Rare Tumors

Published on 25/03/2015 by admin

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Last modified 25/03/2015

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Chapter 500 Rare Tumors

500.1 Thyroid Tumors

See Chapter 562.

Malignant Thyroid Tumors

Pediatric thyroid malignancies are rare tumors that include medullary thyroid carcinoma (MTC) and the differentiated thyroid carcinomas (DTCs): papillary thyroid carcinoma (PTC) and follicular thyroid carcinoma (FTC). The vast majority of childhood thyroid cancers are PTC. The major established environmental risk factor for the development of DTC is exposure to ionizing radiation. MTC is a very uncommon disease in childhood that almost always occurs in the context of 1 of 2 hereditary endocrine tumor syndromes that arise secondary to activating mutations of the RET (REarranged during Transfection) proto-oncogene: multiple endocrine neoplasia type 2a (MEN2A) and type 2b (MEN2B). In addition to the almost complete penetrance of MTC, 50% of patients with MEN2A and MEN2B develop pheochromocytomas, and up to 30% of MEN2A patients have primary hyperparathyroidism. Patients with MEN2B have a distinct phenotype with a characteristic facial appearance, marfanoid body habitus, and a generalized ganglioneuromatosis, manifested most obviously by the presence of oral mucosa neuromas.

Children with thyroid cancer usually present with an asymptomatic thyroid mass and/or cervical lymphadenopathy. Lymph node metastases are present in the majority of PTC cases and lung metastases are identified in up to 20% of cases; metastases to other sites are rare. Despite a more advanced presentation, children with thyroid cancer usually have an excellent prognosis with anticipated survival over decades.

Total thyroidectomy and a compartment-oriented lymph node dissection, as indicated, are best accomplished by surgeons with extensive experience in the management of thyroid malignancies. In DTC, radioiodine (using 131I) is administered a few weeks after surgery to ablate any residual normal thyroid tissue and treat any residual thyroid cancer that is iodine-avid; children with MTC do not require radioiodine ablation. The TSH level is suppressed by giving supraphysiologic levothyroxine doses in DTC, but the TSH is kept normal in MTC. Long-term follow-up involves monitoring of tumor markers (thyroglobulin in DTC and calcitonin/carcinogenic embryonic antigen in MTC) as well as routine imaging based on the initial extent of disease presentation. Neck US is generally the most useful imaging modality. Traditional chemotherapy has generally not been shown to be effective in thyroid cancer, and newer anticancer agents, the oral tyrosine kinase inhibitors, are showing promise in the treatment of children with disease not amenable or responsive to standard therapeutic approaches.

In MEN2, there is a correlation between genotype and phenotype, and the biologic aggressiveness of MTC depends on the hereditary setting in which it develops. With the advent of genetic testing for RET mutations, MTC has become one of the few malignancies that can be prevented or cured via prophylactic thyroidectomy before it becomes clinically relevant. Recent guidelines have updated recommendations regarding the age of prophylactic thyroidectomy in children who are carriers of a RET mutation.