Case #1 2025

Interpretation

Mass at thoracic inlet: Endocrine tumor, likely parathyroid in origin, with concurrent hemorrhage

Explanation

The aspirate contained many dense circular clusters (nests or packets) of uniform cuboidal to low columnar epithelial cells, with rare acinar-like arrangements, in a background containing large amounts of blood. The cells had a moderate amount of light blue slightly grainy cytoplasm with basilar nuclei containing lightly clumped chromatin and no visible nucleoli. Anisokaryosis and anisocytosis were mild. There were also moderate numbers of macrophages, which were erythrophagocytic or contained hemosiderin in their cytoplasm or both (Figures 2-4). The features of the cells are characteristic of a hormone-producing epithelial tumor (e.g. parathyroid, thyroid cytology question 1). With the marked hypercalcemia and hypophosphatemia, a parathyroid tumor of chief cell origin (adenoma or well-differentiated carcinoma) was prioritized (cytology question 2).

The most noteworthy results on the serum biochemical panel were a marked hypercalcemia and hypophosphatemia. This pattern of changes is characteristic of excess PTH from primary hyperparathyroidism¹ or chronic kidney disease.² Although the horse had an increased creatinine concentration with isosthenuric urine, indicating a renal azotemia, the mild nature of the renal disease (based on the degree of increase in the creatinine concentration) did not explain the marked changes in calcium and phosphate concentrations, prioritizing primary hyperparathyroidism as the diagnosis. Other than excess PTH from primary hyperparathyroidism, PTHrP secretion from a non-parathyroid neoplasm producing a paraneoplastic hypercalcemia would be the second main differential diagnosis (i.e. before chronic kidney disease) but is ranked lower than primary hyperparathyroidism because high PTHrP concentrations usually causes hypercalcemia without hypophosphatemia (but can absolutely do both) (chemistry question 1). To differentiate between these causes, imaging studies to look for an underlying parathyroid lesion or tumor producing PTHrP, such as gastric squamous cell carcinoma,³ and a hypercalcemia malignancy panel should be performed as done in this horse (chemistry question 2). Results of the malignancy panel showed a marked increase in free ionized calcium (3.63 mmol/L; reference interval [RI], 1.58-1.90 mmol/L) and PTH (30.8 pmol/L, RI, 0.6-11 pmol/L) and a PTHrP concentration of 0 pmol/L, confirming the presumptive diagnosis of primary hyperparathyroidism.

The hyperglycemia could have been a transient stress-related response (increased epinephrine or endogenous glucocorticoids), however insulin resistance from the diagnosed PPID could also be operative. The increased total and indirect bilirubin concentrations were attributed to the inappetence. The high LDH activity with no changes in liver enzyme (SDH, GLDH, AST) or muscle (CK, AST) activities is an unusual finding and may be secondary to the neoplasm. High LDH activity has been identified as a tumor marker in humans and dogs,^4,5 but not horses. Note that the hypercalcemia may be contributing to the isosthenuric urine by increasing medullary blood flow (resulting in medullary solute washout) and inhibiting the action of ADH in the collecting ducts (via decreasing aquaporin2).

Outcome and additional tests

The mass at the thoracic inlet was surgically excised, fixed in formalin, and submitted for histologic evaluation. Histologic evaluation of hematoxylin-&-eosin-stained slides of the paraffin-embedded fixed tissue showed an expansile mass consisting of medium p0lygonal cells forming trabeculae, cords, and packets, with rare microfollicular arrangements. The cells were supported by a thin fibrovascular stroma. Areas of hemorrhage with hemosiderin-containing macrophages were seen in the stroma, along with dilated blood vessels (Figure 5-7). The tumor cells had large amounts of eosinophilic cytoplasm with round nuclei and displayed mild anisokaryosis and anisocytosis, with 1 mitotic figure in 10 2.37 mm fields. The histologic diagnosis was a parathyroid adenoma. The free ionized calcium concentration decreased to within the reference interval (1.66 mmol/L) 6 days after surgery. The horse was then lost to follow-up.

Discussion

Parathyroid disorders are uncommon in the horse, with reports of primary and secondary hyperparathyroidism^1,6–8 and hypoparathyroidism.^9,10 In a retrospective study of 17 cases of primary hyperparathyroidism in horses,¹ affected animals were mostly older (median, 21 years, range, 10-34 years) and of various breeds, including Quarter Horse, Morgan, Warmblood, mule and pony. The most frequent clinical signs were weight loss, anorexia and lethargy, as seen in the case herein. Of the 17 horses, three horses were presented due to a documented hypercalcemia on blood testing.¹ Affected horses had a total (median, 17.3 mg/dL; range; 14.6-26.6 mg/dL; 17/17) and ionized (median, 2.66 mmol/L; range, 2.14-4.95 mmol/L; RI, 1.44-1.74 mmol/L; 15/15) hypercalcemia on chemistry testing and 71% were hypophosphatemic (range, 0.9-1.9 mg/dL; RI, 2.1-4.7 mg/dL). PTH concentrations were increased in 71% (11.7-119 pmoL/L) or within reference intervals for the remaining horses (3.9-6.7 pmoL/L; RI, 0.6-11.0 pmoL/L). However, a normal PTH concentration is inappropriate in an animal with a high free ionized calcium concentration and still supportive of a diagnosis of hyperparathyroidism. Masses, usually single, were identified with ultrasonography or scintigraphy in 12/14 horses in the thoracic inlet, mid-neck or around the thyroid region. Two horses had multiple masses. As in this case, surgical treatment was elected in 10 horses with single masses and the hypercalcemia normalized after surgery in 5/6 cases with a confirmed histologic diagnosis of parathyroid adenoma. In the remaining 4 surgical cases, histologic results on the removed tissue were reported as normal tissue for the region (thyroid, parathyroid, connective tissue) with one thyroid adenoma (one could speculate this was a parathyroid and not thyroid adenoma, given the hypercalcemia) and one branchial cyst. Common post-operative complications included hypocalcemia, ongoing or new hypophosphatemia, and kidney injury.

Hypercalcemia is an uncommon abnormality in chemistry panels from horses. In small animals, the acryonym of HARDIONS is used to help people remember differential diagnoses for hypercalcemia – H: Hyperparathyroidism, A: Addison’s disease, R: Renal disease, D: Vitamin D toxicosis/excess, I: Idiopathic, O: Osteolytic lesions, N: Neoplasia, and S: Spurious. However, of these conditions, renal disease would be the top differential diagnosis for a horse with hypercalcemia and hypophosphatemia,^2,11 followed by hyperparathyroidism, and hypercalcemia of malignancy, with more rare disorders, such as vitamin D toxicosis (e.g. ingestion of vitamin D-containing plants like Cestrum)¹² and idiopathic granulomatous disease¹³ (the latter may be due to excess vitamin D production¹⁴),  being further down the list. Various neoplasms have been associated with paraneoplastic hypercalcemia in horses, including gastric squamous cell carcinomas³ and multiple myeloma^15,16 with case reports or small case series of a vulvar squamous cell carcinoma,¹⁷ ameloblastoma,¹⁸ lymphoma,¹⁹ a mesenchymal tumor of presumptive ovarian origin,²⁰ and other carcinomas.^21,22 With these neoplasms, paraneoplastic production of PTHrP is thought to be the main mechanism for the hypercalcemia,^16,18 although vitamin D production could be contributing as well. Hypercalcemia and hypophosphatemia is more commonly seen in horses with chronic renal disease² but can be observed in animals with acute kidney injury.²³ These biochemical changes are attributed to the major role of the kidney in excreting the large amounts of calcium normally ingested by horses in their diet. The mechanism for hypophosphatemia in equine renal disease is less clear, but may be due to failure of proximal renal tubular absorption of filtered phosphate or secretion of phosphatonins. With hyperparathyroidism, PTH from the chief cells induces hypercalcemia via promoting osteolysis (via PTH stimulating osteoblasts to secrete factors which stimulate osteoclastogenesis and osteoclast activation) and resorption of calcium in the loop of Henle, distal tubules and collecting duct of the kidney. PTH also promotes the conversion of vitamin D to its active form in the kidney via activating the enzyme 1-α hydroxylase (the latter does not occur in horses, because they lack  1-α hydroxylase). The hypophosphatemia is due to PTH-mediated inhibition of proximal renal tubular resorption of filtered phosphate. Despite the marked hypercalcemia, horses with primary hyperparathyroidism do not appear to have concurrent renal disease (perhaps because of the concurrent hypophosphatemia). It is likely in the horse of this report that the chronic kidney disease was a comorbidity versus a consequence of the hypercalcemia.

The cytologic features of the cells in the smears of the aspirate of the thoracic inlet mass in this horse were classic for an endocrine (or neuroendocrine) tumor, i.e. the cells were found in three-dimensional “packets” or nests, they were of medium size and uniform in appearance, and had indistinct boundaries. Another feature of aspirates from these tumors is “bare” or “naked” nuclei in a background of cytoplasm, because the epithelial cells tend to be fragile and rupture readily during smear preparation. However, naked nuclei are not invariably seen in smears of aspirates of these tumors, as illustrated in this case. Distinction between a parathyroid adenoma or carcinoma requires histologic evaluation of tissue architecture, assessing for vascular and tissue invasion and high mitotic activity, features that were lacking on histologic evaluation of this case. The location of the tumor in the thoracic inlet is aberrant but thyroid and parathyroid tissue can occur in ectopic locations from the usual location in the ventral upper to middle region of the neck to the mediastinum.²⁴

Author: T Stokol

Acknowledgements: Dr. Tran read out the histology report and Dr. Todd-Donato provided the scintigraphic image.

References

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