Radiological manifestations of chondromyxoid fibroma in the zygoma: A case report and literature review

Chondromyxoid fibroma (CMF) is a rare benign bone tumor of cartilaginous origin, with an extremely rare craniofacial occurrence. Considering its rarity, craniofacial CMF presents a diagnostic challenge for radiologists. To our knowledge, only seven cases of zygomatic CMF have been described in the literature, only one of which was in the paediatric age group. Furthermore, none of the currently reported cases include MRI findings of zygomatic CMF. Here, we present a paediatric case of CMF of the zygoma with a comprehensive literature review of the reported cases, focusing on their radiological features and its differential diagnosis.


INTRODUCTION
Chondromyxoid fibroma (CMF) is a rare, benign bone tumour of cartilaginous origin which accounts for <1% of primary bone tumours. 1 CMF was first described by Jaffe and Lichtenstein in 1948 as a bony tumour with a benign clinical course that can be commonly mistaken as chondrosarcoma. 2 CMF most frequently affects the proximal tibia and the distal femur and usually occurs in the second and third decades of life. Craniofacial occurrence of the CMF is extremely rare, reported to account for 5.4% of total CMF cases. 3 Radiological manifestations of CMF are nonspecific and considering its rarity, craniofacial CMF presents a diagnostic challenge for radiologists. To our knowledge, only seven cases of zygomatic CMF have been described in the literature, and none of these have presented the MRI findings of the zygomatic CMF. Also, only one case has been reported in the paediatric age group. Here, we present a paediatric case of CMF of the zygoma, focusing on the radiological features of CMF and its differential diagnosis. To our knowledge, this is the first report that describes the MRI findings of CMF of the zygoma and focuses on radiological findings of this rare presentation.

CASE PRESENTATION
Clinical presentation An 8-year-old girl presented to her paediatrician complaining of atraumatic 'bump' on her right cheek for two weeks. Examination revealed a non-tender firm nodule on the right zygomatic arch without cutaneous abnormalities.

Investigation
Plain facial bone radiographs showed a sclerotic lesion with central lucency in the right zygomatic bone (Figure 1a). A subsequent non-contrast maxillofacial computerized tomography (CT) revealed a well-defined, round, expansile, soft-tissue density lesion arising from the right zygomatic bone, measuring 1.7 × 1.6 × 1.6 cm. The lesion was surrounded by variably thinned cortex with a narrow zone of transition to normal bone. Scalloping of the zygoma and bulging of the expanded zygoma into the orbit were seen. There was no evidence of erosion or invasion of the adjacent soft tissues (Figure 1b, c and d).
To further characterize the lesion, facial magnetic resonance imaging (MRI) with and without contrast was performed and showed, compared to adjacent muscle, a homogeneously hypointense to isointense expansile, lobulated lesion with distinct borders on T1 sequence (Figure 2a). On the post-contrast fat-suppressed T1 sequence, the lesion showed avid enhancement with a curvilinear focus of non-enhancement centrally ( Figure 2b). Adjacent periosteal thickening and enhancement in the lateral orbital wall, as well as along the anterior maxilla were seen (Figure 2b, white arrow). The lesion showed intermediate signal on short-TI inversion recovery (STIR) with a curvilinear focus of hyperintense signal centrally (Figure 2c). A focal area of hyperintense signal in STIR sequence at the orbital process of the zygoma and anterior aspect of the lateral orbital wall likely represent bone marrow oedema (Figure 2d, white arrow).

SUMMARY
Chondromyxoid fibroma (CMF) is a rare benign bone tumor of cartilaginous origin, with an extremely rare craniofacial occurrence. Considering its rarity, craniofacial CMF presents a diagnostic challenge for radiologists. To our knowledge, only seven cases of zygomatic CMF have been described in the literature, only one of which was in the paediatric age group. Furthermore, none of the currently reported cases include MRI findings of zygomatic CMF. Here, we present a paediatric case of CMF of the zygoma with a comprehensive literature review of the reported cases, focusing on their radiological features and its differential diagnosis.

LITERATURE REVIEW AND DISCUSSION
The occurrence of isolated CMF in the zygoma is extremely rare. A literature review was conducted in PubMed and Scopus, using the combination of the following key terms: chondromyxoid fibroma and zygoma or skull. We identified seven published cases of CMF arising from the zygoma summarized in Table 1. [4][5][6][7][8][9][10] This summary includes patient demographics and clinical presentation, lesion location and radiological findings, as well as surgical management, complications and evidence of lesion recurrence after treatment.
The median age of reported patients with CMF of the zygoma, including our patient was 41 years old, ranging from 3.5 to 68 years old. Four patients were female, and three patients were male. CMF affected the left side in five patients and right side in two patients. The lesions ranged in maximal dimension from 1.7 to 3 cm, with the majority of lesions in the case reports being detected at 3 cm (median lesion size 3 cm). Patients presented with various clinical presentations, including painless swelling in four cases and painful swelling in three others.
In part due to heterogeneous imaging findings and its rarity in nonaxial bones, CMF is often misdiagnosed. Zillmer et al reported a case series of 36 patients with CMF: at initial presentation, 67% of patients received the correct diagnosis, 22% of the patients were misdiagnosed, and 11% of the patients had no diagnosis. 11 (when compared to adjacent muscle). Post-contrast images showed peripheral enhancement with a central non-enhancing portion. 14 While these features were identified mostly in CMFs found in long bones, most of the same features are identified in our case of CMF in the zygoma as well. Differential diagnosis of CMF and their radiological manifestations are summarized in Table 2.
Although MRI can help further characterize the lesion and suggest CMF as a possible diagnosis, it is noteworthy that definite diagnosis cannot be made based on radiological features alone, and biopsy is necessary for definitive diagnosis. Histopathologically, CMF has been defined by the World Health Organization (WHO) as "a benign tumour characterized by lobulated areas of spindle-shaped or stellate cells with abundant myxoid or chondroid intercellular material separated by zones of more cellular tissue rich in spindle-shaped or round cells with varying number of multinucleated giant cells of different sizes. " 15 Our case had the same histological features but with relatively greater mitosis. Surgical resection of the CMF is the treatment of choice. Surgical options vary from conservative curettage to en bloc resection. Recurrence rate following curettage of CMF tumour reported to vary from 20-80%, therefore en bloc resection is usually recommended, especially in children who are more prone to tumour recurrence. Radiation therapy is not recommended, since it has been reported to be associated with development of chondrosarcoma. 3 In our literature review of the seven described cases, six patients underwent en bloc resection and one patient underwent curettage. Recurrence did not occur in any of the patients who underwent en bloc resection. However, recurrence occurred in the patient who underwent curettage. Given the high recurrence rate, follow-up is recommended. We do recommend baseline MRI prior to the surgery and follow-up MRI to assess for subtle recurrence changes that might be overlooked by CT scan. Our protocol for these lesions includes the following sequences: T2 and STIR in axial and coronal planes, T1 fat saturated MRI precontrast and post-contrast in all planes.
One of the limitations of the presented literature review is that the focus of previous case reports was not radiological manifestation of CMF, which limits the evaluation of the radiological features of CMF of the zygoma on prior cases.

CONCLUSION
CMF is rare and can be encountered outside of the classical locations, including very rarely in the craniofacial bones. CMF has quite variable and non-specific imaging finding, but when found outside normal locations it appears to broadly follow imaging characteristics of CMF in more common locations. Definitive diagnosis is only possible via histopathology. Ongoing imaging surveillance, ideally with MRI, will be helpful due to the locally aggressive nature of the disease and high likelihood of recurrence. Adding more cases to the literature will enhance radiologists' awareness of the possibility of this pathology when presenting in uncommon locations.

LEARNING POINTS
• Chondromyxoid fibroma (CMF) in the facial bones is rare but when reported has similar imaging characteristics to CMF in the more common locations such as the long bones. • The major roles of imaging in CMF are to determine lesion extension into adjacent structures, to evaluate for residual disease post resection or curettage, and to monitor for disease recurrence. Definitive diagnosis is ultimately accomplished by histopathology. • CMF is a benign bone tumour, but can have aggressive behaviour with high recurrence rate. En bloc resection is favoured over curettage to decrease likelihood of recurrence.

FUNDING
Department of Radiology, University of Texas Medical Branch, Galveston, TX, USA.