Percentage of thyroid nodules with microcalcification who had radiation as child that is malignant?

Biopsy. Thyroid nodules with microcalcifications are considered suspicious and therefore fine needle aspirations should be considered for definitive diagnosis. If fna fails to make the diagnosis, surgical excision may be considered.
High enough. High enough that you should definitely have a thyroid biopsy and be considered for possible surgery. What difference does the exact percentage make?
More risk. Any thyroid nodule is of concern in patients with a history of cervical irradiation. Those with fine calcification can represent papillary cancer. Percentages don't matter as much as needle biopsy proof that the nodule is benign.