Purpose or Case Report: Prenatal screening ultrasound at 18 weeks gestation identified homogeneous enlargement of the distal left leg. There was relative increased blood flow by Doppler, but the bone lengths were symmetric. There were no cysts and no amniotic bands. MRI was performed at 21 weeks gestation for further characterization. MRI demonstrated a lobulated, infiltrative, asymmetric soft tissue signal intensity thickening extending from the knee to the dorsum of the foot. There was no fat content appreciable on T1 or T2 fat saturated images. Given these findings, a vascular soft tissue mass such as kaposiform hemangioendothelioma was the primary differential consideration, with other fibrous tumors considered less likely. Follow up ultrasounds demonstrated commensurate growth of the lesion with the child.

At scheduled cesarean delivery, the otherwise healthy female infant had a leg mass that measured 14 cm, extending from the knee to the dorsum of the foot, as seen on prenatal imaging. The skin overlying the lesion was thick and redundant with hypertrichosis. Postnatal MRI was performed on day 2, unexpectedly showing extensive fat signal, along with edema signal and enhancement of the muscle, fat, and thickened skin. There was no abnormal vascularity and no focal mass. There was mild tibial bowing. Incisional biopsy was performed on day 6, with pathologic diagnosis of spindle cell tumor suggestive of fibrous hamartoma of infancy (FHI).

This apparent case of FHI is an atypical presentation, with interesting imaging implications. FHI is usually smaller (3-5 cm), more common in males (2.4:1), and only rarely involves the lower extremities distal to the knee (0.5% of cases). Only 23% of FHI are present at birth. To our knowledge, there are no case reports describing the fetal MRI appearance of this entity. Hamartomatous lesions and macrodystrophia lipomatosa had been excluded from the differential diagnosis due to the lack of fat signal intensity on fetal MRI. However, the fetus was imaged in early second trimester and normal subcutaneous fat is usually not visible on MRI until late in the second trimester. This case raises questions regarding the timing of appearance of fat in this lesion. As a result, we recommend that fat containing lesions not be excluded on the basis of absence of fat in early fetal MRI. Repeat MR imaging of musculoskeletal tumors in the third trimester to re-assess content may be useful.
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