Polymorphous low-grade adenocarcinoma of the palate: report of a case with electron microscopy

Polymorphous Adenocarcinoma: A Systematic Review of the Literature and Presentation of Two Cases in a Less-Considered Anatomical Site

BACKGROUND

Polymorphous adenocarcinoma (PAC) is the second-most common malignant tumour of the minor salivary glands. Although PAC predominantly affects the palate, it can also involve the buccal mucosa. This systematic review aims to investigate the literature data about PAC. Furthermore, we report two cases of patients affected by PAC in an infrequently considered anatomical site.

METHODS

According to PRISMA guidelines, a systematic review search was conducted on PubMed, Scopus, and Web of Science. Observational studies conducted on patients with a histological diagnosis of PAC were selected and analysed. Furthermore, two cases of patients with PAC affecting the buccal mucosa were reported.

RESULTS

Twenty-nine studies were included, and 143 patients affected by PAC were analysed (62 males, 75 females, and 6 undefined, with a mean age of 57.4 ± 14.5 years). The palate was the most affected site (99/143, 69.2%), followed by the buccal mucosa (12/143, 8.4%). Moreover, we report two cases of patients with PAC affecting the buccal mucosa (one male and one female, with a mean age of 70.5 ± 2.5 years).

CONCLUSIONS

The present study underscores the importance of considering the buccal mucosa as a possible location of minor salivary gland tumours; although it is a less-considered affliction, it is not uncommon.

Historical Evolution of the Polymorphous Adenocarcinoma

The 2017 World Health Organization Classification of Head and Neck Tumors introduced for the first time the diagnostic terminology "cribriform variant of polymorphous adenocarcinoma". This nomenclature attempts to reconciliate the ongoing taxonomical controversy related to cribriform adenocarcinoma of tongue. In order to better understand this classification conundrum, it is imperative for pathologist to comprehend the historical evolution of polymorphous adenocarcinoma formerly known as polymorphous "low grade" adenocarcinoma. This review highlights our understanding of these tumors since their origins.

Immunocytochemistry of myoepithelial cells in the salivary glands

MECs are distributed on the basal aspect of the intercalated duct and acinus of human and rat salivary glands. However, they do not occur in the acinus of rat parotid glands, and sometimes occur in the striated duct of human salivary glands. MECs, as the name implies, have structural features of both epithelial and smooth muscle cells. They contract by autonomic nervous stimulation, and are thought to assist the secretion by compressing and/or reinforcing the underlying parenchyma. MECs can be best observed by immunocytochemistry. There are three types of immunocytochemical markers of MECs in salivary glands. The first type includes smooth muscle protein markers such as alpha-SMA, SMMHC, h-caldesmon and basic calponin, and these are expressed by MECs and the mesenchymal vasculature. The second type is expressed by MECs and the duct cells and includes keratins 14, 5 and 17, alpha 1 beta 1 integrin, and metallothionein. Vimentin is the third type and, in addition to MECs, is expressed by the mesenchymal cells and some duct cells. The same three types of markers are used for studying the developing gland. Development of MECs starts after the establishment of an extensively branched system of cellular cords each of which terminates as a spherical cell mass, a terminal bud. The pluripotent stem cell generates the acinar progenitor in the terminal bud and the ductal progenitor in the cellular cord. The acinar progenitor differentiates into MECs, acinar cells and intercalated duct cells, whereas the ductal progenitor differentiates into the striated and excretory duct cells. Both in the terminal bud and in the cellular cord, the immediate precursors of all types of the epithelial cells appear to express vimentin. The first identifiable MECs are seen at the periphery of the terminal bud or the immature acinus (the direct progeny of the terminal bud) as somewhat flattened cells with a single cilium projecting toward them. They express vimentin and later alpha-SMA and basic calponin. At the next developmental stage, MECs acquire cytoplasmic microfilaments and plasmalemmal caveolae but not as much as in the mature cell. They express SMMHC and, inconsistently, K14. This protein is consistently expressed in the mature cell. K14 is expressed by duct cells, and vimentin is expressed by both mesenchymal and epithelial cells. After development, the acinar progenitor and the ductal progenitor appear to reside in the acinus/intercalated duct and the larger ducts, respectively, and to contribute to the tissue homeostasis. Under unusual conditions such as massive parenchymal destruction, the acinar progenitor contributes to the maintenance of the larger ducts that result in the occurrence of striated ducts with MECs. The acinar progenitor is the origin of salivary gland tumors containing MECs. MECs in salivary gland tumors are best identified by immunocytochemistry for alpha-SMA. There are significant numbers of cells related to luminal tumor cells in the non-luminal tumor cells that have been believed to be neoplastic MECs.

Polymorphous low-grade adenocarcinoma and adenoid cystic carcinoma: distinct architectural composition revealed by collagen IV, laminin and their integrin ligands (alpha2beta1 and alpha3beta1)

AIMS

Polymorphous low-grade adenocarcinoma (PLGA) and adenoid cystic carcinoma (ACC) are malignant salivary gland tumours bearing many similar histological patterns. This study was undertaken to show how the presence and distribution of collagen IV and laminin, and their ligands (integrin alpha2beta1 and alpha3beta1 components), can reveal histoarchitectural differences which distinguish these two entities.

METHODS AND RESULTS

Five cases of ACC and five cases of PLGA from the archives of the Oral Pathology Department of the School of Dentistry of the São Paulo University were submitted to immunostaining with the antibodies to collagen IV, laminin, and integrins alpha2beta1 and alpha3beta1 using the streptavidin-biotin-peroxidase technique. Positive and negative controls were included. PLGA showed a thin line of collagen IV and laminin surrounding structures composed of a single cell layer. Integrins were expressed as a widespread and granular pattern. A thick line of collagen and laminin was observed around the neoplastic structures of ACC. Both integrins were expressed in intercellular spaces and around luminal spaces of tubular structures.

CONCLUSIONS

Collagen IV and laminin, and their integrin ligands, are useful in demonstrating that neoplastic ductal units of PLGA are composed of a single cell layer, being distinct from ACC which contains structures composed of two layers of neoplastic cells.

Salivary gland basal cell and canalicular adenomas: immunohistochemical demonstration of myoepithelial cell participation and morphogenetic considerations

OBJECTIVE

To evaluate cellular composition of salivary gland adenomas using 3 monoclonal antibodies that recognize a smooth muscle phenotype confirmed to be sensitive for myoepithelial differentiation.

DESIGN

Immunohistochemical evaluation of 25 salivary gland basal cell and canalicular adenomas.

SETTING

Archival pathology material from the files of Henry Ford Hospital, Detroit, Mich, and the University of California at San Francisco.

RESULTS

All basal cell adenoma variants exhibit some degree of myoepithelial cell participation with periductal, epithelioid, and spindled (stromal-like) morphologic structures. Only the canalicular adenomas, even if mixed with trabecular and solid patterns, are devoid of staining with these 3 antibodies, suggesting an adenoma composed exclusively of ductal luminal cells.

CONCLUSIONS

There is an overlapping histomorphologic and common cellular composition of the basal cell adenoma variants with other recognized adenomas, such as pleomorphic adenoma and myoepithelioma. Relative differentiation toward 3 cell phenotypes (ductal luminal, basal, and myoepithelial) and the character of extracellular matrix production in varying proportions by the neoplastic myoepithelial cells distinguishes the spectrum of salivary gland adenomas identified in current classification schemes.

The myoepithelial immunophenotype in 135 benign and malignant salivary gland tumors other than pleomorphic adenoma

BACKGROUND

We have previously studied the immunoreactivity of 3 novel smooth muscle-specific proteins, alpha-smooth muscle actin, smooth muscle myosin heavy chains, and calponin, to assess myoepithelial differentiation in pleomorphic adenomas.

OBJECTIVE

To further expand our knowledge of myoepithelial differentiation in other benign and malignant salivary gland tumors.

DESIGN

Formalin-fixed paraffin sections of 135 salivary gland tumors with associated normal glands were stained with monoclonal antibodies using the avidin-biotin complex immunoperoxidase method and enzymatic and microwave heat-induced epitope retrieval.

RESULTS

In adenoid cystic carcinomas and epithelial-myoepithelial carcinomas, all 3 markers exclusively highlighted the myoepithelial cell components and the epithelial cells were entirely negative. No immunostaining was detected in canalicular adenomas, oncocytomas, Warthin tumors, acinic cell carcinomas, mucoepidermoid carcinomas, squamous cell carcinomas, and polymorphous low-grade adenocarcinomas. Salivary duct carcinomas and adenocarcinomas, not otherwise specified had a distinctive pattern of uniform periductal staining of reactive myofibroblastic cells, and in salivary duct carcinomas some ducts retained a peripheral immunoreactive myoepithelial cell layer.

CONCLUSION

Immunoreactivity for these 3 smooth muscle-specific proteins confirms the known neoplastic myoepithelial component of adenoid cystic carcinomas and epithelial-myoepithelial carcinomas. The consistently positive staining pattern in adenoid cystic carcinomas may be diagnostically useful in discriminating histologically similar but consistently negative polymorphous low-grade adenocarcinomas. Periductal linear staining in adenocarcinoma, not otherwise specified and salivary duct carcinomas is distinctive and appears to represent a tight cuff of myofibroblasts associated with the infiltrating glands.

Clinical and therapeutic features of polymorphous low-grade adenocarcinoma

Polymorphous low-grade adenocarcinoma, also known as terminal duct or lobular carcinoma, was first described in two clinical case series in 1983. Before that time most of these neoplasms were diagnosed as benign salivary gland neoplasms including pleomorphic adenomas, variants of monomorphic adenomas, or salivary malignant conditions including malignant pleomorphic adenomas, adenoid cystic carcinomas, and adenocarcinoma not otherwise stated. This neoplasm with few exceptions originates in minor salivary gland tissue of the posterior hard and soft palates or buccal mucosa. It is characteristically slow to enlarge; clinical reports show the neoplasm present for many years before diagnosis. We have evaluated the clinical and microscopic features of 15 cases from the archives of The University of Iowa Surgical Oral Pathology Laboratory and added these to published case reports. A total of 204 cases were evaluated with a female/male ratio of almost 2/1. Forty-nine percent originated in palatal mucosa. Polymorphous low-grade adenocarcinomas arising from pleomorphic adenomas or de novo have been reported within major salivary glands and outside the oral cavity. A 17% recurrence rate was found with a regional metastasis rate of 9%. Five cases had multiple recurrences, and 13 recurrences were at or beyond 5 years after the initial diagnosis. Regional node metastases were identified at the time of initial treatment or at the time of recurrence in 9% of cases in which follow-up data were specified.

Estrogen receptor assay in polymorphous low-grade adenocarcinoma and adenoid cystic carcinoma of salivary gland origin. An immunohistochemical study

An apparent relationship between breast cancer and salivary gland cancer has been observed, and there is one report in the literature that describes estrogen receptors in normal salivary gland and salivary gland cancers. With the use of a monoclonal antibody against estrogen receptor protein and the avidin-biotin immunoperoxidase procedure, we were unable to demonstrate estrogen receptors in formalin-fixed paraffin-embedded sections of either polymorphous low-grade adenocarcinoma or adenoid cystic carcinoma of salivary gland origin.

Polymorphous low-grade adenocarcinoma of minor salivary gland. A comparative histologic and immunohistochemical study

Sixteen polymorphous low-grade adenocarcinomas were reviewed and compared with 17 adenoid cystic carcinomas and with 21 other histologically similar minor salivary gland neoplasms. The polymorphous low-grade adenocarcinomas were for the most part distinctive in their microscopic appearance. Typically they exhibited infiltrative growth by small uniform cells in single-layered ducts. A syncytium of tumor cells was also characteristic, although solid and cribriform patterns were seen, making definitive diagnosis difficult with some tumors. Immunohistochemical staining for S-100 protein, glial fibrillary acidic protein, actin, vimentin, and keratins resulted in relatively distinctive antigenic profiles for the tumors studied. Of significance was strong S-100 protein and weak actin staining of polymorphous low-grade adenocarcinomas, moderate actin staining of adenoid cystic carcinomas, moderate glial fibrillary acidic protein staining of monomorphic adenomas and pleomorphic adenomas, and nonreactivity of monomorphic adenomas for vimentin. It is believed that the immunoprofiles could be useful in the microscopic diagnosis of salivary gland tumors. The identification of antigens found normally in myoepithelial and epithelial cells supports the concept that these tumors are derived from pluripotential reserve cells.

Slow-growing, ulcerated mass of the left posterior hard palate

The importance of a thorough oral and paraoral examination of all patients, with appropriate follow-up of abnormalities, cannot be overstated. The prognosis of any malignant lesion is improved if diagnosis and treatment are rendered early. This patient had an obvious lesion of the hard palate, 4 1/2 years before definitive diagnosis. When the lesion was first noted, prompt diagnostic evaluation may have discovered the malignant process. The lengthy delay in appropriate treatment for this patient has resulted in a potentially tragic outcome.