Two dogs presented with nodular masses on the head and scapula. The masses recurred after excision and were submitted for histopathological examination. Macroscopically, hemorrhage and necrosis were apparent on the cut surface. Microscopically, the lesions were poorly demarcated and incorporated subcutaneous fat and adjacent skeletal muscle fibers. Fibrocytes and fibroblasts were admixed and haphazardly arranged. Both were diagnosed with nodular fasciitis (NF). NF has been rarely reported and called as a ‘pseudosarcomatous’ lesion because of its infiltrative growth and cellular pleomorphism. The present report describes unusual cases of recurrent NF in two young dogs based on histopathologic and clinical features.

Nodular fasciitis is defined by the World Health Organization as a benign proliferative fibroblastic lesion. It was first described in 1955 by Konwaler et al. and also known as “nodular pseudosarcomatous fasciitis” and “infiltrative fasciitis”, which reflect the lesion’s rapid growth, dense cellularity, alarming mitotoic activity and lack of circumscription. Here, we report a case of nodular fasciitis, which appeared as subcutaneous mass of right perizygoma area in 40-year-old Korean female. She had no history of trauma or cosmetic surgery related with the lesion. She disclosed that the mass grew rapidly for last 2 months. Computed tomography showed relatively well-defined spherical mass with 1.2 cm in diameter. Under local anesthesia the mass was resected by intraoral approach. The histological examination revealed a relatively poor-dermacated mass with high cellularity, which was composed with infiltrative spindle-shaped cells without accompanying inflammatory reactions. The tumor cells were partially positive for α-smooth muscle actin and β-catenin, but negative for cytokeratin AE1/3, S-100, desmin, and CD34. Therefore, the lesion was pathologically diagnosed as nodular fasciitis. Although nodular fasciitis is known to pseudosarcomatous benign reactive lesion, the clinician need to follow up the patient periodically. The reasons are as follows; Nodular fasciitis may accompany nuclear accumulation of β-catenin, which imply neoplastic process rather than reactive process of the lesion. Moreover, it has many histological similarities with inflammatory myofibroblastic tumor which have malignant transformation tendency. Further research for mesenchymal tumor could help our understanding for the disease entity of nodular fasciitis

The present case occurred in the cervical area of 51 years old male, who was generally healthy but recently complained of cervical swelling with mild pain. The tumor recently grew rapidly for 5 months. He was referred from local hospital in the diagnosis of metastatic tumor in cervical lymph. In the biopsy procedure, the tumor mass was ill-defined and not associated with cervical lymph node. The microsection showed a highly proliferative fibrous tissue infiltrative growth into the periphery. The spindle tumor cells were stained red in Masson trichrome stain, and strongly positive for PCNA and β-catenine, and partly positive for α-smooth muscle actin (α-SMA), but negative for S-100 and desmin. The differential diagnosis in primary biopsy examination was made as leiomyosarcoma, malignant schwannoma, and mesenchymal sarcoma. Under the diagnosis of sarcoma the patient was operated for tumor resection including cervical lymph nodes. The tumor mass was tightly attached on the lower border of left mandibular body. After the surgical operation the tumor tissue was more investigated with further immunohistochemical stainings, and discussed with several pathologists in Gangneung area. Finally the tumor was turned out to be a nodular fasciitis with pseudosarcomatous proliferation, and also confirmed that the pseudosarcomatous spindle cells belonged to the myofibroblasts originated from the fibrovascular fascial tissue. The present study demonstrated a rare case of nodular fasciitis, which should be differentially diagnosed from the malignant head and neck tumors.