Paul M. Speight - Shear's Cysts of the Oral and Maxillofacial Regions

The relationship between the glandular odontogenic cyst and neoplasia has been speculative and is based largely on its histological similarity to intraosseous mucoepidermoid carcinoma. This is discussed in detail in Chapter 10. This histological similarity can make it very difficult to reach a definitive diagnosis and may result in misdiagnosis of lesions. A number of papers have reported lesions with histological features of a glandular odontogenic cyst that have recurred as mucoepidermoid carcinomas, but some of these are poorly illustrated and in others the diagnostic criteria are not clear. Nevertheless, this has led to suggestions that the glandular odontogenic cyst may be a precursor lesion, or a ‘benign variant’ of mucoepidermoid carcinoma. The debate is further fuelled by the observation that the glandular odontogenic cyst has a high recurrence rate, leading some authors to label it ‘biologically aggressive’ (Greer et al. 2018 ). Mucoepidermoid carcinoma is characterised by specific rearrangements of the MAML2 gene and large studies have shown that glandular odontogenic cysts do not show this change, suggesting that the two lesions are distinct and that the cyst is not a precursor to the carcinoma (Bishop et al. 2014 ). More recently, two studies have reported lesions with the features of glandular odontogenic cyst that have recurred as MAML2 positive mucoepidermoid carcinomas, and have suggested that this is further evidence that mucoepidermoid carcinomas can arise from glandular odontogenic cysts (Greer et al. 2018 ; Nagasaki et al. 2018 ). In both cases, an alternative explanation is that the primary lesions were also mucoepidermoid carcinomas, but were either misdiagnosed or were histologically indistinguishable from cysts. Taken together, however, these studies raise a number of questions and provide sufficient evidence to justify further research into the relationship between glandular odontogenic cyst and mucoepidermoid carcinoma. The two possibilities to be explored are that glandular odontogenic cyst is a benign variant or precursor of mucoepidermoid carcinoma or, more likely, that in some cases the two lesions are histologically indistinguishable. Until these issues are clarified, care must be taken to use strict criteria for diagnosis and histological assessment must be accompanied by careful assessment of the clinical and radiological features (see Boxes 10.1and 10.2, Tables 10.3 and 10.4).

An accurate diagnosis of a cyst requires knowledge, common sense, and a logical approach to the assimilation of all the information available about each lesion. With few exceptions, it is not possible to reach a correct diagnosis without considering the clinical, radiological, and histological features. This is especially relevant to histopathologists, who may be called upon to make a diagnosis on a small biopsy. To do so without considering the clinical and radiological features often leads to diagnostic errors and incorrect management (Barrett et al. 2017 ). In each chapter of this book we present all the features of each cyst type in context and discuss the differential diagnoses and potential errors that might occur if only small biopsies are examined. Throughout we advise on the importance of correlating clinical, radiological, and histological features and suggest that a diagnosis of an intraosseous lesion should never be made without considering the radiology. We also recommend that a diagnosis should never be made on a small biopsy if there is any uncertainty – when in doubt, the pathologist should request a further biopsy. In this chapter, we briefly discuss an approach to making a correct diagnosis and summarise the key features that can assist in making a diagnosis of cysts of the maxillofacial regions ( Tables 2.2– 2.4). With regard to the odontogenic cysts, an accurate diagnosis is facilitated by an understanding of tooth development and the pathogenesis of each cyst type (see ‘Pathogenesis of Cysts’ and Table 2.1)

In most cases, the clinical features of cysts are not specific, since the most common presenting feature is a swelling with few other symptoms. The surgeon therefore must rely on additional features to assist in making a provisional diagnosis. For example, a swelling associated with an absent tooth may suggest a dentigerous cyst, and a small, bluish‐coloured swelling on the lower lip of a child is almost certainly a mucocele. Ultimately, however, a final diagnosis is usually made on histological examination, but before this both the clinician and the pathologist should examine the radiographs or imaging.

The characteristic radiological feature of all cysts of the jaws is a well‐demarcated radiolucency with a well‐defined and often corticated margin. Further features that assist in diagnosis include the shape and size of the lesion and the site, but in most cases it is the relationship to the teeth that provides the best indication of the type of cyst. A conventional plane radiograph is usually sufficient to determine the extent and relationships of jaw cysts, but computed tomography (CT) and magnetic resonance imaging (MRI) are often useful and may be essential for planning surgery of larger lesions. These relationships are discussed and illustrated in each chapter, but here we present an overview of characteristic radiological signs and the basic principles of an approach to interpreting the radiology. Table 2.2shows the cyst types that have characteristic radiological features, provides a cross reference to the figures in each chapter, and summarises the diagnostic utility of each feature.

Table 2.2 Characteristic radiological features that assist in the diagnosis of cysts of the jaws.

Figure 2.1 In the posterior region of the mandible, the course of the inferior dental (ID) canal (hashed lines) allows the tooth‐bearing areas (the alveolar bone) to be clearly distinguished from the basal bone of the mandible. Radiolucencies below the ID canal are not odontogenic in origin (see text for details).