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Correspondence |
Medical Physics Department, York District Hopital, Wigginton Road, York, YO31 8HE, UK
The Editor—Sir,
An article in this journal [1] has given advice on the radiation protection measures that are necessary when a patient leaves hospital following therapeutic administration of a radioactive material. In particular, it addressed the problem of how to determine the period for which other people should avoid prolonged close contact with the patient to keep their radiation doses due to external irradiation acceptably low.
For treatment of hyperthyroidism with radioiodine-131 (131I), suitable restrictions and their appropriate duration have been verified by measurement of the doses received by carers, close family members and the general public [2]. However, for new or infrequently used radionuclide therapies there may be insufficient data on the doses sustained by others to allow appropriate periods of restriction to be based on measured doses.
When normal patterns of close contact with the patient can be ascertained reliably, a dose modelling approach can be used successfully to determine adequate restrictions for keeping doses down to acceptable levels [3]. This is a valuable tool, to be recommended wherever it is applicable. However, circumstances may arise in which the input data required by this method cannot be obtained in sufficient detail.
Nevertheless, radiation protection advice must be given and the article [1] proposed the innovative idea of equating the actual administered activity with an "equivalent activity of 131I". Suitable restrictions could then be chosen from those successfully verified for hyperthyroid patients treated with 131I. Unfortunately, the equation given for "equivalent activity of 131I" has been shown to yield inappropriate periods of restriction for therapies where the effective half-life is different from 6 days [4, 5]. An alternative method of implementing the idea is given below. Clearly, there is no entirely satisfactory substitute for actually measuring the doses arising from close contact with patients receiving a particular type of radioactive treatment. However, the method proposed may assist with determining practicable restrictions while the necessary measurements of doses to family members are being made. (The risks of other radiation-related hazards, such as contamination, would need to be assessed separately.)
Consider a patient discharged from hospital with a fixed biodistribution of radiopharmaceutical that gives a dose rate of D µSv h-1 at a distance of 1 m from the patient. D is assumed to diminish with time as a single exponential with a half-time T. The problem is to determine a suitable period of restriction tR during which close contact with the patient should be disallowed, such that doses to other people are kept below appropriate dose constraints. The dose received by such a person will be given by:
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The value of k1 is controlled by the daily pattern of contact during the period of restriction (i.e. until tR) between the patient and the person that the patient is irradiating. The value of k2 is determined by the pattern of contact after restrictions have been lifted (i.e. after tR). Both constants would be equal to 1 if the person were to remain constantly at a distance of 1 m from the patient.
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).
The minimum acceptable period of restriction may thus be defined as that which keeps the dose received after tR at, or just below, the relevant dose constraint. This criterion can be shown to be consistent with the periods of restrictions recommended when treating hyperthyroidism with 131I [1, 2]. The recommended period of restriction tR is increased in accordance with the amount of 131I administered, AI, so that given an effective half-life TI of 6 days, the quantity:
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The restrictions that have been validated for hyperthyroid patients treated with 131I can be used as a basis for estimating the length of time for which the same restrictions should be imposed for patients undergoing some other radionuclide treatment X. Provided the dose rate resulting from X also decays as a single exponential, with a known half-time TX, the necessary period of restriction tRX may be estimated as follows.
First, the dose rate DX due to therapy X is measured at a distance of 1 m from the patient. Since the dose rate from a thyrotoxic patient is taken to be 0.05 µSv h-1 per MBq of 131I administered [1], an "equivalent activity of 131I", AI, is given by DX/0.05. If the administration had been 131I to a hyperthyroid patient, then a known suitable period of restriction (based on the amount of 131I administered), tRI, would have been imposed. For treatment X, the actual period of restriction tRX needed can be estimated by requiring the doses following the end of restrictions to be the same for each type of treatment, i.e. from the second term in equation [1] we require:
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Example 1: 131I ablation following thyroidectomy
Although the dose rate from 131I decrease bi-exponentially in this case, the clearance is dominated by the slower component after about 3 days [6]. Hence, the dose rate from a patient discharged 3 days or more after treatment will decay effectively as a single exponential with an effective half-life of approximately 4.28 days [6] compared with 6 days when the thyroid is present and hyperactive. If the patient retains 800 MBq at discharge, the starting point is the published guidance for a hyperthyroid patient retaining this activity, namely a period of 14 days during which time anyone other than "comforters and carers" should remain at distances greater than 1 m from the patient, and a concurrent period of 27 days during which close contact should be limited to 15 min per day [1]. Equation (2) is then applied to each of these restrictions in turn, using TI=6 days, TX=4.28 days and tRI=14 days and 27 days, respectively. This yields corresponding periods of restriction for the ablation treatment of 7.9 days and 17.2 days, respectively.
Example 2: palliation of metastatic bone pain using samarium-153 EDTMP
Roberts [4] reports that patients receiving 37 MBq kg-1 of samarium-153 (153Sm) chelated with ethylenediaminetetraethylenephosphonic acid (EDTMP) typically give dose rates of up to 20 µSv h-1 at 1 m a few hours after administration. Since the biodistribution stabilizes within 5–10 h, with about 60% of administered activity fixing in the skeleton and the surplus being rapidly excreted mainly via the urine, Equation (2) can be applied thereafter. The recommended restrictions [1] for 20 µSv h-1, which is "equivalent" to 400 MBq of 131I, are 12 days of maintaining a distance of at least 1 m from the patient concurrent with 25 days of restricting close contact to 15 min per day. Assuming the effective half-life of 153Sm EDTMP is equivalent to the physical half-life of 153Sm (i.e. 
days, TI=6 days and tRI=12 and 25 days, respectively), Equation (2) yields a period of 0.7 days for maintaining a distance of at least 1 m and a concurrent period of 4.9 days for limiting close contact to 15 min per day. This latter period compares well with the range of 1–6 days found empirically to be sufficient by Roberts and co-workers [4].
In contrast to the case of the radioactive thyroid (effectively a point source), the dose rate close (i.e. less than 1 m) to a patient with widespread uptake of activity will not vary in accordance with the inverse-square law, increasing less rapidly as the patient is approached [7]. The proposed method will thus lead to restrictions on close contact that may be somewhat conservative for radionuclides that are distributed throughout the body. It will not, however, underestimate the appropriate duration of restrictions.
References
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