Joint EURADOS-EANM initiative for an advanced computational framework for the assessment of external dose rates from nuclear medicine patients.

BACKGROUND: In order to ensure adequate radiation protection of critical groups such as staff, caregivers and the general public coming into proximity of nuclear medicine (NM) patients, it is necessary to consider the impact of the radiation emitted by the patients during their stay at the hospital or after leaving the hospital. Current risk assessments are based on ambient dose rate measurements in a single position at a specified distance from the patient and carried out at several time points after administration of the radiopharmaceutical to estimate the whole-body retention. The limitations of such an approach are addressed in this study by developing and validating a more advanced computational dosimetry approach using Monte Carlo (MC) simulations in combination with flexible and realistic computational phantoms and time activity distribution curves from reference biokinetic models. RESULTS: Measurements of the ambient dose rate equivalent H*(10) at 1 m from the NM patient have been successfully compared against MC simulations with 5 different codes using the ICRP adult reference computational voxel phantoms, for typical clinical procedures with 99mTc-HDP/MDP, 18FDG and Na131I. All measurement data fall in the 95% confidence intervals, determined for the average simulated results. Moreover, the different MC codes (MCNP-X, PHITS, GATE, GEANT4, TRIPOLI-4®) have been compared for a more realistic scenario where the effective dose rate E of an exposed individual was determined in positions facing and aside the patient model at 30 cm, 50 cm and 100 cm. The variation between codes was lower than 8% for all the radiopharmaceuticals at 1 m, and varied from 5 to 16% for the face-to face and side-by-side configuration at 30 cm and 50 cm. A sensitivity study on the influence of patient model morphology demonstrated that the relative standard deviation of H*(10) at 1 m for the range of included patient models remained under 16% for time points up to 120 min post administration. CONCLUSIONS: The validated computational approach will be further used for the evaluation of effective dose rates per unit administered activity for a variety of close-contact configurations and a range of radiopharmaceuticals as part of risk assessment studies. Together with the choice of appropriate dose constraints this would facilitate the setting of release criteria and patient restrictions.

Thyroid remnant ablation in patients with papillary cancer: a comparison of low, moderate, and high activities of radioiodine.

OBJECTIVE: The consensus about optimal activities of I-131 for thyroid remnant ablation has not yet been achieved. The aim of this study was to compare ablation rates obtained with different I-131 activities. PATIENTS AND METHODS: The study included 466 patients divided into four groups according to I-131 activities given after total thyroidectomy for papillary thyroid cancer: group A [168 patients who received 888 MBq (24 mCi)], group B [125 patients who received 1480 MBq (40 mCi)], group C [65 patients who received 1850 MBq (50 mCi)], and group D [108 patients who received 4440 MBq (120 mCi)]. Ablation outcome was assessed by whole-body scan in hypothyroid state 6-9 months after ablation and finally 18-21 months after the treatment. RESULTS: The rate of successful ablation was similar in the group of patients who received 24 and 40 mCi (75 and 71.2%, respectively). The higher rate of ablation was achieved in the groups treated with 50 and 120 mCi of radioiodine (87.69 and 90.74%, respectively). The ablation rates at the first follow-up examinations (59.5, 67.2, 73.9, 80.6%) were lower than at second control study (75.0, 71.2, 87.7, 90.7%) in all groups. Time required for thyroid remnant ablation seems to be >or=18 months. CONCLUSION: Our study indicates that activity of 50 mCi seems to be optimal to achieve a successful ablation rate (approximately 90%). Low I-131 activities are acceptable for lower risk patients because of satisfactory ablation rate (>70%), lower expense, and minimal radiation burden to patients as well as lower radiation exposure to clinical staff. The ablative use of high activities seems neither justified nor optimized.

Thyroid stunning in vivo and in vitro.

AIM: To review published in-vivo and in-vitro quantitative dosimetric studies on thyroid stunning in order to derive novel data applicable in clinical practice. METHODS: A non-linear regression analysis was applied to describe the extent of thyroid stunning in thyroid remnants, as a function of the radiation absorbed dose of diagnostic radioiodine-131 (I), in thyroid cancer patients investigated in four in-vivo studies. The regression curves were fitted using individual patient absorbed doses or the mean absorbed doses for the groups of patients. Fitted curves were compared with two recent models, the first found in patients with benign thyroid disease and the second found in cultured thyroid cells after I irradiation. RESULTS: The extrapolated absorbed doses for the onset of thyroid stunning were 0 Gy delivered to thyroid cells in vitro, and < or =4 Gy and 34 Gy delivered to thyroid cells in vivo (malignant and benign conditions, respectively). Thyroid stunning amounted to roughly 50% in the case of 2 Gy delivered to thyroid cells in vitro, and in the case of < or =30 Gy and 472 Gy delivered to thyroid cells in vivo (malignant and benign conditions, respectively). CONCLUSIONS: There is no scintigraphically sufficient diagnostic amount of I that can be given prior to I therapy for thyroid cancer that does not cause thyroid stunning, i.e. it is not recommended to deliver pre-therapeutically more than a few gray (<5 Gy) into thyroid remnants. More investigations are required to confirm the proposed in-vitro and benign in-vivo models, but characteristic absorbed doses presented so far for in-vitro vs. in-vivo malignant vs. in-vivo benign thyroid environments differ roughly by an order of magnitude.