These are superficial tumors that are usually painless and freely movable, but may also be attached to the underlying fascia. Involvement of the skeletal muscle is rare. They are most commonly located in the axilla, followed by the upper arm and shoulder, thigh and groin, back, and forearm.
Fibrous hamartomas of infancy are poorly circumscribed masses that consist of a mixture of fibrocollagenous tissue, immature -appearing primitive mesenchymal cells, and mature fat.
Ultrasound is usually the first modality used for evaluation of these lesions, but I can't find a reference for the appearance of these lesions on sonography. MR characteristics reflect the different components discussed above: The fibrocollagenous component is low signal intensity on both T1- and T2-weighted images and the mature fat component is hight signal intensity on both T1- and T2-weighted images. Organized trabeculae of fibrous tissue interspersed with fat should suggest the diagnosis.
When fat is identified in these lesions, the differential diagnosis in infants should include lipoma, lipoblastoma, and an involuting hemangioma.
The images above are from a 1-year-old boy with a right forearm mass (pink arrow). The lateral radiograph reveals an ill-defined mass along the volar surface of the proximal forearm. Ultrasound reveals a heterogeneous, predominantly hyperechoic lesion without significant vascularity on Doppler. MRI reveals a lobular, ill-defined mass that is predominantly isointesnse to skeletal muscle on the T1-weighted image and hyperintense on the T2-weighted image, but has internal areas of fat signal. Thin fibrous sepata are seen on the T1-weighted image (black arrow). Mild enhancement is seen.
- Hashimoto H. Fibrous hamartoma of infancy. in Pathology and Genetics of Tumours of Soft Tissue and Bone. Fletcher CDM, Unni KK, Mertens F (eds). IARCPress Lyon, 2002. pp 58-59.
- Laffan EE, Ngan BY, Navarro OM. Pediatric soft-tissue tumors and pseudotumors: MR imaging features with pathologic correlation: part 2. Tumors of fibroblastic/myofibroblastic, so-called fibrohistiocytic, muscular, lymphomatous, neurogenic, hair matrix, and uncertain origin. Radiographics. 2009 Jul-Aug;29(4):e36.