Should the solid variant of odontogenic keratocyst and keratoameloblastoma be classified as the same entity? A clinicopathological analysis of nine cases and a review of the literature

An Unusual Gingival (Peripheral) Tumor with Features of Keratoameloblastoma with Cytologic Atypia or Possible Malignant Transformation Exhibiting ARID1A Mutation

BACKGROUND

Keratoameloblastoma is a poorly characterized and rarely reported odontogenic neoplasm that can exhibit overlapping histopathologic features with conventional ameloblastoma and keratocystic odontogenic tumor (KCOT), with an ambiguous relationship to the so-called solid KCOT.

METHODS

A peripheral maxillary tumor causing bone saucerization in a 54-year-old male is described and investigated with immunohistochemistry and Next-Generation Sequencing (NGS).

RESULTS

Microscopically, the tumor comprised of a predominantly plexiform proliferation of odontogenic epithelium with central keratinization and evidence of surface origin. Peripheral cells exhibited nuclear palisading with variable reverse polarization, while stellate reticulum-like areas were observed internally. A few follicles and a few foci in the lining of cystic spaces revealed increased cellularity with cells exhibiting small but conspicuous nucleoli, focal nuclear hyperchromatism, and a few mitoses mostly seen in the peripheral outer cell layer. Nuclear staining for ki-67 was increased in those areas when compared with the other cystic, follicular, and plexiform areas. These features were interpreted as cytologic atypia suggesting also the possibility of a malignant process. Immunohistochemically, the tumor was positive for CK19 and negative for BRAF VE1, calretinin, and CD56. Ber-Ep4 was only focally positive. By sequencing, an ARID1A c.6527_6538delAG frameshift mutation (VAF: 5.8%), classified as likely oncogenic, and an FBXW7 c.1627 A > G missense mutation (VAF: 8.0%), classified as a variant of uncertain significance, were detected. Two mutations, probably germline (VAF ~ 50%), were recorded for RNF43 and FBXW7. No pathogenic variants were identified in PTCH1, BRAF, NRAS, HRAS, KRAS, FGFR2, or SMO genes.

CONCLUSION

The significance of an ARID1A variant in keratoameloblastoma is uncertain since this variant has not been reported in ameloblastoma or KCOT, to date. Alternatively, it may characterize malignant transformation in the present case since ARID1A mutations have been encountered in various cancers. Sequencing of additional cases is necessary to determine whether this may represent a recurrent genomic event.

Solid variant of odontogenic keratocyst in a 22-year-old man: Report of a case

Key Clinical Message

In conclusion, we can focus on histologic features such as stellate reticulum, reverse polarization of basal cell layer nuclei, and luminal lamellated keratinization as distinguishing factors of ameloblastoma and SKCO. If there is any clinically doubt, molecular testing could be helpful.

Abstract

"Solid odontogenic keratocyst" is a rare variant of odontogenic keratocyst, which usually involves mandible. This case was presented as a unique variant of odontogenic keratocyst in an unusual site of left maxilla with extension to the maxillary sinus.

Keratoameloblastoma: A Report of Seven New Cases and Review of Literature

BACKGROUND

Keratoameloblastoma (KA) is an uncommon and controversial variant of ameloblastoma exhibiting central keratinisation. Due to their rarity, there is limited information in the literature on their clinical, radiologic and histologic features. This study adds seven additional cases of KA to the literature, and reviews the current published literature on this rare entity.

METHODS

KAs were retrospectively reviewed over a 20-year period from three Oral and Maxillofacial Pathology Laboratories. Included cases were examined and the diagnosis confirmed under conventional microscopy. Immunohistochemistry with the use of a monoclonal antibody against calretinin was performed on included cases. The clinical, radiologic and histologic features of the seven new cases of KA were analysed and compared to existing cases in the literature.

RESULTS

KAs presented at a mean age of 40 years with a nearly equal gender distribution and a mandibular predilection (65%). The majority (92%) of cases presented with localised swelling with associated pain in 32% of cases. Mixed density or internal calcifications were noted in 40% of cases. All tumours presented with bony expansion, with cortical destruction noted in 62% of cases. Histologically, all tumours consisted of solid and cystic follicles with surface parakeratinisation and lamellated accumulations of central keratin. In areas the cystic follicles had an epithelial lining suggestive of an OKC. There were focal luminal areas of loosely arranged polygonal cells reminiscent of the stellate reticulum. The basal cells consisted of columnar cells with evidence of palisading and prominent subnuclear vacuolisation. Of the cases treated via tumour resection, 27% presented with tumour recurrence.

CONCLUSION

This case series reports seven additional cases of KA, taking the total to 26 reported cases. The identification of subtle histologic features, including focal stellate reticulum-like central areas, subnuclear vacuolisation and lamellated-type central keratinisation, are key in diagnosing KA. The radiologic features will often indicate signs of aggressiveness such as cortical destruction, differentiating KA from OKC. All cases were completely negative for calretinin IHC, limiting its use in distinguishing KA from OKC. Further large series are needed to expand the current understanding of this rare variant of ameloblastoma.