This report illustrates a case of intrathyroid parathyroid carcinoma that masqueraded as a thyroid primary tumor both cytologically and clinically. The patient was first seen by an orthopedic doctor for evaluation of a back pain of increasing intensity. X-rays studies of the vertebral column and a laboratory blood test work-up followed. On physical examination a thyroid nodule was also palpated and the patient was seen by an endocrinologist. Ultrasound evaluation documented a solid, hypo-echoic nodule in the lower third of the left lobe of thyroid gland which was readily examined by FNA biopsy.
On microscopic evaluation of the aspirate our first impression was that of a medullary thyroid carcinoma (MTC) but the following cytologic features seemed not to fit with the diagnosis: uniform cell size and lack of significant cellular pleomorphism; complexity of cellular arrangement with frequent three-dimensional tight clusters and tendency to cell aggregation; abundant monomorphic naked nuclei; concomitant presence of both oncocytoid and clear cells; lack of spindled or triangular cells. Moreover, the tumor was located in the lower thyroid pole, which is unusual for MTC.
In fact, the above cytologic features suggested a parathyroid lesion as an alternative diagnosis. Laboratory data which were made available soon after FNA biopsy sampling showed a calcemia of 16.1 mg/dL, (n.v. 8.4-10.5), a phosphoremia of 2.27 mg/dL (n.v. 2.5-4.5) and signs of slight renal insufficiency. These findings were consistent with a condition of hyperparathyroidism and strongly supported the above contention. Our FNA cytologic diagnosis was “parathyroid tumor, uncertain if benign or malignant”. Preoperative values of serum PTH were 2002 pg/ml (normal value 8.0-76.0). The patient underwent right hemithyroidectomy with isthmus resection.
Histology of surgical specimen demonstrated a well circumscribed and encapsulated intrathyroid nodule Foto_6
; microscopically, the architecture of tumor growth was either solid structureless, or multinodular, or trabecular with thick fibrous bands incompletely dividing the tumor. The tumor consisted of a population of medium sized polygonal oxyphilic and transitional oxyphylic parathyroid cells Foto_7
with occasional mitotic figures. Immunostaining for PTH was positive Foto_8
. There were occasional necrotic areas. Foci of capsular disruption and pericapsular invasion into thyroid parenchyma in addition to intravascular tumor growth Foto_9
and minimal foci of extrathyroid invasion were also seen. These latter findings demonstrated the malignant nature of the tumor.
It is well known in the Literature that the distinction of parathyroid from thyroid cells in FNA samples can be challenging, and incorrect identification of parathyroid as thyroid lesions has been reported in a significant proportion of cases in the largest published series 1 2 3 4 5 6
. Potential trap is due to the finding in parathyroid lesions of aggregation patterns and cellular features, including intranuclear holes (cytoplasmic inclusions) 7
, which overlap with those seen in thyroid follicular lesions 2 5 6
or Hurthle cell neoplasms 5 6
, or papillary carcinoma 6 7
. If the tumor exhibits a clear cell cytology it can be confused with a primary clear cell carcinoma of the thyroid. Naked nuclei can be misinterpreted as lymphocytes and suggest the diagnosis of lymphocytic thyroiditis 8
. Finally, as in the present case, plasmacytoid or oncocytoid cellular morphology and intranuclear holes can simulate the FNA cytological picture of medullary thyroid carcinoma 6 9
I want to thank all the colleagues for their comments and congratulate my friend M. Spinelli for making the correct suggestion.