Oncocytic Salivary Gland Tumours: Cytological Features and Diagnostic Pitfalls

Abstract

Oncocytes are epithelial cells having abundant eosinophilic cytoplasm. The presence of oncocytes in salivary glands pose a diagnostic challenge since they can be present in lesions ranging from non-neoplastic, benign to malignant. FNAC is a simple procedure which can aid in the pre-operative diagnosis of these lesions. This study is an eight year retrospective study in which salivary gland aspiration cytology cases having oncocytic cells and with available corresponding histopathology were included. These slides were reviewed for features like cellularity, presence of oncocytic cells, glandular elements, squamoid cells, nuclear atypia, mitosis, lymphoid tissue, necrosis. Twenty cases were included in the study. The mean age of presentation was 60 years showing male preponderance with parotid gland being the most common site of involvement. Concordant diagnosis on cytology and histopathology was seen in 16 cases and discordance was seen in 4 cases. All the discordant cases were reported as benign on cytology but on histopathology they were labelled as acinic cell carcinoma, squamous cell carcinoma, mucoepidermoid carcinoma and an intraparotid lymph node respectively. Review of discordant cases showed subtle findings like ill-formed acini, cytoplasmic vacuolation, goblet cells and dysplastic foci raising suspicion of a different diagnosis. The potential areas of pitfall and cause of discrepancy have been discussed in this study. It is crucial to be aware of the spectrum of lesions in which oncocytes are seen, to enable an accurate diagnosis on cytology. Careful evaluation of smears for subtle clues can minimize errors.

Introduction

Oncocytes are epithelial cells with abundant, densely granular eosinophilic cytoplasm due to the presence of numerous mitochondria. These cells have a central round enlarged nuclei with distinct nucleoli. Oncocytes are seen in various organs and tissues like salivary gland, thyroid, parathyroid, kidney, pituitary, liver, pancreas, lacrimal glands, buccal mucosa, oesophagus, and others. Within the salivary glands, presence of oncocytes pose a diagnostic challenge as they could be part of diverse processes like hyperplasia, metaplasia, or neoplasia. Fine needle aspiration cytology (FNAC) is a simple, cost effective, easy, and reliable procedure for preoperative diagnosis of various lesions with some limitations. Studies have reported a good sensitivity and specificity of FNAC in evaluation of salivary gland tumours. The sensitivity ranges from 86 to 100% and specificity between 90 and 100% in differentiating non-neoplastic from neoplastic lesions and primary from metastatic lesions [1, 2]. The diagnostic accuracy varies widely between 48 and 94% when FNAC is used in subclassification of salivary gland tumours [3, 4]. This is usually attributed to the considerable morphological overlap between various benign and malignant entities. The presence of oncocytic cells, basaloid cells, metaplastic squamous cells seen in various benign and malignant lesions, add to the diagnostic dilemma [5]. Various studies have attempted to provide a simple reporting format from a two-tiered to a six-tiered system with terminologies like atypical, suspicious, and malignant to classify and guide further management and ascertain risk of malignancy. The Milan system of reporting salivary gland cytopathology provides a comprehensive and reproducible approach for the same; however, overuse of terminologies like” atypia of undetermined significance” tends to dilute the diagnostic effectiveness [6].

The present study aims to delineate the cytomorphological features of salivary gland tumors having oncocyte predominant smears with emphasis on potential diagnostic pitfalls.

Material and Method

This is an eight year (2013–2020), retrospective, descriptive study, done in the pathology department of a tertiary care center. All salivary gland aspiration cytology cases having oncocytic cells and with available corresponding histopathology were included in the study. The demographic data and site were obtained from the medical records. The Papanicolaou and May Gruenwald Giemsa-stained slides of these cases were reviewed for features like cellularity, presence of oncocytic cells, glandular elements, squamoid cells, nuclear atypia, mitosis, lymphoid tissue, and necrosis.

Results

A total of 574 salivary gland cytology cases, over a period of eight years (2013–2020) were reviewed. Thirty-five cases had a sizeable oncocytic cell population. Of these, 20 cases with a corresponding histopathology diagnosis were included in the study. The age at presentation ranged from 21 to 80 years (median: 60 years). A male predominance was seen (M:F—2.3:1). The most affected gland was parotid (80%) with a mean size of 3.19 cm. A concordant benign and malignant diagnosis was seen in 16 (80%) cases. Four (20%) cases with a benign cytological diagnosis were proved malignant on histopathological examination of the resected specimen. Tables 1 and 2 shows the cytological features of concordant and discordant cases, respectively.

Table 1.

Cytologic features of concordant cases

Concordant cases (n)	Oncocytic cells	Squamoid cells	Glandular cells	Nuclear pleomorphism	Nucleoli	Mitosis	Lymphocytes	Cyst macrophages	Background
Warthin tumor (9)	9	0	3	0	3	0	9	5	Necrosis- 2 Hemorrhage-4 Proteinaceous-2
Oncocytoma (3)	3	0	0	0	0	0	1	1	Necrosis -1 Proteinaceous-1
Adenocarcinoma (Salivary duct carcinoma-3, Adenocarcinoma NOS-1)	3	1	2	4	4	1	1	0	Necrosis- 3 Hemorrhage -3

Table 2.

Cytologic features of discordant cases

	Histopathology Diagnosis	Cytology Diagnosis	Cellularity	Oncocytic cells	Squamoid cells	Glandular cells	Nuclear pleomorphism, nuceloi, mitosis	Lymphocytes	Cyst macrophages	Background
1	Acinic cell carcinoma	Benign oncocytic lesion	Cellular	Present	–	Acini + (on review)	–	–	–	Proteinaceous
2	Metastatic squamous cell carcinoma	Warthin tumour	Cellular	Occasional	Present	Acini +	Single foci of pleomorphic cells (on review)	+  +  +	+	Hemorrhage
3	Mucoepidermoid carcinoma (Low-grade)	Warthin tumour	Cellular	Occasional	Occasional	Goblet cells (on review)	–	+  +	+	Proteinaceous
4	Intraparotid lymphnode	Warthin tumour	Scant epithelial elements	Occasional	–	–	–	+  +  +	–	Proteinaceous

Amongst the concordant cases, benign entities included Warthin tumor (n = 9) which radiologically showed well-defined, cystic, heterogenous enhancing mass with hypodense areas, consistently showed clusters of oncocytic cells, lymphocytes, and cyst macrophages on cytology smears. Oncocytoma (n = 3) presented as a well-defined, heterogeneously enhancing mass on radiology and showed sheets of oncocytic cells with scant other elements on cytological examination. The malignant cases were either labelled positive (n = 3) or suspicious for adenocarcinoma (n = 1) on cytology. These cases showed a poor delineation, infiltration along with hypodense areas on radiology. All these cases showed a dual population of epithelial elements including oncocytic cells, squamoid cells and glandular cells. Nuclear pleomorphism, prominent nucleoli and a necrotic background were other consistent features on cytology smears.

Amongst the discordant cases, Case 1 was labelled as benign oncocytic lesion based on sheets and acinar arrangement of oncocytic cells and radiology findings of a well-defined heterogeneously enhancing mass; however, the histopathology of resection specimen revealed an acinic cell carcinoma. Review of cytology slides showed occasional foci of vague acini formation and cells with cytoplasmic vacuolation, pointing to a possibility of acinar cell population (Fig. 1). Case 2, 3, 4 presented as well-defined lesions with hypodense regions and were called Warthin tumour on cytology owing to presence of oncocytes, squamoid cells and abundant lymphocytes; however, they were diagnosed as metastatic squamous cell carcinoma, mucoepidermoid carcinoma and an intraparotid lymph node respectively on histopathology. On review of case 2, a single foci of cell cluster with atypical nuclear features was noted raising the suspicion of a malignant diagnosis. Review of case 3, did yield occasional goblet cells and occasional cells with mild nuclear changes and case 4 few oncocytic cells but no atypical nuclear features, were seen (Figs. 2, 3).

Fig. 1.

Cytology smears of discordant case 1 showing clusters and of oncocytic cells (a). The cells are polygonal with round central/eccentric nulceli, prominent nucleoli and abundant granular cytoplasm (b). Few cluster with vague acini formation (c) and cytoplasmic vacuolation (d), which are cytological indicators of an acinic cell population. (Pap, 200×)

Fig. 2.

Cytology smears of discordant case 2 showing foci of clusters with peripheral cells showing nuclear atypia (a), nuclear hyperchromasia and pleomorphism (b). Squamoid polygonal cells with granular cytoplasm (c) and anucleate squames (d). The presence of squamous cells is a potential pitfall as they can be a reactive population in Warthin tumour and a dominant population of metastasis from a well differentiated squamous cell carcinoma. (Pap, 200×)

Fig. 3.

Cytology smears from discordant case 3 showing low grade nuclear changes in the squamous cells (a), cyst macrophages, goblet cells (b) and squamous cells (c), which could be a possible indicator of a low grade mucoepidermoid carcinoma. Smears from case 4 with a predominant lymphoid population and few oncocytic cels cluster (d). A smear from a cystic Warthin tumour with predominant lymphoid cell population, few oncocytic cell clusters (e). A cluster of ciliated cells (e, inset) is an important feature used for diagnosis of Warthin tumour and its differentiation from mimics. (Pap, 200×)

Discussion

Oncocytes in salivary gland lesion can be broadly grouped under the categories of metaplastic, hyperplastic, or neoplastic. The tumours which show predominant oncocytic differentiation include oncocytoma and Warthin tumour. Other tumours like acinic cell carcinoma, mucoepidermoid carcinoma, and rarely salivary duct carcinoma can also show an oncocytic differentiation [7].

Warthin tumour (WT) is the second most common benign salivary gland tumour. It is primarily seen in the parotid gland and periparotid lymph nodes due to the proliferation of the ductal structures entrapped in the parotid associated lymph nodes during development [8, 9]. Smoking has a strong causal association and can cause metaplasia of the parotid duct epithelium. The typical features seen on cytology include the presence of oncocytic cells in sheets and papillary fragments, lymphocytes, cyst macrophages and necrotic background, though the predominance of one component over the other leads to challenges in diagnosis [10]. Lymphoid component was present in all cytologically diagnosed cases of WT in the present study.

Lymphocyte predominance can be seen in aspirates from lymphoepithelial lesion, intraparotid lymphnodes, lymphoepithelial cysts, lymphoma or lymphoid infiltrates surrounding the tumour [8]. A careful observation of the cytology smears for a mixed lymphocyte population, and an epithelial element e.g.- oncocytes, acinar cells, ductal cells and ciliated cells should be done along with radiological features to confirm or rule out a WT [9]. On the other hand, presence of scant lymphocytes and sheets of oncocytes can suggest a diagnosis of oncocytoma, but WT should still be considered in the differential. Occurrence of singly scattered oncocytes and mild epithelial atypia favour a diagnosis of oncocytoma over Warthin tumour.

The presence of squamoid cells is an area of caution. Squamous metaplasia is a frequent finding in WT. Presence of reactive changes, atypical squamous cells, and necrosis with scant oncocytes, and lymphocytes can be confused with infected epidermal inclusion cyst or a metastatic squamous cell carcinoma. Degenerated oncocytes may look atypical and mimic squamous cells posing a diagnostic challenge [8]. Oncocytes themselves can be a metaplastic finding in the salivary gland with sialadenitis surrounding a metastatic or primary malignancy. Primary squamous cell carcinoma of parotid gland is rare and most of these carcinomas are metastatic deposits in an intraparotid lymph node. A clinical correlation with the history of a head and neck squamous cell carcinoma can help to narrow down the diagnosis [9].

Low grade mucoepidermoid carcinoma is another area of diagnostic difficulty owing to certain overlapping features like cystic change, presence of squamous cells, bland cytological features, lymphocytes, and scant oncocytes. In such a case a careful observation should be made for epidermoid cells, intermediate cells, goblet cells and mucin in the background [11]. In the present series the review of discordant case 3 did show occasional clusters with nuclear dysplasia in the squamoid cells. Such a finding could be a potential pointer to a low grade malignancy as well as a potential pitfall as it could a reparative feature as well. Caution should be exercised in reporting of such isolated features [8, 11].

Oncocytoma is a rare benign tumour of salivary gland characterised cytologically by sheets of oncocytic cells, the background is usually clear, and lacks lymphocytes. Such a cytomorphology overlaps with several nonneoplastic and neoplastic entities [9]. Oncocytic metaplasia is a common finding in the ductal cells in old age, and chronic sialadenitis. Oncocytosis is a nodular or diffuse proliferation of oncocytic cells and lack a defined capsule. Oncocytic variant of pleomorphic adenoma shows oncocytic cells along with myoepithelial cells, and chondromyxoid matrix [8, 12]. A radiological finding of a circumscribed heterogeneously enhancing mass along with cytology lacking myoepithelial cells favors the diagnosis of oncocytoma in such a scenario [12]. Malignant entities with oncocytic cytology include oncocytic carcinoma, acinic cell carcinoma, oncocytic variant of mucoepidermoid carcinoma, and salivary duct carcinoma. Most high-grade malignancies can be differentiated on cytology by presence of nuclear atypia, mitosis, and a necrotic background. Similar features were seen in the present study with salivary duct carcinoma and adenocarcinoma NOS having a concordant cytology and histopathology diagnosis [8, 12].

Acinic cell carcinoma is cytologically characterized by presence of clusters and sheets of polygonal cells with eccentric nuclei and abundant granular, vacuolated cytoplasm along with lymphocytes and stripped nuclei in the background [9]. Low power examination may prove difficult to differentiate between acinic cell carcinoma and oncocytoma, however a careful high-power examination to see cytoplasmic vacuolation, and stripped nuclei in the background can be helpful features. Oncocytic carcinoma can result from malignant transformation of a preexisting oncocytoma and show considerable nuclear atypia, mitosis, and necrosis on cytology smears [8, 12].

FNAC is a safe, cost effective procedure which has emerged as a useful tool for preoperative assessment and planning management of salivary gland tumors. Although it has an overall accuracy of around 81–98%, there are grey zones when it comes to specific diagnosis on cytology [5]. Salivary gland tumors owing to their considerable overlap in morphology prove to be challenging in this regard. An adequate cytological sampling including multiple passes to different regions of the tumour or a guided procedure can help to reduce the effect of tumour heterogenicity on the cytological diagnosis [3]. Furthermore, FNAC induced changes like tumor infarction, cystic change, squamous metaplasia, giant cell reaction can prove to be deterrent in diagnosis in further repeat FNAC and frozen sections. A knowhow of the overlapping cytomorphological features, along with clinical and radiological findings can aid in reducing fallacies and promote better patient management [4, 5].

Conclusion

The presence of oncocytes in cytology smears from salivary gland lesions are a diagnostic challenge. It is crucial to be aware of the spectrum of lesions in which these cells are seen, to enable an accurate diagnosis on cytology. Careful evaluation of smears for subtle clues as discussed above can minimize errors, thereby helping in the management of the patient.

Author contribution

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by PP, NR, NVK and SS. The first draft of the manuscript was written by VKS and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

The authors did not receive support from any organization for the submitted work.

Declarations

Conflict of interest

All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.