Radiotherapeutic agents: properties, dosimetry, and radiobiologic considerations

Measurement of 131I activity in thyroid of nuclear medical staff and internal dose assessment in a Polish nuclear medical hospital

This paper presents results of 131I thyroid activity measurements in 30 members of the nuclear medicine personnel of the Department of Endocrinology and Nuclear Medicine Holy Cross Cancer Centre in Kielce, Poland. A whole-body spectrometer equipped with two semiconductor gamma radiation detectors served as the basic research instrument. In ten out of 30 examined staff members, the determined 131I activity was found to be above the detection limit (DL = 5 Bq of 131I in the thyroid). The measured activities ranged from (5 ± 2) Bq to (217 ± 56) Bq. The highest activities in thyroids were detected for technical and cleaning personnel, whereas the lowest values were recorded for medical doctors. Having measured the activities, an attempt has been made to estimate the corresponding annual effective doses, which were found to range from 0.02 to 0.8 mSv. The highest annual equivalent doses have been found for thyroid, ranging from 0.4 to 15.4 mSv, detected for a cleaner and a technician, respectively. The maximum estimated effective dose corresponds to 32% of the annual background dose in Poland, and to circa 4% of the annual limit for the effective dose due to occupational exposure of 20 mSv per year, which is in compliance with the value recommended by the International Commission on Radiological Protection.

131I effective half-life (Teff) for patients with thyroid cancer

Nuclear medicine procedures that use I activity represent significant sources of potential absorbed dose to medical staff, volunteers, and the general public. The potential exposures are due principally from exposures to excreted and retained activities in the patients' bodies. In general, exposure rate decreases in a simple exponential manner. The average effective half-life found for all patients in the study reported here is 11.41 +/- 0.02 h; this information may be used in guidelines on the management of patients in thyroid cancer therapy and for general radioprotection practice.

Evaluation of the potential absorbed doses from patients based on whole-body 131I clearance in thyroid cancer therapy

The evaluation of the absorbed dose from radioactive patients during the treatment of thyroid disease is an important factor in establishing precautions in these procedures, and the I retention/excretion by patients' bodies provides additional information to medical and radioprotection service. In 94 patients, the measurement of exposure rates was performed over 7 d following NaI administration, and the rates permitted the study of the dynamics of excretion and the potential dose evaluation. The administered activities ranged from 3.7 GBq (100 mCi) to 16.65 GBq (450 mCi), and the results proved that the majority of the activity is excreted by patients in the first 3 d after NaI administration. The average (131)I activity excreted at 24, 48, 72, 96, and 120 h after oral administration was (72 +/- 10), (91 +/- 6), (97 +/- 3), (98.9 +/- 1.5), and (99.6 +/- 0.7)%, respectively. According to the administered activity, the evaluation of the accumulated absorbed dose from patients ranged from 3.0 +/- 0.7 to 8.4 +/- 1.1 mSv at 1 m and 1.2 +/- 0.4 to 3.2 +/- 0.4 mSv at 2 m. The data reported here are important to radioprotection policy and to add to and improve on the guidelines reported in U.S. NRC Regulatory Guide 8.39.

Track structures and dose distributions from decays of (131)I and (125)I in and around water spheres simulating micrometastases of differentiated thyroid cancer

The disintegration of the radionuclides (131)I and (125)I and the subsequent charged-particle tracks left behind in water (as a model substance for a biological cell) are simulated by the Monte Carlo track structure simulation code PARTRAC, using new inelastic electron scattering cross sections for condensed water. Every photon and electron emitted was followed in detail, event by event, down to 10 eV. From the spatial information on the track structures, absorbed dose distributions per (131)I and (125)I decay were calculated in and around water spheres simulating micrometastases as well as in the tissue surrounding such metastases. These radionuclides were assumed to be distributed uniformly inside spheres of different diameters (0.01, 0.03, 0.1, 0.3, 1.0 and 3.0 mm). The respective electron degradation spectra, the nearest-neighbor distance distributions between inelastic events, and the distance distributions for all activations for both iodine radionuclides were calculated. The absorbed fractions of the initial electron energies, absorbed doses and energy depositions, and single-event distributions, F(1)(epsilon), inside the six water spheres described above and in the surrounding tissue were also calculated. The absorbed doses per decay inside the six water spheres, i.e., the calculated S values (listed from 0.01 to 3.0 mm), were 6.8 x 10(-4), 7.2 x 10(-5), 5.5 x 10(-6), 4.9 x 10(-7), 3.1 x 10(-8) and 1.8 x 10(-9) Gy Bq(-1) s(-1) for (131)I, and 3.4 x 10(-3), 1.7 x 10(-4), 5.1 x 10(-6), 2.0 x 10(-7), 5.6 x 10(-9) and 2.2 x 10(-10) Gy Bq(-1) s(-1) for (125)I. It is concluded that, in the treatment of thyroid cancer, the geometrical track structure properties of (125)I might be superior to those of (131)I in micrometastases with diameters less than 0.1 mm; however, in this medical context, many other factors also have to be considered.

Optimising the radioimmunotherapy of malignant disease: the broadening choice of carrier and effector moieties

The treatment of cancer by radioimmunotherapy remains an experimental approach successful only in a limited number of selected disease conditions. One ground for optimism over the future of radioimmunotherapy lies in the fact that where cures have been obtained it has been despite the design of the immunoconjugate rather than because of it. As the choice of available functional components for conjugate construction increases, the process of evaluation and optimisation is underway. The replacement of the commonly used 131I with radionuclides possessing radiation characteristics more suited to particular disease states, and tailored to the behaviour of the carrier vehicle, should bring improved energy deposition within tumour while reducing whole body radiation burden. Similarly, the introduction of chemically or genetically engineered targeting molecules in place of conventional antibodies may bring improved pharmacokinetic characteristics and higher tumour accumulation. Optimisation of the therapeutic and carrier moieties employed in radioimmunotherapy should bring distinct improvements in clinical efficacy.

Polyclonal 90Yttrium labeled antiferritin for refractory Hodgkin's disease

Six patients with chemotherapy resistant Hodgkin's disease were treated with intravenous polyclonal 90-Yttrium (90Y) labeled antiferritin. Eighteen days after isotope infusion, patients received an autologous bone marrow transplant that was cryopreserved prior to initiation of treatment. Ten (one patient), 20 (four patients), or 30 mCi (two patients) were used. One patient received three cycles, three patients received two cycles, and two patients received one cycle. The same antibody labeled with 111-Indium (111In) was helpful in documenting the absence of anti-antibodies in six out of six patients, the presence of tumor targeting in six out of seven patients, and allowed for dose estimates in two out of six patients. One patient with a complete response received approximately 20 Gy to the tumor, whereas a second patient with 20 Gy to the tumor showed progressive disease. A total of three patients obtained a complete response, one had a partial response, and two patients progressed on treatment. Acute toxicity was limited to bone marrow aplasia, without a clear-cut beneficial effect for transplantation after 20 mCi 90Y and the suggestion of a positive effect after 30 mCi. One patient died in complete remission 26 months after treatment with chronic lung insufficiency, probably unrelated to the isotope treatment. The early observations are that 90Y-labeled antiferritin has a pronounced antitumor effect as a single agent and less normal tissue toxicity than other treatment modalities for Hodgkin's disease, such as chemotherapy, external beam radiotherapy, or autologous bone marrow transplantation after high dose chemo/radiotherapy.

Radiation dose assessments in radioiodine (131I) therapy. 1. The necessity for in vivo quantitation and dosimetry in the treatment of carcinoma of the thyroid

In order to destroy thyroid cancer metastases by radioiodine an average tissue dose of 80-300 Gy is needed. Such high doses can be expected, following the administration of the conventional 5.5 GBq of 131I, only if both the percentage uptake per gram in the target tissue and the effective half life of the radioiodine in it are higher than well-defined threshold values, and if every dimension of the tissue exceeds several millimeters. The fulfillment of such favourable conditions in actual clinical cases can only be confirmed by in vivo quantitation of the absorbed dose achieved as a result of the administration of radioiodine.

Considerations on 131I-metaiodobenzylguanidine therapy of six children with neuroblastoma

Six children affected by neuroblastoma at stages III and IV were treated with high-specific-activity 131I-metaiodobenzylguanidine (MIBG). After 131I-MIBG treatment three patients died at 12, 10, and 12 weeks, respectively; the other three were still living at 21, 16, and 24 weeks, respectively. Although the assumptions for this therapy were propitious, the results obtained do not correspond to those expected. It is supposed that large tumor volume and previous chemotherapy and/or radiotherapy may impair the effectiveness of 131I-MIBG therapy. Consequently, 131I-MIBG therapy is recommended even if the spread of disease is not proved-only, however, when the tumor is small.