Parathyroid Carcinoma Camouflaging as Secondary Hyperparathyroidism- A Case Report

Introduction:
Parathyroid carcinoma is a rare endocrine tumor accounting for less than 1% of all hyperparathyroidism. Mostly it manifests as primary hyperparathyroidism. However, parathyroid carcinoma arising on the background of secondary hyperparathyroidism is unusual which if overlooked could increase the risk of morbidity and mortality.

Case Description:
A 35-year-old gentleman diagnosed with end stage renal disease (ESRD) two years back on hemodialysis presented with bilateral fore feet pain for 5 months. Clinical examination was unremarkable except for pale conjunctiva. No neck mass was palpable. On workup he was diagnosed to have secondary hyperparathyroidism secondary to vitamin D3 deficiency resulting from CKD. He was being managed medically but despite that his PTH levels remained high 2399.1 pg/ml. On further workup ultrasound neck showed hypoechoic, non vascular lesion measuring 1.6 x 1.3 cm outside the left lobe of thyroid gland and in between the left lobe of thyroid gland and carotid vessels, suggestive of enlarged parathyroid gland which was further supported by sestamibi scan. Hence, he underwent minimally invasive left parathyroidectomy and biopsy report revealed low grade parathyroid carcinoma measuring 2 x 1 cm showing capsular, vascular and focal invasion into the adipose tissue and no perineural invasion. The Ki67 proliferative index was 5-7%. After surgery his symptoms persisted and PTH levels continued to rise. Sestamibi scan 6 months after parathyroidectomy revealed enlarged right upper parathyroid gland. His whole body scan was negative for metastasis and the bone scan showed superscan suggestive of metabolic bone disease. He subsequently underwent enbloc resection of left side along with near total parathyroidectomy whose biopsy revealed right parathyroid adenoma.

Discussion:
Elevated PTH levels in CKD is mostly attributed to secondary hyperparathyroidism. However, rare cases have been reported where parathyroid carcinoma was discovered in CKD patients with underlying secondary or tertiary hyperparathyroidism. CKD is one of the risk factor for cancer leading to 1.2 to 1.5 fold increased cancer risk. ESRD in our patient might have led to the development of parathyroid carcinoma.

Parathyroid carcinoma are usually hyperfunctioning and symptomatic like this case and less than 10% are asymptomatic and non functional. 30-75% of cases present as a neck lump which was not found in this case. Distinguishing between parathyroid adenoma versus carcinoma is crucial as the extent of surgical resection depends on it. Though histopathology of excised tumor help in diagnosing malignancy but at times it confronts with diagnostic challenges. Presence of one of the features such as vascular invasion, perineural invasion, adjacent structure invasion and metastasis strongly suggest parathyroid malignancy some of which were seen in our case. Our patient had PTH levels three times the upper limit of normal which are commonly seen in parathyroid carcinoma. Grossly, malignant parathyroid have whitish to grey hue versus reddish brown color in case of benign etiology which also directs surgical approach.