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Background Parathyromatosis, described as hyperfunctioning, benign parathyroid tissue scattered throughout the soft tissues of the neck, is a rare cause of recurrent hyperparathyroidism. Fewer than 40 cases have been described in the literature. We describe a case of a woman with recurrent hypercalcemia secondary to parathyromatosis; additionally this is the first reported case of a papillary pattern in parathyromatosis. Clinical Case A 45 year old woman with a history of a left parathyroid adenoma, status post focused parathyroidectomy 10 years ago, presented with constipation, polyuria and abdominal pain. Initial examination demonstrated a well healed cervical scar with no palpable neck masses. Admission laboratory findings were significant for a calcium level of 13.6 mg/dL (n 8-10.4 mg/dL) , ionized calcium 7.7 mg/dL (n 4.6-5.2 mg/dL), phosphorus 2.2 mg/dL (n 2.7-4.5 mg/dL), 25 OH-vitamin D 9.6 ng/mL (n 30-100 ng/mL), PTH 395 pg/mL (n 14-72 pg/mL), PTHrP <0.7 pmol/L (n <2.0 pmol/L) and urine calcium 484 mg/24 hr (n 50-250 mg/24hr). No obvious enlarged parathyroid was localized on neck ultrasound or sestamibi scan. Neck CT showed two 6 mm nodular lesions in the upper mediastinum. The patient had refractory hypercalcemia despite maximal medical therapy. She was taken to the operating room for reoperative neck exploration. She was noted to have diffuse exophytic fronds of tissue covering her left thyroid lobe, which prompted removal of the lobe. Intraoperative PTH decreased from 454 pg/mL to 39 pg/mL. Microsopic examination demonstrated multifocal hypercellular parathyroid nodules in skeletal muscle and perithyroid adipose tissue consistent with parathyromatosis. An unusual pathologic finding was its unique papillary configuration, raising consideration for a parathyroid carcinoma on frozen section. Final pathology confirmed parathyromatosis with no features of carcinoma (vascular invasion, fibrosis or high mitotic activity) with diffuse immunohistochemical reactivity for PTH and non-reactivity for Thyroid Transcription Factor-1. The patient's calcium level was normal post operatively (9.2 mg/dL) with resolution of symptoms. Conclusion Parathyromatosis is a rare disease that raises challenges in diagnosis and management. Preoperative localization of parathyromatosis is difficult, leading to a high surgical failure rate. Scarring and fibrosis from previous neck exploration leads to technical difficulty, since the most common cause for parathyromatosis is seeding of hypercellular parathyroid tissue during previous parathyroidectomy. Medical management is essential during the post-operative period to maintain a normocalcemic state. Our case presents with a pathologic finding of papillary pattern in parathyromatosis, which has not been previously described and can pose a diagnostic challenge in separation from papillary carcinoma on frozen section examination