Oncocytic Lesions of Salivary Glands

INTRODUCTION

Oncocytes are enlarged polygonal shaped epithelial cells with abundant, granular, intensely eosinophilic cytoplasm, a central pyknotic, round uniform nucleus with distinct nucleoli, and a low nucleus-cytoplasm ratio. The prefix onco- (e.g., oncology, oncocyte) comes from the Greek onkoustai, meaning “to swell.” Ultrastructurally, oncocytes are crammed with numerous mitochondria of various sizes marked with mitochondrial hyperplasia, pleomorphism and paucity of other organelles.^1,2 Oncocytes have been described in a multitude of human tissues, including salivary gland, lacrimal gland, ocular caruncle, parathyroid(oxyphil cells), thyroid(Askanazy/Hurthle cells), pituitary, oesophagus, liver, pancreas, kidneys, adrenals, testicles, and ovaries.³
The histological distinguishing feature of this ‘swollen cell’ is the intensely eosinophilic granular cytoplasm, typically due to increased numbers of mitochondria. In the early forms, there is a great increase in the number of mitochondria, which are typical in morphology. In later oncocytes, these organelles undergo striking changes in form and size. These mitochondrial changes are accompanied by the gradual disappearance from the oncocyte of other cytoplasmic membrane systems and of plasmalemmar specializations.³ As other cytoplasmic structures can cause acidophilia, additional distinguishing criteria have been proposed. These include: high levels of oxidative activity; variations in the size and shape of mitochondria: absence of dense granules in the intra-mitochondrial matrix: and regression of electron microscopical features such as brush borders and basal infoldings. However, Ellis & Gnepp (1988) regard the light microscopical appearance of a well demarcated mass of large eosinophilic cells, with small dark nuclei and arranged in an organoid pattern as diagnostic for oncocytoma.⁴ Radiologically, oncocytes concentrate technetium (99mTc), and therefore, tumors comprised of oncocytes are “hot” by radionuclide scanning.

HISTORY

First described by Schaffer in 1897 as ductal and acinar elements of salivary glands in tongue, esophagus and pharynx as a result of degenerative phenomenon in salivary gland parenchymal cells. The German pathologist Hu¨rthle was first to describe oncocytes as constituents of normal canine thyroid glands in 1894. Later, Zimmermann also noted these cells in sublingual gland and referred to as Pyknocytes because of their condensed nuclear chromatin/pyknotic nuclei. Jaffe first used the term Oncocytoma although differently from the present connotion. The first case report of oncocytoma was by Mc Farland in 1927, when it was classified as a variant of pleomorphic adenoma. In 1931, Hamperl first applied the term oncocyte to the Large granular cells in tissues of salivary gland, kidney, thyroid, parathyroid, pituitary and adrenal glands. The first salivary tumour composed entirely of oncocytes (oncocytoma) was described by McFarland (1927), Hamperl (1962), reviewing the English and German literature found 36 cases of oncocytoma, to which he added a further.¹¹ Since then, oncocytic and oncocyte-like cells have been reported in other primary salivary tumours and hyperplasias and the term oncocytoma appears to have been used to describe a wide variety of lesions.^4,5

HYPOTHESIS FOR MITOCHONDRIAL HYPERPLASIA

Schaffer believed the oncocyte was the result of a degenerative phenomenon in salivary gland parenchymal cells. Hamperl initially considered oncocyte to be a peculiar degeneration that was irreversible with age. Later, he expanded his position to suggest that, individual cells/cell aggregates could undergo permanent modification into oncocytes as metaplastic process or could proliferation as hyperplastic or neoplastic process. In contrast, Blanck and his co-investigators proposed that, oncocytoma is not neoplasm, rather is a nodular hyperplasia.⁶
Some studies suggest that, salivary gland oncocyte as an Adaptive/ compensatory hyperplastic cell that occurs secondary to an undetermined somatic mutation, rather than a purely degenerative process.⁵ Mitochondrial dysfunction and defective cellular metabolism have been proposed as the basis for the characteristic findings of oncocytic mitochondrial hyperplasia and pleomorphism. Hamperl suggested that oncocytic metaplasia represents a final common pathway of cellular differentiation due to functional enzyme exhaustion (Burned out/spent cell)³ or mitochondrial enzyme. Mitochondria produce energy through the manufacture of adenosine triphosphate. Aging is thought to cause a functional exhaustion of mitochondrial enzymes, resulting in a compensatory mitochondrial hyperplasia that occupies approximately 60% of the cellular cytoplasm.² Mitochondriopathy can result from nuclear DNA defects or from exposure to certain environmental toxins; however, mitochondrial DNA (mtDNA) mutations themselves have been implicated in an increasing number of human pathologic conditions, including encephalomyopathy, cardiomyopathy, optic atrophy, seizures, diabetes mellitus, Parkinson’s disease, sensorineural hearing loss, and various neoplasms.³ Frequently, discussions on salivary gland tumorigenesis begin with the salivary duct secretory unit. Previous reports have implicated the striated duct cell (multicellular theory) and the intercalated duct reserve cell (bicellular theory) as the origin of oncocytic neoplasms of the parotid gland.^7,8 In 1989, Linnane et al advocated that aging is the accumulation of mtDNA errors leading to “mitochondrial respiratory failure” and multisystem degeneration. The resultant mitochondrial hyperplasia can be thought of as the cell’s compensatory attempt to overcome an energy-deficient state. Together, these theories form the current understanding of the pathogenesis of oncocytic neoplasms. If oncocytic neoplasms are the result of an acquired mitochondriopathy due to mt DNA error(s), one can consider all parotid oncocytic morphologies (oncocytic metaplasia, oncocytosis, oncocytoma, and oncocytic carcinoma) as a discreet collection of pathology. This collection, dubbed parotid oncocytopathy, is conjectural because no molecular data yet confirm a link between oncocytic morphologies or that progression from one form to another occurs. Synchronous morphologies have been noted in single parotid specimens and oncocytomas have reportedly progressed to malignancy. Both suggest that molecular “confirmation” (e.g., a shared mtDNA alteration) should exist.⁹ It has been hypothesized that the presence of oncocytes with their characteristic mitochondrial hyperplasia and pleomorphism is the result of mitochondrial dysfunction causing defective cellular metabolism.¹⁰

CLASSIFICATION

1. The World Health Organization(WHO) histologic classification of salivary gland tumors divides parotid oncocytic neoplasms into 3 categories: Oncocytosis, Oncocytoma, and Oncocytic carcinoma.⁹
2. According to Dardick, oncocytic lesions can be:

• Typical oncocytoma
• Clear cell oncocytoma
• Hybrid oncocytoma/ Warthin’s tumour

• Multifocal nodular oncocytic hyperplasia
• Diffuse oncocytosis

• Aggresive/atypical oncocytoma
• Oncocytic adenocarcinoma

3. Working classification:

• ONCOCYTIC LESIONS which include- Oncocytosis, Oncocytoma, Oncocytic carcinoma
• BENIGN LESIONS WHICH SHOWS ONCOCYTIC metaplasia- like Warthin’s tumour, Pleomorphic Adenoma, basal cell adenoma, myoepithelioma, cystadenoma, canalicular adenoma
• MALIGNANT LESIONS EXHIBITING ONCOCYTES- which include muco-epidermoid carcinoma and acinic cell carcinoma, polymorphous low grade adenocarcinoma.

Criteria used for the diagnosis of the principal oncocytic lesion:

• Nodule entirely of oncocytic cells
• Solid trabecular/acinar growth pattern without a prominent peripheral palisade
• Light and dark cells
• Internal fibrous stroma (little)
• Fibrous capsule

• All acini affected
• Light and dark cells
• No nodules or tumor formation

• Multiple nodules of oncocytic cells in gland in lobular distribution
• Typical clear cells with few light and dark cells
• No capsule around nodules

ONCOCYTOSIS

Small regions of oncocytic metaplasia are frequently seen in normal salivary glands. By the age of 70 years, oncocytes can be universally found in all parotid glands. As with any cell, the oncocyte has the ability to proliferate. This cellular activity has been classified into 3 histologic patterns. The diffuse hyperplastic oncocytosis (DHO) and the multifocal nodular oncocytic hyperplasia (MNOH) are thought to be hyperplastic in nature, whereas the third (oncocytoma) is considered to be a true neoplasm. Clinically observed oncocytic proliferation, whether it reflects hyperplasia or neoplasm, represents 0.5% to 1.2% of all parotid gland tumors.^4,5,10 The parotid gland is predominantly involved, with infrequent inclusion of the other salivary glands. Bilateral growths can occur synchronously or metachronously in 4% to 7% of these oncocytic growths. The oncocytic lesions are reported to have a female predilection; this fact is confirmed by those cases of DHO where the gender is known. The peak incidence of all oncocytic lesions seems to occur between the seventh and ninth decades of life, with the DHO patients having a mean age of 67. 4 years.¹¹ Because the lesions are painless, patients endure their presence and their duration has been reported to vary from 4 months to 25 years. Both the DHO and MNOH follow a benign course separate from the oncocytoma. They are best considered to represent oncocytic tumor-like hyperplasias.^12,13 The high frequency of multi-nodularity and the occasional demonstration of a bilateral occurrence support the theory that these two lesions are benign hyperplasias.¹¹ The process of oncocytic change occurs in many cell types, potentially any cell containing mitochondria. Oncocytosis, or nodular oncocytic hyperplasia, occurs as a multifocal collection of oncocytes. It is a benign proliferation, that in rare instances can replace the entire parotid gland. It has been estimated to account for 0.1% of parotid tumors.
Because oncocytes and their corresponding neoplasms occur in a multitude of anatomic locations apart from the parotid gland, it is less important to discuss tumorigenesis with regard to a salivary duct progenitor cell.⁴

Diffuse oncocytosis

The affected glands also showed evidence of atrophy, such as fatty infiltration and loss of acini. which was in keeping with the ages of the patients. Unlike MNOH, ducts were present in the oncocytic areas: one even showed an attached sebaceous gland.⁴

Multifocal nodular oncocytic hyperplasia

WHO 2017 has added Nodular oncocytic hyperplasia under other epithelial lesions in the histopathological classification of salivary gland tumors. The most striking feature was clear cell change together with a paucity of characteristic light and dark cells. Oncocytic changes were seen in ducts which were unrelated to nodules. Occasionally, normal acini were included at the periphery of large nodules of MNOH, and their presence may be a useful feature in distinguishing between this lesion and an oncocytoma. One case contained an area of spindle cell proliferation within a large nodule of oncocytic hyperplasia. Which additionally showed moderate nuclear pleomorphism and occasional calcospherites. One case of uncomplicated MNOH was associated with a pleomorphic adenoma which itself showed oncocytic change.⁴

ONCOCYTOMA

Oncocytoma, also called Oxyphilic Granular Cell Adenoma, Acidophilic Granular Cell Tumor, Oncocytic Adenoma, Oncocytic Cell Adenoma forms approximately 3% of all head and neck neoplasms originating in the parotid gland, and fewer than 1% of these are oncocytic neoplasms. Despite their original description nearly 75 years ago, little is known about these rare tumors.⁹ The oncocytoma is considered the benign neoplastic counterpart of the DHO and MNOH. As such, the oncocytoma is clinically detectable as a single mass or as the prevailing nodule in a multinodular setting. It can occur bilaterally, and it usually has a capsule, which may be thin and incomplete.¹¹ These well-circumscribed tumors are comprised of monotonous sheets of oncocytes. A benign clinical course is the rule, but literature review yields an approximate 20% recurrence rate due to occult multifocality or incomplete excision.^13,14 Interestingly it has been suggested that radiation exposure could be a risk factor for the development of oncocytomas; whether this could eventually progress to diffuse oncocytosis is open to speculation. Although oncocytes are thought to result from the transition of a previously normal secretory cell to a less active cell, this does not appear to impair its ability to divide and thus oncocytes can divide in sufficient numbers to produce tumours.³
Oncocytomas most often occur as asymptomatic, well-circumscribed, solitary, painless masses usually measuring 3 to 4 cm but may reach up to 7 cm in diameter. Rarely, they may present with pain or discomfort. They may also occur as multifocal or bilateral neoplasms.
Grossly oncocytomas are solid, well-circumscribed, tan to red-brown, nodular or multinodular lesions. Fibrous encapsulation has been a criterion used to distinguish oncocytoma from oncocytic hyperplasia, although in some oncocytomas it may be minimal.¹⁶
Microscopically the tumor is seen as solid clusters or cords of tightly packed oncocytes separated by thin strands of fibrovascular stroma with scattered lumina of variable sizes, some with associated eosinophilic intraluminal secretions. The cells are large cuboidal to columnar and arranged in an organoid pattern with prominent eosinophilic, finely granular cytoplasm and uniform round, centrally placed nuclei. The eosinophilia is variable and hence there may be an admixture of light and dark stained cells. Oncocytomas may occasionally show extensive cystic change.^17,18
The clear cell variant of oncocytoma has the same architectural pattern as that of conventional oncocytoma. The clear cell change in oncocytomas may range from forming a minor component to being the predominant cell type of the tumor. In this variant, the clear cytoplasm is due to the accumulation of cytoplasmic glycogen with margination of mitochondria in the oncocytes. These clear cells are positive for glycogen with PAS staining. Conventional eosinophilic oncocytes are often evident among the clear cells. An accurate recognition of the clear cell predominant oncocytomas is essential since other clear cell malignancies can closely resemble this benign tumor. The prognosis for oncocytoma is excellent and the mainstay of treatment is complete surgical excision. Recurrence rates range from 0 - 30% usually between 0.5 – 13 years after initial diagnosis. Multifocal tumors and incomplete excisions are associated with increased risk of recurrence. The differential diagnosis for oncocytic lesions in the parotid gland include; papillary cystadenoma lymphomatosum (Warthin tumor), acinic cell carcinoma, clear cell carcinoma, oncocytic carcinoma and mucoepidermoid carcinoma.¹⁸ The extraordinary feature of the oncocytic metaplasia is that it occurs in the same form in epithelial cells of different organs, hence all these cells look very much alike. It is, in fact, sometimes impossible to distinguish an oncocyte of a salivary gland from an oncocyte of the thyroid or parathyroid. In biology this behavior is referred to as “convergent differentiation.” The same difficulty exists in distinguishing the oncocytomas of different organs; it is practically impossible to tell the difference between a benign oncocytoma of the salivary gland and of the kidney or between those of the parathyroid and thyroid glands.¹⁹

ONCOCYTIC CARCINOMA

The terms oncocytic carcinoma, oncocytic adenocarcinoma, malignant oncocytoma and malignant oxyphilic adenoma are synonymous. Oncocytic salivary gland carcinoma is an uncommon tumor representing only 0.05–0.4% of salivary gland neoplasms and about 5% of oncocytic neoplasms.5 It was first reported in 1953 by Bauer and Bauer and is the rarest morphology of oncocytic neoplasia. It is characterized by oncocytes with marked cellular atypia, frequent mitoses, destruction of adjacent structures, perineural or vascular invasion, and distant or regional lymph node metastases. In salivary gland ductal epithelium, the appearance of oncocytes (oncocytic metaplasia) is rare before the age of 50 but nearly universal beyond age 70. In 1962, Hamperl suggested that oncocytic metaplasia represents a final common pathway of cellular differentiation due to functional exhaustion of one or more mitochondrial enzymes.¹⁹
Similar to their benign counterparts, nearly 80% occur in the parotid gland. Interestingly, the majority is presumed to arise in a pre-existing oncocytoma but they also may occur de novo. Necrosis, peri-neural spread, pleomorphism, intravascular invasion, and distant metastasis to the cervical lymph nodes, kidneys, lungs, and mediastinum are the main features of this high grade malignant tumor. FNAC is less sensitive for oncocytic neoplasms, perhaps due to the rarity of these tumors and diagnostic pitfalls previously associated with FNAC (for example, sampling errors and over interpretation of paucicellular specimens). Approximately one-third of patients with oncocytic carcinoma of the parotid develop a painful mass or experience facial paralysis. The skin overlying the gland is occasionally discolored or wrinkled. Diagnosis is usually made 1 to 2 years after the onset of disease.²⁰
The oncocyte itself is a large polygonal cell characterised by a granular and eosinophilic cytoplasm, which reflects the accumulation of mitochondria. Other features include a round uniform nucleus, prominent nucleolus and frequent mitoses. Criteria for malignancy include destruction of adjacent structures, perineural or vascular invasion, and distant or regional lymph node metastases. The role of imaging in the assessment of oncocytic carcinoma is to define the intraglandular versus extraglandular location, to identify malignant features, to assess the extent of local extension and to detect nodal metastases or systemic involvement.²¹
The diagnostic criteria for oncocytic carcinoma dictate first that the tumor cells be identified as oncocytes. Secondly, the diagnosis of malignancy should be based not only on cellular and nuclear pleomorphism, but also on local infiltration, increased mitotic activity, vascular or perineural infiltration and metastasis. In cases where the histological and ultrastructural investigations are not sufficient to distinguish between oncocytoma and oncocytic carcinoma, the clinical findings become essential. Clinically, recurrences and lymphatic involvement raise suspicion of a malignancy. Necrosis is present in 20% of oncocytic carcinomas.¹⁸

SPECIAL STAINS

In addition to electron microscopy, histochemical stains like phosphotungstic acid-hematoxylin (PTAH), Novelli, Cresylecht violet V and Kluver-Barrera Luxol fast blue reveal the mitochondria in the cytoplasm of the oncocytes. Other suitable staining methods for mitochondria are the Champy-Kull method and Altmann acid-fuschin picric acid method. Enzyme histochemistry can also be used to demonstrate mitochondria.
Barnes and Bedetti reported that immunohistochemistry using an anti-mitochondrial antibody against cytochrome c oxidase was a very sensitive and specific method for identifying mitochondria within the cytoplasm of oncocytes by light microscopy. This method can be used in paraffin embedded tissues. Oncocytomas are positive for cytokeratin and basal cells stain with p.63 These basal cells are not identifiable with routine light microscopy. Oncocytomas are negative for some myoepithelial markers like smooth muscle actin, calponin, S-100 protein and GFAP (Glial fibrillary acidic protein).¹⁸

CONCLUSION

Oncocytic lesions of the salivary gland is a rare form of disease with uncertain etiology. Diagnostic accuracy of FNA for oncocytic neoplasms is low, warranting further investigations using anti-mitochondrial immunohistochemical methods. Histopathology is the corner stone for the diagnosis. Multiple oncocytic morphologies present in a single parotid specimen lends support to the concept of transition between oncocytic forms and raises the notion of a progression model for parotid oncocytic neoplasms that starts with oncocytic metaplasia. Although oncocytopathy due to acquired mitochondrial dysfunction is a plausible mechanism, the molecular mechanism for tumorigenesis remains unknown. Complete surgical excision is the treatment of choice while the role of radiotherapy or chemotherapy is still controversial, and careful long-term follow-up is necessary.

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