[Clinicopathologic and molecular characteristics of myxoid angiomatoid fibrous histiocytoma]

Objective: To investigate the molecular genetic and clinicopathologic characteristics, immunophenotypes, diagnostic and differential diagnostic features of myxoid angiomatoid fibrous histiocytoma (MAFH). Methods: Three cases of MAFH were collected from the archives of Zhejiang Provincial People's Hospital between January 2015 to August 2018. The clinical and radiologic features, histomorphology, immunohistochemistry, molecular genetics and prognosis were analyzed. Results: Patients consisted of 2 women and 1 man aged 37 years, 46 years, and 57 years, respectively. The clinical manifestations of 3 patients were presented as a painless, slowly-enlarged mass with a duration ranging of 2 weeks, 2 months and 50 years. These tumors were located at the deep somatic soft tissue of extremities or limbs (right hip, left forearm, left wrist, respectively) and 2 were preoperatively considered as ganglion cyst or giant cell tumor of tendon sheath by imaging examinations. The diameter of circumscribed mass lesion was ranged from 3.0 to 7.5 cm, which exhibited a gray white to tan and gelatinous cut surface. Extensive hemorrhage and cystic changes were observed in 2 cases. Under low magnification, all tumors showed a dense fibrous pseudo-capsule with a peritumoral lymphoplasmacytic cuff and a multi-nodular growth pattern. Blood-filled cystic spaces were observed in 2 tumors. The myxoid stroma occupied 60.0%, 80.0% and 90.0% area of the entire tumor, respectively. Within the myxoid areas, tumor cells were oval to stellate and arranged in cord-like, microcystic and reticular growth patterns. Transitions of myxoid tumor components to more solid areas with typical histology of angiomatoid fibrous histiocytoma (AFH) were observed at least focally in all the three cases. The tumor cells exhibited minimal atypia and scarce mitoses (1 to 2/50 HPF) without necrosis, and prominently focal intracytoplasmic vacuoles were identified in one case. The results of immunohistochemistry staining showed that, 2/3 cases focally expressed desmin, 2/3 focally expressed epithelial membrane antigen (EMA), and 1/3 focally expressed CD99. The positive index of Ki67 was approximately 1% to 5%. Fluorescence in situ hybridization analysis showed that EWSR1 gene rearrangement occurred in all of the three cases. During the period of follow-up, one case showed local recurrence at 15 months, one case showed postoperative recurrence at 24 months, and the recurrent tumor slowly grew for 120 months until the second resection, without recurrence at the following 2 months. The left case showed a disease-free survival at 32 months. Conclusions: MAFH is a rare subtype of AFH with a low-grade behavior and may lead to diagnostic confusions. Carefully searching for the typical AFH histomorphology and combining with EWSR1 gene rearrangement test can help to distinguish MAFH from other mimickers.