A Subset of Salivary Intercalated Duct Lesions Harbors Recurrent CTNNB1 and HRAS Mutations: A Molecular Link to Basal Cell Adenoma and Epithelial-Myoepithelial Carcinoma?

Correlation between basal cell adenoma and basal cell adenocarcinoma of the salivary gland: a histomorphological and molecular review of 129 cases

Basal cell adenoma (BCA) and basal cell adenocarcinoma (BCAC) are salivary gland tumors with biphasic differentiation, composed of luminal ductal cells and abluminal basal cells with a high nuclear-to-cytoplasmic ratio. While BCA is a relatively common benign tumor, BCAC is a rare malignancy, and its genetic context and relationship with BCA remain unclear. We investigated 93 BCA and 36 BCAC cases to further characterize these two tumor entities from histological and molecular perspectives. BCA/BCAC proliferated in a mixture of tubular, trabecular, solid, cribriform, and membranous patterns. A jigsaw puzzle pattern, peripheral palisading, S100-positive stroma, cystic change, and sclerosis were observed in approximately 50% of the cases. BCAC demonstrated the following malignant features: infiltration to surrounding tissue, tumor necrosis, and increased mitotic activity (81%, 22%, and 22%, respectively). The nuclear expression of β-catenin was frequently observed in both BCA and BCAC (89% and 60%), and CTNNB1 hotspot mutations were detected in 46% and 48% of BCA and BCAC cases, respectively. Tubular patterns of growth, jigsaw puzzle patterns, peripheral palisading, S100-positive stroma, and cystic changes were more common in β-catenin-positive BCA/BCAC than in β-catenin-negative BCA/BCAC. Among the β-catenin-negative BCA/BCAC cases, one case each harbored PLAG1 and MYB rearrangements. We concluded that β-catenin-positive BCA and BCAC share common histologic and molecular features, and BCAC is considered a malignant counterpart of BCA. β-Catenin-negative BCA/BCAC might include morphological mimickers, which can be genetically classified into other tumor types, including pleomorphic adenoma and adenoid cystic carcinoma.

Novel Histologic Features in Ameloblastoma With RASQ61R Mutation

Ameloblastoma is characterized histologically by evidence of ameloblastic differentiation and molecularly by MAPK pathway alterations, most frequently BRAFV600E mutation and RAS mutations, as well as by SMO mutations. This mutational profile is present across all histologic variants, including those occasionally lacking overt histologic evidence of ameloblastic differentiation, such as desmoplastic ameloblastoma and granular cell ameloblastoma. Recently, we have come across 4 cases of maxillary ameloblastoma demonstrating peculiar histologic features not accounted for by recognized histologic variants. Three intraosseous tumors were remarkably similar in histologic appearance and demonstrated a proliferation of spindled to basaloid cells in solid/sheet-like, cystic, and ribbon-like growth patterns within dense fibrous connective tissue. One case had numerous squamous morules and only 1 case, focally, demonstrated ameloblastic differentiation, yet all 3 cases harbored NRASQ61R mutation. A fourth case harbored HRASQ61R mutation and arose peripherally, in palatal (maxillary) gingiva, as a follicular-patterned neoplasm with bland squamoid morphology and scattered foci of ameloblastic differentiation. RAS Q61R immunohistochemistry was positive in both the tumor and overlying surface epithelium, in support of surface derivation. These 4 cases demonstrate that ameloblastoma may occasionally present with non-traditional histologic features, lacking categorization into known histologic variants and sometimes lacking any evidence of ameloblastic differentiation. In this setting, the differential diagnosis may be broad and include more indolent odontogenic neoplasms such as adenomatoid odontogenic tumor or squamous odontogenic tumor, odontogenic carcinomas, and non-odontogenic neoplasms. A high index of suspicion, followed by confirmatory molecular testing or mutation-specific immunohistochemistry, is necessary for accurate diagnosis.

Malignant transformation in pleomorphic adenoma: the impact of DNA methylation profiling and pathogenic mutations

Salivary gland tumors (SGTs) are a rare and heterogeneous group of lesions with diverse microscopic appearances and variable clinical behavior. The most common SGT subtype, pleomorphic adenoma (PA), can undergo malignant transformation into carcinoma ex pleomorphic adenoma (CXPA). Carcinomatous transformation from PA to CXPA is a highly progressive and multi-step process. Distinguishing a PA with some atypia from a low-grade intracapsular or minimally invasive CXPA is primarily subjective as no mitotic count thresholds exist to help pathologists distinguish between them in difficult cases. In this prospective study, we collected 140 cases encompassing both PA and CXPA to study their molecular signatures. The primary objective was to investigate the use of DNA methylation profiling as a potential molecular tool for differentiating between these two entities. Methylation analysis was performed on 33 PA cases and 33 CXPA cases. We were able to demonstrate that based on their methylation profiles, PA and CXPA could be classified into three distinct clusters that we called benign, intermediate, and malignant. We also revealed that the presence of TP53, HRAS, PTEN and/or TERT pathogenic mutations were exclusively present in CXPA cases; and that chromosomal alteration on chromosomes 5 and 8 are potentially associated with malignant transformation. In conclusion, our study provides a comprehensive molecular framework for PA and CXPA. The presence of a pathogenic mutation in TP53, HRAS, PTEN, or pTERT or HER2 amplification could be integrated into a molecular diagnosis of CXPA for tumors within the PA CXPA spectrum. In the future, we aim to improve our methylomic classification to make it a precision medicine diagnostic tool for the treatment management of tumors within the PA-CXPA spectrum.

Mucoepidermoid Carcinoma Associated with an Intercalated Duct Lesion: Precursor or Coincidence?

BACKGROUND

Mucoepidermoid carcinoma (MEC) typically occurs in isolation. We report an unusual case of MEC coexisting with an intercalated duct lesion (IDL), both harboring MAML2 rearrangements.

CASE PRESENTATION

An 82-year-old male with a left parotid mass underwent superficial parotidectomy.

METHODS

Microscopic examination of immunohistochemical stains and fluorescence in situ hybridization (FISH) studies were performed.

RESULTS

The parotid demonstrated an intermediate grade MEC and an adjacent IDL with the typical abluminal ΔNp63 (p40) and ductal SOX-10 immunopositivity, but absent LEF-1 and nuclear β-catenin staining. FISH confirmed MAML2 rearrangements in both lesions.

CONCLUSION

This case expands the spectrum of IDL-associated tumors and suggests IDLs may serve as MEC precursors.

12q Amplification Characterizes a Distinctive Salivary Gland Tumor with Bizarre Myoepithelial Atypia

BACKGROUND

Over the past two decades, an increased understanding of molecular alterations has greatly refined salivary gland tumor classification. Many tumors that were previously difficult or impossible to classify have been recognized to represent emerging entities based on shared histologic, immunophenotypic and molecular characteristics. While initial attention was given to carcinomas, more recently molecular discoveries have shed light on salivary gland adenomas as well. We present a series of biphasic salivary gland tumors characterized by striking bizarre myoepithelial atypia, unified at the genetic level by evidence of 12q amplification.

METHODS

Salivary gland tumors demonstrating bizarre but degenerative-appearing atypia were identified. Immunohistochemistry, MDM2 and HMGA2 fluorescence in situ hybridization (FISH), and/or targeted next-generation sequencing (NGS) were attempted on the cases.

RESULTS

Seven cases were identified. The tumors arose in the parotid gland (3 of 7), oral cavity (3 of 7) and submandibular gland (1 of 7). The patients were 5 women and 2 men, ranging from 53 to 83 years (mean, 65.7 years). Histologically, the tumors appeared well-circumscribed and partially encapsulated. They were highly cellular and solid, with no chondromyxoid stromal component. The tumors were biphasic, with a population of eosinophilic ducts in a background of basaloid cells with variable clear cell change and spindling. The most striking feature in all cases was the presence of scattered cells with bizarrely atypical nuclei with smudgy chromatin. These bizarre cells maintained low nuclear:cytoplasmic ratios and lacked mitotic activity. The biphasic nature of the tumors was demonstrated by the basaloid cells staining with S100, p63 and p40, while the ducts were positive for CD117 and AE1/AE3 (strong). The bizarre cells had a myoepithelial immunophenotype. The Ki67 index ranged from 1 to 10%, with most of the markedly atypical cells not labeling. DNA NGS was successful in 4 cases, demonstrating 12q copy number increase and 5q copy number loss in all 4 cases. RNA sequencing was able to identify increased MDM2 and HMGA2 expression in one additional case, while amplification of MDM2 and/or HMGA2 was also demonstrated in 6 of 7 cases by FISH. In summary, all 7 cases exhibited evidence of 12q amplification by at least one technique.

CONCLUSION

Salivary gland neoplasms with bizarre myoepithelial atypia are consistently associated with evidence of 12q amplification. Although this histologic alteration may be alarming, the smudgy nature of the chromatin and paucity of mitotic activity and Ki67 labeling suggest that this finding may not necessarily be indicative of malignancy.

Update of newly-recognized salivary gland neoplasms: molecular and immunohistochemical findings and clinical importance

With the advancement of molecular testing and the routine use of immunohistochemical stains, salivary gland tumours previously categorized as adenoma or adenocarcinoma, not otherwise specified, are being reclassified with distinct diagnoses. Newly recognized benign entities include: sclerosing polycystic adenoma, keratocystoma, intercalated duct hyperplasia and adenoma, and striated duct adenoma. Newly recognized malignant salivary gland tumours include: microsecretory adenocarcinoma, sclerosing microcytic adenocarcinoma, and mucinous adenocarcinoma. Additionally, rare subtypes of mucoepidermoid carcinoma have been described, including Warthin-like and oncocytic. Understanding of intraductal carcinoma continues to evolve. Correctly distinguishing these lesions from mimickers can be crucial for appropriate patient care and prognostication, as well as future conceptualization of salivary disease.

Infarction of basal cell adenoma of the parotid gland: First case report and literature review on CTNNB1 I35T mutations in salivary basal cell neoplasms

Infarction has rarely been reported in some types of salivary gland tumors. In this study, we present the first case of infarction occurring in salivary basal cell adenoma. A 62-year-old male presented with swelling in the left parotid region. Histopathological examination revealed extensive central necrosis surrounded by a rim of viable tumor tissue showing the typical histology of basal cell adenoma. Nuclear β-catenin expression and the CTNNB1 p.I35T (c.104 T > C) mutation were identified in the tumor. A diagnosis of basal cell adenoma with central necrosis was made, and the postoperative period was uneventful. In addition, we review the literature on CTNNB1 I35T mutations in basal cell neoplasms of the salivary glands. Awareness of the possible occurrence of infarction and the high frequency of the unique mutation in basal cell adenoma may help in the differential diagnosis of salivary gland tumors.

[New information about tumours of the salivary glands : WHO classification 2022]

The WHO 2022 classification of head and neck tumours contains another slight increase in the number of listed benign and malignant tumour entities of the salivary glands. This includes conceptual changes and alterations in the terminology of some entities. While some new features are regarded as preliminary or provisional, others are strongly disputed (for example the terminology of intraductal carcinoma). The impact of molecular findings, mainly recurrent gene fusions, continues to increase rapidly and some have been included in the definition of certain tumour entities. The significance of molecular findings is, however, still largely restricted to diagnostic aspects. Newly included entities include microsecretory carcinoma (defined by an SS18::MEF2C fusion), sclerosing microcystic adenocarcinoma (similar to skin adnexal tumours of the same name) and mucinous adenocarcinoma (characterized by AKT1 mutations with heterogeneous morphology).

KRAS codon 12 mutations characterize a subset of de novo proliferating "metaplastic" Warthin tumors

Warthin tumor (WT; synonym: cystadenolymphoma) represents one of the most frequent salivary gland tumors with a frequency equaling or even outnumbering that of pleomorphic adenomas in some series. Histologically, the tumor displays tall columnar oncocytic cells, arranged into two cell-thick layers lining variably cystic glands within an organoid lymphoid stroma. Tumors with exuberant squamous metaplasia in response to FNA-induced or other types of tissue injury/infarction have been referred to as "metaplastic WTs." However, the same terminology was used for tumors with variable mucinous cell and solid or stratified epidermoid proliferations (occasionally mimicking mucoepidermoid carcinoma), although the "metaplasia concept" has never been proven for the latter. We herein investigated 22 WTs showing prominent mucoepidermoid-like or solid oncocytoma-like proliferations without prior FNA or histological evidence of infarction/ trauma using the TruSight Tumor 15 gene panel and KRAS pyrosequencing. As a control, we tested 11 conventional WTs. No statistically significant differences were observed between the two subcohorts regarding patient's age and tumor size. Six of 22 (27%) proliferating/ metaplastic WTs revealed oncogenic KRAS mutations clustering at codon 12 (exon 2), while all conventional tumors lacked these mutations. Our findings are in line with a neoplastic nature of the epidermoid/ mucoepidermoid proliferations in non-injured "metaplastic" Warthin tumors. We propose the descriptive term "de novo proliferating Warthin tumor" for this variant to distinguish it from infarcted/inflamed genuine metaplastic Warthin tumor.