Can male breast cancer reoccur in the thyroid after 8 years of mastectomy, radiotherapy and chemotherapy?

Less likely: The thyroid would be an unusual spot for breast cancer to spread. More common sites would be bone, liver, lung and brain. But with cancer, anything is possible. If there is a nodule on the thyroid the best thing to do would be an ultrasound guided biopsy if it looks concerning.

Yes: Male breast cancer often goes undetected until tumors have been present for a long time and though it would be unusual to recur in the throid your description suggests this was an aggressive breast cancer when first discovered. That would make late metastasis much more likely.

Very unlikely: Very few cancers metastasize to the thyroid. I'll bet you it's a new tumor. The pathologist can tell for sure.

That would: be exceedingly unlikely. If you have a thyroid nodule, it is probably unrelated to your breast cancer.