed between the number of counts of the 59Fe and 99mTc images and the corresponding 99mTc and 59Fe activities. To assess the clinical feasibility, the consistency of the data resulting from the drawing of the ROIs was verified and those of conventional ferrokinetics were introduced and their consistency verified. These assessments suggest that the scintigraphic method can be used in place of the conventional external counting method.