A 67-year-old man presents with a fixed palpable mass in the left parotid region, with pain and trismus, without clinical signs of infection or a recent history of tooth extraction. After an ultrasound showing normal parotid gland and buccal fat pad, a CT scan and an MRI were requested.
Pre- and post-contrast-enhanced computed tomography (CT) of the maxillo-facial region revealed a large multi-loculated tumor centered in the masticator space, involving the angle and ramus of the mandible, and protruding into the masticator muscles. The lesion showed soft-tissue and cystic densities, leading to heterogeneous enhancement after contrast administration. The lesion replaced part of the angle and ramus of the mandible, extending up into the mandibular condyle and abutting the zygomatic arch, with some linear, peripheral, cortical fragments. The mandibular contours showed only some small erosions without permeative destruction or periosteal reaction.
On magnetic resonance (MR) imaging the tumor was isointense to muscle on T1-weighted images (T1W) and heterogeneously hyperintense on T2-weighted images (T2W), with multiple small cystic areas, well delineated from adjacent muscles of mastication, and showing peripheral heterogeneous enhancement after gadolinium. No lymphadenopathies or invasion of the parotid or parapharyngeal spaces was present.
Odontogenic Keratocyst (OKC), also known as Keratocytic Odontogenic Tumour, is a benign developmental odontogenic lesion that arises from remnants of the dental lamina, lined by a thin regular parakeratinized epithelium.  It accounts for 10-20% of all odontogenic cysts. In the mandible is the third most common cyst. [1, 2] Almost 80% of OKC are found in the mandible. [1, 2] Peak incidence is between the second and third decades of life, with a smaller peak between fifth and seventh decades, with a male predominance. [1, 3]
Although it’s considered benign, is a locally aggressive and infiltrative lesion, which accounts for a recurrence rate around 28%. [4, 5] Usually, is located in the mandibular ramus and posterior body. [1-3] In the maxilla it’s preferentially located in the canine region.6 When multiple and in younger patients, Gorlin-Goltz syndrome should be suspected. [1-5] Malignant transformation is rare, occurring in 1% of all odontogenic cysts. 
Clinically they are usually painless, incidentally found lesions, until cortical expansion ensues and leads to tooth dislocation, root resorption, cortical disruption and/or extraosseous soft-tissue extension. In these circumstances, patients tend to present with pain, swelling and, eventually, trismus. [2, 4]
Radiologically, OKC can present as unilocular or multilocular lesions. Unilocular lesions can be mistaken for periapical and dentigerous cysts (may be associated with an unerupted tooth) or for an ameloblastoma, accounting for almost three quarters of all OKC. [2, 4] When multiloculated and large, these tumours expand and scallop the cortical margins, sometimes with more aggressive features of cortical disruption, erosions, bone reabsorption, and even extraosseous soft-tissue extension. [3, 4] Periosteal reaction or aggressive bone destruction is never present. Daughter cysts are a key-finding , and a fusiform type of bone expansion is typical. 
OKC may have a soft-tissue density as high as 50 HU, and on MR, depending on the protein content, can present with low to high signal intensity on T1W and usually heterogenous high signal intensity on T2W images. Peripheral and septate-like enhancement is typical, although heterogenous enhancement can also be found. [2-4]
Treatment of choice is complete surgical resection, usually by enucleation. Recurrence rate of OKC is higher than other odontogenic cysts, related with its incomplete removal or presence of daughter cysts, associated with OKC’s natural infiltrative pattern. [1, 2]
OKC is usually a painless incidentally found tumour of developmental odontogenic origin, that can present as a unilocular or multilocular cystic lesion in the posterior mandible, with an infiltrative pattern and a high recurrence rate.
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Differential Diagnosis List
Glandular Odontogenic Cyst
Giant Cell Tumours