Abstract

Inflammatory myofibroblastic tumor (IMT) is a rare spindle cell neoplasm of borderline malignancy that typically arises in the deep soft tissues of the abdominopelvic region, followed the lung, heart, and genitounary tract. Although IMT is most commonly seen in children and young adults, rare cases have been reported as congenital tumors. Herein, we report a case of congenital IMT in a 38-week male infant who presented with an 8.5 cm subcutaneous mass in the left axilla and upper back region. A biopsy revealed bland spindle cells without necrosis or atypical mitoses, with a mitotic count of less than 1/mm². Immunohistochemical studies showed that the lesional cells were variably positive for WT1, smooth muscle actin (SMA), and CD34, while they were negative for keratin, desmin, myogenin, and S100. CD45 highlighted scattered infiltrating lymphocytes, while CD34 highlighted the prominent vasculature. Initially, the overall morphologic features and clinical presentation suggested a fibrous hamartoma of infancy (FHI), and complete excision was recommended. However, resection of the mass revealed large areas of fascicles of myofibroblastic cells with an associated inflammatory cell infiltrate and hyalinized collagen. These cells were strongly positive for anaplastic lymphoma kinase (ALK) protein, expressed SMA in a myofibroblastic pattern, and were negative for caldesmon. No areas of mature adipose tissue were identified. Next-generation sequencing evidenced an ATIC::ALK fusion, leading to a final diagnosis of IMT. Our case highlights the importance of considering IMT in the differential diagnosis of congenital fibroblastic tumors.