Aggressive paediatric myofibromatosis is an autosomal recessive disease characterized by fibroblastic proliferation from cells originated in muscle-aponeurotic tissue. Its etiology is unknown, and the average age of the reported cases is 7 years old. The tumor exhibits rapid painless growth and appears attached to muscle tissue and/or bone. The treatment of choice is conservative surgical excision despite of early relapses has been reported.

A 2-year-old patient, with no morbid history, presented with a large swelling in the left submandibular region, firm, neither defined limits nor inflammatory characteristics. Its size doubled 2 months after an incisional biopsy. CT images showed great compromise of the left mandibular body with expanded and thinned cortical bone. The MRI showed extension towards the pharynx. Histopathological findings were elongated fibroblastic and ovoid cells arranged in bundles and fascicles within fibromyxoid stroma, an image consistent with the diagnosis. The treatment consisted in a conservative exeresis of the tumor, preserving the jaw. Control 1 year after surgical removal shows no signs of relapse and the mandibular structure has been restored.

The large size of the lesion and bone involvement at such an early age evidenced a very aggressive lesion, however, supported by a previous biopsy, we performed a conservative treatment, which only caused the loss of a dental germ, impossible to take off from the intraosseous tumor. The control of this type of lesions requires a longer follow-up.