cyst. This feature may be seen in lesions of cemento‐osseous dysplasia, cemento‐ossifying fibroma, and cementoblastoma. The dentigerous cyst embraces the crown of an unerupted tooth and cannot be confused with radicular cyst, but the corticated margin is continuous with the lamina dura (Figures 5.5 and 5.6).
Histopathological Examination of Cysts
In most cases the responsibility for a final diagnosis lies with the histopathologist who must examine samples of tissue. As stated above, it is important that the pathologist does not make a final diagnosis without first considering the clinical and radiological features of the lesion. These may be stated on the pathology request form, but often the pathologist should read the radiology report, consult with the radiologist, or personally examine the radiographs. Many cysts may reach a large size and it is good practice to establish a diagnosis before definitive surgery. This means that the histopathologist is often presented with a small incisional biopsy of a large lesion. In most cases, consideration of the radiological and histological features together is sufficient to establish a diagnosis. The key features summarised in Tables 2.2 and 2.3 should assist decision making in most cases. Occasionally a definitive diagnosis is not possible on a small biopsy and a final diagnosis must await examination of the whole specimen. Pathologists must not be afraid to withhold a final diagnosis until it has been possible to examine sufficient tissue.
Histopathological examination of a cyst begins with examination and sampling of the whole specimen. If an associated tooth is also removed, then the relationship of the cyst to the tooth can be directly observed and is of particular value in the diagnosis of a radicular cyst (located at the tooth apex), dentigerous cyst (attached at the cementoenamel junction; Figure 5.18), and paradental cyst (attached to the disto‐buccal aspect of the tooth; Figure 4.6). In all cases it is of value to examine the cyst in its entirety and also to dissect it and examine the cut surface and the lumen. Most cysts are unilocular with a thin regular lining, but careful examination of the gross specimen will show evidence of multilocularity and reveal areas of thickening or luminal nodules if present. Cysts that are typically multilocular on gross examination include the botryoid odontogenic cyst (Figures 8.3, 8.9, and 8.10) and glandular odontogenic cyst (Figure 10.6). Thickening of the wall or luminal nodules are seen in lateral periodontal cyst (Figure 8.6), glandular odontogenic cyst (Figures 10.6 and 10.8), and calcifying odontogenic cyst (Figures 11.8 and 11.9). Calcifying odontogenic cyst may also have calcified material in the wall or be associated with an odontoma. Representative samples of the cyst wall, including any areas of thickening, should be taken for histological examination and any hard tissue should be decalcified and sampled for histology.
On dissection, most cysts contain small amounts of serosanguinous fluid, but the odontogenic keratocyst and orthokeratinised odontogenic cyst usually contain a ‘cheesy’ or ‘buttery’ keratinaceous material that is cream or yellow coloured, and may have a characteristically unpleasant odour. Such contents will be familiar to many pathologists as a characteristic feature of epidermal cysts of the skin.
The histological features of each cyst and the histological differential diagnosis are described in detail in each chapter. Very few cysts have histological features that are absolutely diagnostic or pathognomonic, and diagnosis is usually made by considering a combination of features in the context of the radiology. The only possible exception to this is the odontogenic keratocyst, which shows a thin regular lining of parakeratinised epithelium with features that are unique to this cyst type (see Figures 7.15–7.17). Table 2.3 provides an overview of characteristic histological features and their diagnostic utility for different cyst types.
Immunohistochemistry and Molecular Pathology
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.
| Histological feature | Cyst type(s) | Diagnostic utility | Figure references |
|---|---|---|---|
| Proliferative epithelium with an arcading pattern | Radicular cyst Inflammatory collateral cysts | Typical feature of radicular cyst and of inflammatory collateral cysts But proliferative arcading epithelium may be seen in any odontogenic cyst that is secondarily inflamed | Figures 3.7, 3.12 (radicular cyst), 4.7 (paradental cyst), 5.22 (dentigerous cyst) |
| The epithelial lining is attached to an unerupted tooth at the cementoenamel junction | Dentigerous cyst | Virtually diagnostic of dentigerous cyst. This feature may be seen on macroscopic examination of an intact specimen or in decalcified sections Note: there have been reports of odontogenic keratocyst or orthokeratinised odontogenic cyst attached at the cementoenamel junction, but this is very rare and is thought to be due to a tooth ‘erupting’ into a cyst (see discussion in Chapter 12) | Figures 5.18 and 5.19 |
| Thin regular parakeratinised epithelium with a corrugated surface and prominent basal layer | Odontogenic keratocyst | Diagnostic of odontogenic keratocyst. This typical epithelium is not seen in any other jaw cyst | Figures 7.15–7.17 |
| Epithelial plaques or thickenings with a whorling pattern |
Lateral periodontal cyst Botryoid odontogenic cyst Gingival
|