Tid Bits - Calcifying odontogenic cyst and Ameloblastoma

A reading of some of the lessons, in the Chapter/Section "Odontogenic Cysts and Odontogenic Tumors" is recommended before jumping into this lesson.

Q: Is Calcifying Odontogenic cyst, a cyst or a tumor?

A: COC is classified as a developmental odontogenic cyst.

HISTORY TO RECENT UPDATES

Since this lesion was first described, there has been much confusion with regards to the nature or behavior of this lesion and its terminology. Reports have shown this lesion to have sufficient variation in its clinical behavior and histopathological features.

The 1971 and the 1992 WHO reports classified this lesion under the odontogenic tumor category. Interestingly, though classified as a tumor, the report described the lesion to be a non-neoplastic cystic lesion fueling confusion. However, the 1992 report went on to state that the terms dentinogenic ghost cell tumor or odontogenic ghost cell tumor could be proposed for certain variants of this lesion showing a solid growth with ameloblastomatous lining, ghost cells and dentinoid.

Later in the 2005 WHO report, this lesion was described as a benign cystic tumor or a cystic neoplastic lesion. This meant that though the lesion was cystic, its nature was that of a benign neoplastic tumor. The consensus report did not consider it to be a non-neoplastic cyst. Hence the lesion was retained under the tumor category, but it was renamed as calcifying cystic odontogenic tumor. Its solid variant dentinogenic ghost cell tumor however, was considered to be a distinct entity on its own.

However, the most recent WHO report in 2017, states that most of the COC lesions are cystic in nature and rarely are they solid tumors. Also they have a clinical course that is not aggressive and rarely recur. Hence, the report states that there is sufficient evidence to deem these cystic lesions as developmental cysts and not tumors. Pathologists have agreed to go back to the lesion’s previous terminology and call it calcifying odontogenic cyst.

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Q: Why were the ameloblastoma subtypes changed recently (2017)?

Ameloblastoma - subtypes

Ameloblastoma was previously thought to comprise of the following subtypes

• Solid/multicystic,

• Unicystic,

• Desmoplastic and

• Peripheral

However, in the latest (4th) edition of the World Health Organization Classification of Head and Neck tumours, ameloblastoma subtypes include

• Ameloblastoma (Conventional)

• Unicystic

• Peripheral

• Metastasizing

🤨 Isn’t “Metastasizing ameloblastoma” a malignant odontogenic tumor while ameloblastoma benign? How has it been included as a subtype of ameloblastoma, then?

Metastasizing ameloblastoma is defined as a histologically benign typical ameloblastoma which metastasises to distant sites. Both the primary lesion and the metastasis must have histological features of benign ameloblastoma."
- Speight PM, Takata T. New tumour entities in the 4th edition of the  World Health Organization Classification of Head and Neck tumours:  odontogenic and maxillofacial bone tumours. Virchows Arch. 2018  Mar;472(3):331-339.  

💡This is much like how “metastasizing pleomorphic adenoma” is considered to be a variant or a complication of pleomorphic adenoma.

🤔 The term "Solid/multicystic" and subtype "Desmoplastic" have been removed from the classification

Solid/Multicystic 
Most ameloblastomas are the “conventional type” and were synonymously called solid/multicystic. Conventional ameloblastoma is a well-recognized entity. 
💡Moreover the term “multicystic” caused confusion with the unicystic variety. Hence the term “solid/multicystic” has been removed.

Desmoplastic 
This was previously thought to be a distinct clinicopathologic entity by itself. 
💡However, desmoplastic ameloblastoma, though histologically distinct does not show any difference in clinical behaviour from conventional ameloblastoma. Hence this subtype has been removed and is now only considered to be a histological pattern of conventional ameloblastoma.

📖I would recommend going through the article, New tumour entities in the 4th edition of the World Health Organization Classification of Head and Neck tumours: odontogenic and maxillofacial bone tumours, by Paul M.Speight and Takashi Takata. The article takes you through the major changes that have been made recently, in the classifications of Odontogenic Cysts and Tumours.

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REFERENCES AND FURTHER READING

1. Speight PM, Takata T. New tumour entities in the 4th edition of the World Health Organization Classification of Head and Neck tumours: odontogenic and maxillofacial bone tumours. Virchows Arch. 2018 Mar;472(3):331-339.

2. Barnes L, Eveson JW, Reichart P, Sidransky D (2005) Odontogenic Tumours. Ch 6, WHO classification of tumors: pathology and genetics of head and neck tumours. IARC, Lyon.

3. Kramer I, Pindborg J, Shear M (1992) WHO International Histological Classification of Tumours. Histological typing of odontogenic tumours, 2nd edn. Springer-Verlag, Berlin.

4. Tomich CE. Calcifying odontogenic cyst and dentinogenic ghost cell tumor. Oral Maxillofac Surg Clin North Am. 2004 Aug;16(3):391-7.

5. Reichart PA, Philipsen HP. Odontogenic tumors and allied lesions. Quintessence Publishing; 2004

6. Shear M, Speight PM. Cysts of the Oral and Maxillofacial Regions. 4 th ed. Blackwell Munksgaard; 2007.