PA21 A rare case of diffuse infantile fibromatosis

Abstract

Fibromatoses are a heterogeneous collection of tumours of the skin and soft tissue with a large variability in presentation. We present a case of diffuse infantile fibromatosis in a 17-day-old infant boy. He was born at 40 + 7 weeks gestation with a vacuum-assisted delivery following an uncomplicated pregnancy. On physical examination, there were two well-circumscribed circular plaques that coalesced centrally on his left elbow. The plaque measured 2 × 1 cm and was firm, nontethered and nontender. Full skin examination was otherwise normal. Histopathological examination showed hyperkeratotic skin with an attenuated epidermis overlying a dermally based bland-appearing spindle cell proliferation. The lesion was infiltrative throughout the dermis and was arranged in short fascicles and interdigitated with subcutaneous fat, which was extensively trapped by spindle cells. There was no significant atypia and mitotic activity was identifiable but not brisk. Immunohistochemical stains showed strong and diffuse reactivity for smooth muscle actin. Infantile fibromatosis is the childhood counterpart of musculoaponeurotic fibromatosis. It is a benign mesenchymal tumour of infancy and early childhood with diffuse proliferations of oval-to-spindled fibroblasts that infiltrate regional structures. There is a male predominance and in most cases is noted within the first 8 years of life. Clinically, it presents as an asymptomatic, white-yellowish mass that is nontethered to the overlying skin. Although any site may be involved, the majority of cases affect the upper limbs, head and neck region. Histological findings include a wide morphological spectrum. In infants, small oval cells appearing between primitive mesenchymal cells and fibroblasts are in a myxoid background. The cells are loosely arranged and infiltrate skeletal muscle and other regional structures. Peripheral lymphocytic inflammation blends with more cellular proliferation composed of spindle-shaped fibroblasts arranged in bundles and fascicles. There is no well-documented immunohistochemical profile. An important differential diagnosis to exclude is congenital/infantile fibrosarcoma, which, characteristically, has high cellularity, uniform herringbone pattern, high mitotic rate, and zones of haemorrhage and necrosis. In terms of management, as there is a high likelihood of local recurrence in 65% of cases, evidence supports complete excision with documented margins. Where the lesion is diffusely infiltrative, a partial resection to maintain the best postoperative function is recommended.