Deep fibrous histiocytomas are histologically benign fibrohistiocytic tumors usually found in the subcutaneous or deep soft tissues which can metastasize.

Epidemiology

Deep fibrous histiocytomas are rare tumors. They make up for less than 1% of all fibrohistiocytic tumors and occur over a wide age range. Men are slightly more often affected than women ^1,2.

Diagnosis

The diagnosis of deep benign fibrous histiocytoma is established by location and histological features ¹.

Diagnostic criteria

Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5^th edition) ¹:

• well-defined lesion arising in a subcutaneous or deep/visceral location
• mixed fascicular and whorled or storiform growth pattern
• monomorphic spindle or histiocytoid cell population
• branching hemangiopericytoma-like vessel pattern

Clinical presentation

The most common complaint is an indolent slowly-growing mass ¹.

Pathology

Deep fibrous histiocytomas are well-defined proliferations with a mixed fascicular and storiform cell pattern characterized by monomorphic histiocytoid or spindled cells with interposed branching vessels ^1-3.

Etiology

The etiology of deep fibrous histiocytomas is unknown ¹.

Location

Deep fibrous histiocytomas most frequently arise from the subcutaneous tissues of the extremities and the head and neck region, however, almost 10% are located in the deep visceral soft tissue areas like the retroperitoneum, the mediastinum or the pelvis ^1-3. Uncommon locations are within the muscles and visceral organs are a rare site of origin ¹.

Macroscopic appearance

Macroscopically deep fibrous histiocytomas are well-defined lesions sometimes adhering to the adjacent fascia or tendons ¹.

Microscopic appearance

The histological spectrum of deep fibrous histiocytomas includes the following features ^1-3:

• spindle cells or histiocytoid cells with eosinophilic cytoplasm and plump vesicular appearing nuclei
• storiform growth pattern
• no nuclear pleomorphism
• often hyalinized stroma and less frequently myxoid changes with staghorn pattern of vascularity
• occasional cystic change or hemorrhage

Immunophenotype

Immunohistochemistry stains will be positive for CD34 and smooth muscle actin in almost one-half of the cases. Deep fibrous histiocytomas usually do not express STAT6 ¹.

Radiographic features

On imaging deep fibrous histiocytomas are often described as oval, well-delineated tumors with definable borders with an intervening fat plane to the muscular fascia, if found in a subcutaneous location ^4,5.

Ultrasound

On ultrasound deep fibrous histiocytomas have been described as well-circumscribed and shown to be hypoechoic ⁴.

CT

CT appearance has been described as non-specific soft tissue mass with avid enhancement ⁵. 

MRI

Deep fibrous histiocytomas are reported to be mostly homogeneous masses that might show central vascularity, internal hemorrhage or necrosis and the following appearance ^5,6:

Signal characteristics

• T1: low intensity
• T2: high intensity
• T1 C+ (Gd): avid peripheral enhancement 

Treatment and prognosis

The treatment of deep fibrous histiocytomas usually is surgical excision. Local recurrence is observed in about one-fifth of the cases. There are occasional metastases reported in up to 5% ^1-3.

History and etymology

The lineage of fibrous histiocytomas has been initially described by the American pathologists Shirley L Kauffman and Arthur Purdy Stout in 1961 ^7,8.

In 1990 the British pathologist Christopher DM Fletcher published a series of 21 cases of deep benign fibrous histiocytomas ⁹ of which 5 were later reclassified as solitary fibrous tumors due to changes in classification criteria ¹⁰.

Differential diagnosis

Tumors or other conditions which can mimic deep fibrous histiocytomas are the following ^4,10:

• cutaneous benign fibrous histiocytomas
• undifferentiated pleomorphic sarcoma
• cutaneous leiomyosarcoma
• dermatofibrosarcoma protuberans
• tenosynovial giant cell tumor
• solitary fibrous tumor