Effective dose from radiopharmaceuticals

EuropeanJournalof

Nuclear Medicine Effective dose from radiopharmaceuticals

Dear Sir, We note with interest the paper by Lennart Johansson et al. in the November issue o f your journal [1]. What concerns us most is the apparent uncritical acceptance of the "effective dose", as defined by the I C R P in 1991, as a reliable method of measuring the radiation risk to patients from radiopharmaceuticals. Both the I C R P and the BEIRV report, on which ICRP's evaluations are based, have attracted searching criticism by eminent radiobiologists and nuclear medicine physicians [2]. That the I C R P assumptions are an oversimplification, if not patently untrue, can be shown by calculating the "effective d o s e " for a 370 MBq dose of iodine-131 (thyroid uptake 35%) (Table 2, p 937, line 12), namely 8.8 Sv. This dose, which is almost double the LDs0 for humans, taken with the concomitant risk estimates, is grossly at odds with the observed, factual incidence of leukaemia and other cancers in over 1.5 x 106 patients treated with radioiodine for thyrotoxicosis since 1945, i.e. slightly less than that observed in the general population [3]. No doubt this will evoke the usual mitigatory res p o n s e that the effective dose calculation is limited to

Reply

Dear Sir, We would like to thank Drs. Macleod and Kemp for their letter [1] regarding our paper [2]. Effective dose from radiopharmaceuticals. In order to estimate the risk for stochastic effects from an inhomogeneous exposure o f the total body, the most correct way is to consider each individual organ on the basis of its absorbed dose and radiation sensitivity. This is, however, impracticable and the I C R P has therefore introduced a system for weighting the organ doses from an inhomogeneous exposure o f the body [3, 4]. The weighting factors describe the relative distribution b e t w e e n different organs of the risk from a whole-body exposure. The result of this weighting procedure is a figure that gives the homogeneous whole-body dose, "effective dose", which results in the same total risk. One has to remember, however, that the intention behind the effective dose concept is to obtain a quantity which can be used for radiation protection purposes, rather than a quantity which exact-

Letter to the editor diagnostic investigations only and not therapeutic applications. But why? Does the physiological model of 1311 uptake kinetics change because the therapy dose is greater? It is in a situation like this that one wonders if there is not more than a modicum o f truth in the old adage, " t h e r e are lies, damned lies and statistics" ! M.A. Macleod P.M. Kemp

Department of Nuclear Medicine Royal Naval Hospital Haslar, Gosport Hampshire POI2 2AA, UK

References 1. Johansson L, Mattsson S, Nosslin B, Leide-Svegborn S. Effective dose from radiopharmaceuticals. Eur J Nucl Med 1992; 19:933938 2. Newsletter. J Nucl Med 1990; 31 : 13A-19A 3. Hall P, Boice JD, Berg GB, Bjelkengren G, Ericson U, Hallquist A, Lidberg M, Lundell G, Mattison A, Tennvall J, Wiklund K, Holm L. Leukaemia incidence after I131 exposure. Lancet 1992; 340:1-4

ly describes the magnitude of the radiation risk. The concept should also be adequate for use in different exposure situations (internal or external irradiation with varying dose rate etc.) Only stochastic effects have been considered in the derivation of the weighting factors for the effective dose. The effective dose concept is thus not applicable for therapy since this means that the absorbed dose in the organ of interest is so large that non-stochastic effects are present (which is the aim o f the therapy, and necessitates dose estimations based on measured biokinetic data for the individual patient). With a weighting factor of 0.05 to the thyroid, energy absorbed in this organ accounts for more than 99% of the effective dose from iodine-131 given in the form of iodide. The absorbed dose relevant for risk estimations in organs other than the thyroid is, in this case, thus two orders of magnitude lower than what can be calculated from the reported effective dose per unit activity administered, 24 mSv M B q - 1 [2]. Observations on patients treated or diagnosed with radioiodine, as reported by Hall et al. [5], excluded with high assurance an excess leukaemia risk of more than 25% in the group of patients studied (n = 46 988). With a natuEur J Nucl IVied (1993) 20:448-449

Vol. 20, No. 5, May 1993 - © Springer-Verlag 1993

449 ral incidence o f 0.4%, derived f r o m data reported by the same authors, this figure corresponds to an absolute risk f r o m the radiation of 0.1%. The average activity administered to this group of patients was 185 M B q 131I, and the m e a n thyroid uptake was 4 3 % ; this leads to an absorbed dose in the red bone m a r r o w of 18 m G y [6]. Applying the I C R P risk factor for leukaemia (0.7% S v - 1) on this figure results in a risk for leukaemia o f 0.01%. When comparing these two estimates it is evident that the data f r o m Hall et al. [5] cannot be used to contradict the I C R P risk estimation. L. Johansson, S. Mattsson, B. Nosslin, S. Leide-Svegborn

Radiation Physics Department, Umea University Hospital, S-90185 Umea, Sweden

European Journal of Nuclear Medicine Vol. 20, No. 5, May 1993

References 1. Macleod MA, Kemp PM. Eur JNucl Med 1993; 20:448-449 2. Johansson L, Mattsson S, Nosslin B, Leide-Svegborn S. Effective dose from radiopharmaceuticals. Eur J Nucl Med 1992; 19:933938. 3. ICRP Publication 26. Recommendations of the ICRP. Oxford: Pergamon, 1977. 4. ICRP Publication 60. 1990 recommendations of the International Commission on Radiological Protection. Oxford: Pergamon, 1991. 5. Hall P, Boice JD Jr, Berg G, et al. Leukaemia incidence after iodine-131 exposure. Lancet 1992; 340:1-4. 6. ICRP Publication 53. Radiation dose to patients from radiophar-] maceuticals. Oxford: Pergamon, 1988.