Answer to Question #9091 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Therapeutic Nuclear Medicine

The following question was answered by an expert in the appropriate field:

Q

I want to know the amount in percentage of sweating excretion (perspiration) of an administered 131I dose (for example: 3,700 MBq). We know the fecal excretion is about 10%, renal is about 70%, but I haven't found the percentage in perspiration and salivary fluid.

A

First of all there is a difference between the 131I kinetics for treatment of hyperthyroidism and thyroid cancer. Since you have mentioned 3,700 MBq, I assume you are looking for the pertinent values after oral sodium 131I treatment for thyroid cancer. If this is the case, the percentages you cite for urinary and fecal excretion appear to be incorrect; 70% is too low and 10% is too high. Unfortunately, you will be hard pressed to find percentages for excretion via perspiration and saliva, rather than measured values at various times post-administration. I refer you to the following two publications for measured levels of 131I from thyroid cancer and hyperthyroid patients:

  1. Ibis E et al. Iodine-131 contamination from thyroid cancer patients. J Nucl Med 1992; 33:2110–2115; and
     
  2. O'Doherty MJ et al. Radiation dose rates from adult patients receiving 131I therapy for thyrotoxicosis. Nucl Med Commun 1993; 14:160–168.

For example, according to the Ibis article, the amount of 131I excreted via perspiration is maximal at about 24 hours postadministration and measured activities at this time have varied from 11.1 to 248 Bq, depending on the body part sampled (forehead, chest, neck, hand). As values at 4, 24, and 48 hours are given you could add them all up at each sampling time, make some assumptions about the remaining nonsampled areas of the body, and then integrate this activity-time curve to very roughly estimate the total percentage of 131I excreted via perspiration.

Given that only Bq quantities have been measured, only a very small percentage of the administered activity will most likely be found to be associated with excretion through perspiration. Salivary activity has been similarly determined; however, this latter estimation method for salivary excretion cannot be used since the saliva is nearly completely swallowed by the patient and the 131I is reabsorbed into the blood.

Jeffry A. Siegel, PhD

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