Case 15
Clinical history: Slow growing thigh mass in a 30 year old man. 

Diagnosis: “Superficial CD34 positive fibroblastic tumor”
 
Discussion:

This is a very rare soft tissue tumor with unique characteristics: nuclear pleomorphism, paradoxically rare mitotic activity, diffuse CD34 expression, and frequent keratin expression. They usually present in middle-aged adults as a slow-growing skin or subcutaneous mass of less than 5 cm (range: 1.5 – 10 cm), most commonly in the lower extremity. The clinical and microscopic features of the current case fit very well for this entity. Per the 5th edition WHO Bone and Soft Tissue Tumors book (published in 2020), the prognosis of this tumor is excellent; out of the 30 cases with clinical follow up reported in the literature so far, there were no local recurrences, no distant metastases, and no death from disease; 1 patient had a regional lymph node metastasis 7 years after presentation.     
 

References:
·       Yamaga et al. Oncol Lett. 2017 Sep; 14(3): 3395–3400.
·       Carter et al. Mod Pathol. 2014 Feb;27(2):294-302.
·       Hendry et al. Pathology – Journal of the RCPA. 2015 Aug;47(5):479-82
·       WHO Classification of Tumours Editorial Board. Soft Tissue and Bone Tumours. 5th ed. Lyon, France: IARC Press, 2020.
 
 
Illustration:
15-1.    Sections show a tumor filling the dermis and extending into the subcutis.
15-2.    It is composed of spindled to epithelioid cells with abundant eosinophilic often granular cytoplasm arranged into hypercellular sheets and fascicles.
15-3.    A subset of the tumor cells shows prominent nuclear pleomorphism. Despite the prominent nuclear atypia, mitotic figures are very rare (<1/50 HPF).
15-4.    Xanthomatous foamy cells are a common finding. Focal nuclei show cytoplasmic nuclear pseudoinclusions (bottom left).
15-5.    On immunostains, the tumor cells are diffusely and strongly positive for CD34. There was also focal weak staining for pancytokeratin AE1/AE3 (not shown). The tumor cells were negative for S100 protein, calponin, SMA, desmin, and CD31. Nuclear INI-1 (SMARCB1) expression was retained (normal).