I saw my surgeon this morning and got more detail. The tumor I could feel was larger than I thought: 2.3cm x 1.6cm x 0.8cm. They also found small (~0.2cm) satellite tumors. They all tested positive for classic papillary thyroid carcinoma, as expected.
The doctor said this variety of thyroid cancer cannot migrate on its own to soft tissue and there is no evidence that it has transformed to a more aggressive variety. He said the only explanation he could think of is "tumor spillage" (band name?) during surgery or a needle biopsy; but given that I didn't have a needle biopsy before my original surgery, and given the details of the original surgery, this doesn't seem like a plausible explanation, so basically he said "this can't happen but it happened." He also said he finds this case fascinating.
I'm waiting to see how my thyroglobulin levels look. If it drops off to zero, then they likely got all (or almost all) of the cancer. If it stays relatively elevated, I might still have some cancer floating around somewhere else, and it might make sense to attempt to treat it with radioactive iodine. Or not.
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