Residency Program - Case of the Month

Answer:

Fibrous hamartoma of infancy

Histological description:

Histological sections showed a subcutaneous poorly circumscribed soft tissue mass characterized by three distinct components forming organoid structures. The three components consisted of: 1) well-defined intersecting trabeculae of dense fibro-collagenous tissue composed of fibroblastic and myofibroblastic spindle cells with bland nuclei, separated by dense collagen; 2) islands of immature-appearing small, round to stellate, primitive mesenchymal cells with scant cytoplasm embedded in a myxoid matrix, and 3) mature fat interspersed among the other two components. The tumor cells were positive for vimentin and focally for CD34. All these morphological and immunohistochemistry findings were consistent with the diagnosis of fibrous hamartoma of infancy.

 Discussion:

Fibrous hamartoma of infancy (FHI) is a rare, benign, neoplasm with subcutaneous fibrous proliferations showing unique clinical and pathological features. These tumors are more common in boys and are usually seen within the first two years of life (1). About 15-25% of cases are present at birth (2) and there is no apparent familial or syndromic association. FHI is almost always a solitary lesion (3), not painful, and usually presents as a rapidly growing, freely movable mass in the subcutis or dermis. The tumor is most frequently located in the axilla, the upper limbs, upper trunk, neck, inguinal region, buttock, foot or external genital area. Involvement of the hand and wrist is rare (4).

Fibrous hamartomas of infancy are poorly circumscribed masses composed of gray-white fibrous tissue intermixed with islands of fat (5). The histological appearance is characterized by the presence of three distinct components: well-differentiated spindle cells in fibroblastic or myofibroblastic trabeculae having variable amounts of collagen deposition, mature adipose tissue that may be seen as a major component or only at the periphery, and immature cellular areas resembling primitive mesenchyme. In some cases, extensive sclerosis replaces much of the lesion, resembling neurofibroma or disorderly fibrosis. The immature areas and well-differentiated fibroblastic parts are positive for vimentin. Actin and rarely desmin are positive in spindle cell areas.

Fibrous hamartoma of infancy is a benign tumor and requires local excision (5, 6). Rarely, recurrence occurs, which is cured by reexcision (6). The most important differential diagnoses are infantile myofibromatosis and sarcomas of soft tissue (7).   It is important to distinguish FHI from other forms of fibrous proliferation of infancy because is a benign lesion curable by local resection. The diagnosis can usually be established by attention to the combination of clinical characteristics and specific histological patterns.

References:

1. Jebson PJ, Louis DS. Fibrous hamartoma of infancy in the hand: a case report. J Hand Surg Am. 1997;22:740–2.
2. Dickey GE, Sotelo-Avila C. Fibrous hamartoma of infancy: current review. Pediatr Dev Pathol. 1990; 2:236-243.
3. Sotelo-Avila C, Bale PM. Subdermal fibrous hamartoma of infancy: pathology of 40 cases and differential diagnosis. Pediatr Pathol 14: 39-52.
4. Song YS, Lee IS. Fibrous hamartoma of infancy in the hand: unusual location and MR imaging findings. Skeletal Radiol 2010
5. Rosai and Ackerman's Surgical Pathology, 9th edition, 2004
6. Pathology and Genetics of Tumours of Soft Tissue and Bone (IARC WHO Classification of Tumours), 2002
7. McGowan J 4th, Smith CD, Maize J Jr, Cook J.Giant fibrous hamartoma of infancy: A report of two cases and review of the literature. J Am Acad Dermatol. 2010 Jun 30