Painless swelling in the right cheek (ECR 2011 Case of the Day)

Clinical History

A 66-year-old woman with previous thyroidectomy for follicular carcinoma of the thyroid gland presented with a painless swelling in the right cheek, gradually growing in size over the previous two months. Physical examination revealed a palpable mass with no associated skin changes or superficial lesions. No neck nodes were palpable.

Imaging Findings

An orthopantomography showed a large expansive lesion in the right side of the mandible with lytic characteristics.
A head and neck CT without contrast medium (Fig. 1) showed a 4 x 5.5 cm mass lesion situated in the right masticatory space, originating from the posterior mandibular ramus. The mass was cystic with a unilocular pattern and scalloped borders associated with thinning and defects of the bone cortex.
An MRI with contrast medium (Fig. 2) was performed in order to better evaluate the morphology of the lesion. The lesion showed a high signal intensity in T2 weighted sequences and low signal intensity in T1 weighted sequences, due to fluid content. It had a cyst-like appearance with an enhancing peripheral rim showing an intramural enhancing soft tissue nodule on the medial aspect of the cystic wall.

Discussion

Ameloblastoma is an odontogenic tumour thought to originate from central mandibular or maxillary odontogenic epithelium [1, 2]. It represents about 1% of oral tumours and is the most common odontogenic tumour (35%). Its peak incidence is in the 3rd-4th decades, while other cystic lesions of the mandible and maxilla (e.g. aneurismal bone cyst) are more common during childhood [3]. Similar to dentigerous cysts and odontogenic keratocysts it most commonly occurs in the mandible (80%), particulary in the posterior mandibular ramus [1, 4]. There are several histological sub-types, however, radiologically we are not able to distinguish them all [2, 5].
The tumour is often clinically asymptomatic and slow-growing. In most cases it is histologically benign, but can be locally aggressive with increased risk of local recurrence if not adequately treated (estimated recurrence rate 33%). Late signs include loose teeth and bleeding. It is often associated with an unerupted 3rd molar and in most cases >2 cm in size at diagnosis. Malignant transformation is rare (1%).
Radiologically we can classify them in 3 groups: peripheral, unicystic and multicystic. The multicystic type is the most common (86%), with various cysts grouped together or separated by osseous septa [1, 4]. The most relevant diagnostic pattern is a ‘bubbly’, multilocular, mixed cystic-solid mass. In these multicystic cases it is easier to distinguish between ameloblastoma and odontogenic cystic abnormalities (example dentigerous cysts and odontogenic keratocysts) which often present as unilocular cystic lesions. In cases of unicystic ameloblastomas this distinction is harder. Contrast-enhanced CT (and MRI) could show either small contrast-enhancing nodular lesions within the cystic wall or larger lesions with surrounding soft-tissue enhancement mixed with cystic areas. These enhancing nodules are an important feature for differential diagnosis, particularly with dentigerous cysts when also associated with an unerupted 3rd molar. MRI is indicated to assess involvement of neurovascular structures and extraosseous components, even though they are rare. T1-weighted images show a mixed signal intensity or typical low signal intensity. T2-weigthed images usually show high signal intensity due to fluid content. MRI is useful for differential diagnosis with odontogenic keratocysts since fluid content of odontogenic keratocysts is more proteinacious and viscous, and therefore does not show such a high signal intensity [6]. Differential diagnosis between larger dentigerous cysts, odontogenic keratocysts and unicystic ameloblastoma may be difficult, and sometimes can only be confirmed histologicaly [6].
Complete surgical excision is indicated for small lesions while en bloc removal is recommended for larger ones [1, 7].

Differential Diagnosis List

Final Diagnosis

Ameloblastoma

URL:	https://www.eurorad.org/case/9905
DOI:	10.1594/EURORAD/CASE.9905
ISSN:	1563-4086