Abstract: 【Abstract】 Objective To delineate clinical characteristics of infantile myofibroma/myofibro-matosis （IM）. Methods A retrospective analysis was conducted on histologically confirmed IM cases from Beijing Children's Hospital, Henan Children's Hospital, and Maternity and Child Health Care of Guangxi Zhuang Autonomous Region between August 2014 and July 2021. The clinical, pathological, imaging features, and outcomes were analyzed and summarized. Results A total of 44 IM patients were included, comprising 28 males （63.6%） and 16 females （36.4%）. Their ages at onset （M［Q1, Q3］） were 14.8 （4.7, 42.4） months, and 26 patients （59.1%） developed IM before the age of 1 year. Seven patients （15.9%） were initially diagnosed with IM. Skin involvement occurred in 42 patients （95.5%）, of whom 30 （71.4%） presented with a solitary lesion and 12 （28.6%） with multiple lesions. Skin lesions mainly manifested as painless, firm nodules or masses. The most commonly involved extracutaneous site was the skeletal system （21/44, 47.7%）. Histopathological examination of all 44 IM cases revealed a biphasic architecture pattern, characterized by the coexistence of two distinct morphologies or cell types within the tumor tissues （including spindle cell areas composed of fascicularly and densely arranged myofibroblasts, and primitive mesenchymal cell areas composed of small, round undifferentiated cells. Immunohistochemical study was performed in 42 cases; 40 （95.2%） were positive for smooth muscle actin, and 20 （47.6%） were positive for CD34. Genetic testing was conducted in 3 cases, and NOTCH mutations were identified in 2. Among the 44 patients, 30 patients （68.2%） underwent surgical excision, 5 patients （11.4%） received intralesional injections of triamcinolone acetonide, 1 patient （2.3%） received chemotherapy for intestinal involvement, and 8 patients were managed expectantly. During the follow-up of 49 （36, 60） months, lesions completely resolved in 42 cases （95.5%）, while 2 cases died of pulmonary infection following chemotherapy or postoperative airway compression. Conclusions IM predominantly affected infants and young children, with the skin and skeletal system being the most commonly involved sites. Skin lesions often manifested as firm nodules or masses, and histopathological examination was crucial for definitive diagnoses. Most IM cases exhibited favorable outcomes.

Key words: Myofibroma, Myofibromatosis, Infant, Clinical manifestations, Therapy, Prognosis