I-131 refractory DTC can be defined as an inability of malignant/metastatic tissue to take up I-131 from the start of treatment or loss of ability to take up I-131 after previous evidence of uptake. But what happens when there is uptake and no longer tumor response? Which is the limit of cumulative radioiodine therapy to consider radioiodine resistance?
It is not a common experience that there is uptake of iodine and no response to treatment in the lesion. The Thyroglobulin levels usually show a decreasing trend indicating response. There are however cases where the thyroglobulin levels are around 20 to 30 and on the scan a solitary focus is noted. These foci show no change on iodine administration and the thyroglobulin levels also do not show definite change. If this is the scenario you are encountering then the possible explanation is that the disease focus is too small for effective ablation by the beta particles which might be going beyond the tissue. The fact that the tissue is small is also represented by the relatively low Tg levels (though these are detectable) In such cases it is better to treat the patient and forget about the numbers. If the patient is asymptomatic anf Tg levels on follow up show no definite increase it might be better to watch and wait instead of subjecting the patient to repeated doses of radiation.
Many authors are considering that 600 mCi of cumulative activity would be a limit to consider RAI refractoriness. Durante et al. clearly showed that 96% of patients with DTC and advanced disease will respond with this activity. But it should be defined case by case. So the question is: do you continue to treat with radioiodine in spite of reaching this limit?
I think the main concern in treating with high doses would be the incidence of second malignancy. Literature with respect to this suggests that a cumulative dose more than 1000mCi increases the rate of second malignancy. (PMID: 22341041, PMID: 21368600). So if the patient is responding then the above mentioned dose should be reasonable to achieve. The other thing would be FDG avidity of the disease. If they are FDG negative then they would be slowily progressing but on the other hand if FDG positive then the benefit of treating with higher dose would be more.
The issue of FDG avidity calls for a PET scan. I am working on FDG avid PET scans but there are not much evidence based litterature to guide and my personal experience alone cannot justify me to make a statement at this point of time though i am a senior faculty and practicing endocrine surgeon
I concur that the literature in relation to FDG avid thyroid cancer is scarce. It is a common experience (supported extensively in literature) that among a given type of tumors, those with higher uptake of FDG show poorer prognosis