Rarely, thyroid nodules in children turn out to be medullary thyroid cancers, a type of cancer we discuss in Chapter 9. More often, a family is already known to have at least one member diagnosed with medullary thyroid cancer (called an index patient), and the child's blood tests have shown that he or she has a mutation in the RET proto-oncogene responsible for causing this cancer. In this case, it takes only one parent with the mutation to pass it on to a child (called an autosomal dominant mutation). Furthermore, the likelihood that the mutation will cause cancer (known as penetrance) is nearly 100 percent. That means that if an individual has this mutation he or she is expected to develop this cancer at some time in his or her life. Usually, the blood relatives of the index patient should have their blood tested to see if they also have this specific gene mutation. Children in the family should be tested as early as possible. Should these tests show the presence of this mutation, to prevent the development of this cancer or to treat it at an early stage, an experienced surgeon should remove the person's entire thyroid gland. Children as young as three years of age should have their thyroid removed if their genetic tests are confirmed positive. This is because metastatic (spreading) inherited medullary thyroid cancers have been documented in very young children. Some of these children are also at risk for developing tumors of their adrenal glands making too much adrenaline, calledpheochromocytomas, and need to be checked at regular intervals.
Only around 20 percent of medullary thyroid cancers of all age groups are inherited in this fashion. On the other hand, most childhood medullary thyroid cancers are likely to have been inherited from a gene mutation, since nongenetic medullary thyroid cancers are more common in older adults.
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