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I would really appreciate your advice about an 18-year-old girl with papillary carcinoma of thyroid and extensive miliary pattern of metastases throughout both lung fields. An added complication is that she was given contrast for a CT scan just before a total thyroidectomy. Since I have not treated someone with such extensive lung disease, I have some concerns.
1. I plan to measure a 24-hour urine for iodine, creatinine, and sodium one month from the time of the CT scan after she has been on a low iodine diet for three weeks, and if the urinary iodine is less than 150 mcg per 24 hours, this suggests that most of the dye has been excreted.
2. Our treatment with I have been usually been empirical; although one of the nuclear medicine physicians uses a method to calculate a safe dose to the lungs, he does not calculate the dose that would result in an 80 mCi retained dose at 48 hours. Since he is out of the country for several weeks, I am unsure of his method. Is empirical dosing with eg. 200 mCi a reasonable dose, or should I plan to send her somewhere where dosimetry is done to ensure a maximum safe dose? She is otherwise healthy with normal renal function.
3. I plan to get pulmonary function tests and involve a respirologist in her care.
Because of the danger of acute radiation pneumonitis, are there any precautions I should take or any prophylactic treatments such as corticosteroids? I have not seen or treated anyone with acute radiation pneumonitis and would appreciate any references.
4. Should I plan to use Lithium to increase retention of I in metastases or might this worsen the pneumonitis?
Sincerely,
D. W. Ingram, MB, FRCPC, FACP
St. Clare's Mercy Hospital, St. John's, NL
Response
A general rule is that you want to have less than 65-70 mCi accumulated in the lungs, in order to prevent radiation pneumonitis. It is possible to estimate that using a 2mCi 131-I scan at 48-72 hr, visualizing the lungs with a gamma camera, and relating the uptake to a standard such as 10% of the same dose, counted at the same time. While it is unusual, extensive mets in a young untreated person could accumulate a high amount of 131-I, and could easily exceed 65 mCi with a 200mCi dose.You should do this test very close to the treatment, since you want it to reflect the actual conditions when treated.
I am not aware of pre-treatment for this problem, although it is done with cranial or cord mets. However steroids are certainly the usual treatment if problems arise. The pneumonitis can be severe and even fatal, so precautions are highly appropriate.
Waiting 4 weeks prior to testing urine iodine should be enough. Your cut-off may be a bit low, but would certainly mean that the iodine load was already excreted. Most CT contrast is excreted quite rapidly- in one - two weeks, but the effect on the thyroid lingers on.
I have no personal experience with lithium, and would not use it. If you do, you must do the exact protocol with both a test dose and the actual treatment, so that you will understand what its effect is on the treatment dose given.
Regards,
L De Groot, MD