[A case of solitary bone cyst in the skull increasing in size after trivial head injury].

A 12-year-old boy had been known to have a small swelling in the left high vertex for several years. After a trivial head hit to the site of the swelling, the swelling enlarged gradually. A bone window CT scan showed a lesion having bubble-like lytic change in the left parietal bone. Similar changes, but small, were able to be pointed out in a CT scan taken seven years previously. In the following 13 months CT scans eventually revealed sequential increases to 3.5 cm in diameter. Surgical exploratory resection of the mass was performed. Intraoperatively, partial destruction of the outer skull table and a simple cyst with serous yellowish brown colored fluid were identified. There was no finding adherent to the diploic structure. The bone defect after excision was reconstructed by using a titanium plate. The patient was followed up for 2 years after the surgery. Bone window CT showed bony development of normal appearance. Histological examination showed the cyst wall consisted of fibrous connective tissue but there were neither epithelial nor endothelial cells. The histopathological diagnosis of SBC was most likely. SBC is relatively common in long bones, but rarely in flat bones. Only several cases of the SBC of cranial bone have been reported. Although a craniectomy for total excision followed by cranioplasty by resin was common, in cases of children, cyst removal with titanium plate application would be an alterative. SBC increasing in size after head injury is extremely rare, but clinicians may need to be aware of cystic skull bone tumors increasing in size after head injury.