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

There are very many publications reporting the expression of different proteins in odontogenic cysts. In most cases the purpose has been to shed light on the pathogenesis and mechanisms of growth of the lesions, but many papers have attempted to determine whether particular patterns of expression can provide accurate diagnostic markers for each cyst type. Studies of keratin expression and proliferation markers are particularly numerous and the odontogenic keratocyst has been the subject of the majority of studies. Overall, however, immunohistochemistry has only a very small role to play in the diagnosis of cysts of the maxillofacial regions. Although each cyst type may show a different pattern of cytokeratins, the expression demonstrated by immunohistochemistry merely reflects the type of keratinisation that is easily and clearly visible on examination of a routine H&E (haemotoxylin and eosin)‐stained section. The best example of this conundrum is the odontogenic keratocyst. Many studies have been undertaken to compare the cytokeratin profile of keratocysts with other cysts, but almost without exception, the specimens used have been selected as typical histological examples of each cyst type. It is not surprising therefore that if the features are typical, there is no need for any additional staining beyond a good H&E‐stained section to make the diagnosis. One key area of diagnostic difficulty is when the pathologist must examine a small biopsy of an inflamed cyst. When heavily inflamed, any cyst type, including a keratocyst, may become lined by proliferative epithelium identical to that seen in a radicular cyst. The lining becomes non‐keratinised and studies have not been able to identify immunohistochemical markers that can differentiate between an inflammatory cyst and an inflamed developmental cyst. This issue is discussed in detail in Chapter 7. Our experience, supported by a number of studies (Rao et al. 2015 ), suggests that a good H&E‐stained section is the most specific marker for patterns of keratinisation and is usually sufficient to make an accurate diagnosis.

Table 2.3 Characteristic histological features that assist in the diagnosis of cysts of the maxillofacial regions.

Despite these limitations, there are a few instances where immunohistochemistry can help establish a diagnosis and differentiate between lesions with similar histological features. A number of diagnostically useful applications are summarised in Table 2.4.