Aneurysmal bone cyst (ABC), a term now regarded as a misnomer, is a non-neoplastic bony lesion that consists of cystic cavernous cavities without lining endothelium filled with blood and that occurs commonly in teenagers or young adults. ABCs occur mostly in long bones and the spinal column, and craniofacial involvement is a very rare occurrence. ABCs are either primary or secondary; secondary ABCs develop in preexisting bony pathologies primarily including giant cell tumor, osteoblastoma, angioma and chondroblastoma and less commonly including fibrous dysplasia (FD), ossifying fibroma and osteosarcoma.

FD is also a rare pathology that represents 2.5% of all bone tumors and 7% of benign bone tumors [1] and occurs mostly in the craniofacial skeleton of adolescent and young adult patients. FDs are benign non-neoplastic lesions commonly presenting with craniofacial disfigurement or compressive cranial neuropathies such as vision loss. A few cases have been documented with oculomotor palsy due to involvements of the neural foramina at the base of the skull. Most FDs are self-limiting, with stabilization after puberty.

Reports of secondary ABC occurring in craniofacial FDs are extremely rare in the literature, accounting for less than 20 cases. Patients usually present with a rapidly enlarging, soft, painful mass in the preexisting FDs, for which treatment is a curative resection of the ABC with a conservative resection of the FD or a conservative treatment that includes percutaneous sclerosing therapy. The case review presented here was conducted with the approval of the Institutional Review Board (KUH10700028).

A 25-year-old female, with no significant medical or familial history, presented with rapidly growing right forehead mass beginning 2 months prior to admission. She had been followed for a diffuse forehead bulging since early in her second decade of life at a neurosurgical center in Kazakhstan. She underwent a magnetic resonance imaging (MRI) scan to assess the soft bulging in her right frontal region 2 months prior to admission. The mass grew rapidly and she began to feel pain when the mass reached the size of a tennis ball. The patient was transferred to our hospital for definite diagnosis and treatment. The physical examination was normal except for a bulging, soft mass measuring 5.5 cm on the right forehead. She was free of neurological symptoms, and routine laboratory examinations were within normal limits. A computed tomography (CT) scan revealed characteristic findings of FD with a heterogeneous ground-glass appearance of both the frontal, sphenoid, and ethmoid bones, as well as a discrete cystic mass in the right frontal bone (Fig. 1). MRI also showed findings of FD in the bilateral craniofacial bone and indicated multi-septated fluid-fluid levels suggestive of ABC (Fig. 2). We performed a preoperative ophthalmological evaluation, in which visual acuity was 1.0 in both eyes, and there were no visual field, fundus, or eye movement abnormalities. For evaluation of the vascularity of the cystic mass and embolization of feeder arteries prior to surgery, a trans-femoral cerebral angiography was done. The angiography demonstrated moderate vascularity along the rim of the cystic mass and embolization via the right middle meningeal artery and right superficial temporal artery was performed. After embolization, the vascularity of the mass was markedly reduced (Fig. 3).

The patient underwent right frontal craniectomy with gross total resection of the brownish soft-tissue mass with multiple septated, brownish fluids and sediment-filled, cystic cavities followed by simple cranioplasty. The outer table thinned out as fine as paper, and the inner table was nearly absent. The histopathological findings of the skull lesion indicated FD with degenerative changes, forming typical ABC-like areas (Fig. 4). There was no evidence of malignancy in the surgical specimen. The FD was partially removed around the ABC, and after surgery (Fig. 5), the patient's cranio-facial deformity was dramatically improved (Fig. 6). No recurrence of the ABC nor progression of the FD was found at 6 months postoperatively.

FD, Langhans cell histiocytosis (Histiocytosis X), and ABC are the three most common juvenile craniofacial bony lesions. Among these, FD is the most common proliferating craniofacial bony lesion. In FD, abnormal growth of fibroblasts replaces medullary bone with fibrocellular tissue [23]. The biologic behavior of FD is known to be self-limiting, with no progression after puberty [456]. According to reported cases, in FD patient, presented with painful swelling over a short period of time, which may clinically represent the very unusual sarcomatous transformation of FD [7].

ABC is neither an aneurysm nor a true cyst but is made of channels containing flowing blood. The pathogenetic mechanism of ABC remains unclear. Other report suggested that it may arise from abnormal hemodynamic conditions causing an increase in venous pressure, which then leads to hemorrhage[8]. ABC may occur as either a primary (70%) or a secondary (30%) bone cyst [9]. Diseases known for predisposing the patient to lesions of ABC include osteoclastoma, osteosarcoma, osteoblastoma, and hemangioma [10]. ABC arising from FD is extremely rare, especially in the craniofacial bone. This lesion was first reported by Branch et al. [10] in 1986, and fewer than twenty cases have been reported since then [39101112131415161718192021]. We summarized the case reports of ABC with FD in the craniofacial bone in a literature search from PubMed in Table 1. In our patient and in seven of the reported cases, a growing craniofacial mass was a presenting symptom. The rapidly distended contour of a secondary ABC may create suspicion of a malignant change in the pre-existing, benign bone tumor. The incidence of malignant transformation in FD has been reported to be between 0.5% and 4% [7]. When a patient presents with a rapid growth of the skull, the possibility of malignant change of the benign bone pathology and the development of secondary ABC should both be considered, with the latter given enhanced consideration in younger patients. Screening through imaging may provide useful clues as to the correct preoperative diagnosis. The presence of fluid-fluid levels on a CT scan or MRI is a typical feature of ABCs, although it has also been reported in many other osseous lesions containing hemorrhage [22]. When fluid-fluid levels are demonstrated in FD, this outcome may be associated with simple cystic degeneration or sarcomatous transformation of FD [23].

In a literature review, the treatment of choice for an ABC with FD is total resection of the ABC and cranioplasty. Close follow up is mandatory due to the likelihood of recurrence of the lesion. Where possible, embolization of the feeding artery has been reported to reduce bleeding during surgery and enable maximal resection, as seen in our case [15].

We are reporting on a rapidly developing ABC in a pre-existing FD that presented with painful swelling and disfigurement in the right frontal region. Because subtotal resection is associated with a 50% recurrence rate, the patient underwent aggressive total resection of the ABC with partial removal of the FD followed by cranioplasty [24252627]. At the six-month postoperative follow up with CT and MRI images, there was no recurrence.

In conclusion, if a patient complains of a rapidly growing, soft mass with pain and pre-existing FD, secondary ABC development should be considered. In our case, preoperative angiography with embolization of the feeders was helpful in reducing intra-operative bleeding during an aggressive total resection.