26 October 2006 – Case of the Week #63
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We thank Dr. Sharon Bihlmeyer, University of Vermont, Burlington, Vermont (USA), for contributing this case. We invite you to contribute a Case of the Week by sending an email to NPernick@PathologyOutlines.com with microscopic images (any size, we will shrink if necessary) in JPG or GIF format, a short clinical history, your diagnosis and any other images (gross, immunostains, EM, etc.) that may be helpful or interesting. We will write the discussion (unless you want to), list you as the contributor, and send you a check for $35 (US) for your time after we send out the case. Please only send cases with a definitive diagnosis.
Case of the Week #63
A 53 year old man presented with a 4 month history of a protuberant and painful left anterior pretribial mass. Eleven years previously, he had an excision of a similar mass in the same location. Metabolic laboratory studies were within normal limits, including a phosphorus level of 4.4 mg/dL. An excisional biopsy was performed.
What is your diagnosis?
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Phosphaturic Mesenchymal Tumor
Phosphaturic Mesenchymal tumor is extremely rare. It affects women slightly more commonly than men, with a median age 53 years, but a wide range of 9-80 years.
This tumor often produce oncogenic osteomalacia, characterized by low serum phosphate, reduced tubular reabsorption of phosphate and a low or inappropriately normal 1,25 dihydroxy Vitamin D. In children, it may cause rickets (Pediatr Dev Pathol 2000;3:61).
These tumors are usually benign, and complete excision typically causes dramatic reversal of the endocrine signs and symptoms (AJSP 1989;13:588)
Grossly, the tumors may be up to 14 cm, and arise in soft tissue and bone. They are hypocellular, composed of bland spindled cells with small nuclei and indistinct nucleoli, as well as osteoclast-like giant cells. The stroma exhibits myxoid change, a distinctive “grungy” calcified matrix, metaplastic bone and fat. There may be hemangiopericytoma-like vessels, microcysts and hemorrhage. The tumors are infiltrative, but there is typically no/rare mitotic activity, and no atypia. Cases are considered malignant if they have nuclear atypia, high cellularity and 5+ mitotic figures/10 HPF.
Immunohistochemistry is not routinely used. However, the tumor cells are immunoreactive for fibroblast growth factor-23 (AJSP 2004;28:1) and dentin matrix protein 1 (Mod Path 2004;17:573), which are thought to mediate their metabolic effects, and may be useful in difficult cases.
The differential diagnosis includes other mesenchymal tumors, such as hemangiopericytoma, osteosarcoma and giant cell tumor.
This case showed a normal phosphorus, which is not typical, and some mitotic figures. However, the diagnosis was confirmed by Dr. Christopher Fletcher at Harvard.
Nat Pernick, M.D., President
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