Introduction

Occurrence of multiple tumours in salivary gland either synchronously or metachronously has been reported. These multiple tumours can be unilateral or bilateral. The definition of hybrid tumours is not clear. When two histologically different tumour types occur in the same topographic area and share common origin, the term hybrid tumours are used. The two or more neoplasms in the hybrid tumours are sharply demarcated, possess distinct histologic, molecular and prognostic features and occur in one organ. ^[1] In parotid gland, the incidence of hybrid tumours is rare, accounting for fewer than 0.1% of the tumours.^[2]

In the hybrid tumours, most of the times, both the tumours are malignant. Few reports of combination of benign and malignant tumours have been reported. Frequently encountered combinations include adenoid cystic carcinoma (ADCC), epithelial myoepithelial carcinoma (EMC) and salivary duct carcinoma (SDC). ^[3] While EMC has been reported to occur more frequently in combination with other malignant tumours like adenoid cystic carcinoma, basal cell adenocarcinoma and salivary duct carcinoma, ^[1,4,5] occurrence of EMC with benign tumours is rare. ^[6,7]

FNAC diagnosis of EMC is usually challenging due to difficulty in defining the bimodal nature of the lesion on smears in view of the fragile cytoplasm of myoepithelial cells which are seen as naked nuclei in the background. Likewise, in the presence of another tumour in the same topographical area, the diagnostic challenge is amplified. During FNAC, the needle may pass through both the lesions, with smears showing representation from both, misleading the cytopathologist.

Herein, we report a cytologic dilemma encountered in a case of a hybrid tumour of parotid gland comprising of epithelial myoepithelial carcinoma and Warthin’s tumour which was misinterpreted as mucoepidermoid carcinoma with pleomorphic adenoma at cytology.

Case Report

A 48-year male presented to the surgery out patient department with history of left pre-auricular mass for the last 20 years duration. Initially the lesion was small with recent history of rapid enlargement in size since the past three months. The swelling was associated with pain since past three months. There was no history of fever, sudden weight loss or loss of appetite. There was no history of any discharge or itching from the skin overlying the lesion. He had a medical history of type 2 diabetes mellitus for two years and is on medications. He is a known smoker and alcoholic for past 13 years. His general examination findings were within normal limits. On local examination, a mass was palpable of size 5 x 4 x 3 cm located in the left angle of mandible. (Figure 1A) The mass was non-tender, firm to hard in consistency. Skin over the swelling was unremarkable. There was no evidence of weakness or neurologic deficit in locoregional area. Lymph nodes were not palpable. Oral examination did not reveal any significant finding.

Figure 1 A & B: Smears showing myoepithelial cells in epithelial myoepithelial carcinoma (misinterpreted as intermediate cells of mucoepidermoid carcinoma) (Hematoxylin and Eosin, x 100)

A magnetic resonance imaging (MRI) scan of neck with contrast showed a lesion which was well encapsulated, homogenously enhancing intra-parotid round lesion in the superficial lobe of left parotid suggestive of a mucoepidermoid carcinoma.

Fine needle aspiration cytology (FNAC) was performed from multiple sites within the tumour by standard technique. Haematoxylin and eosin stain, Papanicolaou stain and May Grunwald Giemsa stained slides were prepared. FNAC smears were moderately cellular comprising of cells arranged in small clusters and singles. The individual cells were small with round to oval nuclei and moderate eosinophilic cytoplasm. Admixed with these were keratinized cells in singles, oncocytes in small clusters and few histiocyte like cells. Background was dirty with lymphocytes, few neutrophils, many bare nuclei and mucin like extracellular material present in abundance. One smear showed, fibrillary myxoid stroma. A diagnosis of mucoepidermoid carcinoma in a background of pleomorphic adenoma was made. (Figures 2 to 4)

Figure 2: A & B: Smears showing cyst macrophage and oncocytes in Warthin’s tumor (misinterpreted as mucin secreting cells and oncocytes in mucoepidermoid carcinoma) (Hematoxylin and Eosin, x 100)

Figure 3: A: Smears showing basement membrane material in epithelial myoepithelial carcinoma (misinterpreted as fibrillary stroma of pleomophic adenoma).B: Basement membrane in epithelial myoepithelial carcinoma (misinterpreted as thick mucin of mucoepidermoid carcinoma) (Hematoxylin and Eosin, x 100)

Figure 4: A: Clinical image of the lesion. B: Gross: Cut surface showing solid, homogenous, whitish, friable tumour mass, with irregular margins.

Surgery was planned. Parotidectomy with radical neck dissection was performed. Gross: The parotidectomy specimen measured 6 x 4.5 x 4 cm. External surface showed an intact capsule. Cut surface showed a well circumscribed tumour located 1 cm away from inferior margin. The tumour was solid, homogenous, whitish, friable, measuring 5 x 4 x 2 cm. (Figure 1B) A capsule was noted surrounding it.The tumour showed a small suspicious area of breach in the capsule in one foci. There was no evidence of necrosis, haemorrhage or cystic change within the tumour. (Figure 4) Neck dissection specimen yielded 23 lymph nodes largest measuring 1x 1 cm and was unremarkable on gross appearance. Specimen margins were inked and were free from tumour on gross examination. Distance of tumour from closest margin (inferior) was 1 cm.

Representative bits were processed by routine processing, paraffin blocks prepared, sections obtained and stained with haematoxylin and eosin and special stains.

Microscopy: Sections studied from the tumour showed a well circumscribed, encapsulated tumour, comprising of neoplastic cells arranged in predominantly glandular pattern separated by fibrous stroma. The glands were lined by inner cuboidal epithelium having round nuclei with coarse chromatin and peripheral columnar epithelium with pale eosinophilic to clear cytoplasm. The tumour cells showed mild nuclear pleomorphism with high nucleo-cytoplasmic ratio, coarse chromatin and prominent nucleoli. The tumour nests were surrounded by distinct basement membrane. Few atypical mitosis were noted. Foci of capsular invasion was noted. (Figure 5) Periodic acid Schiff stain showed positive staining of basement membrane material surrounding the glands. (Figure 6A)

Figure 5: Sections from epithelial myoepithelial carcinoma show A: Biphasic pattern, B: Basement membrane surrounding the glands, C: Capsule with adjacent normal parotid gland and D: Capsular invasion. (Hematoxylin and Eosin, x 100)

Figure 6: A: Section shows basement membrane highlighted by PAS stain in epithelial myoepithelial carcinoma. B:Epithelial component composed of oncocytes and subepithelial lymphoid aggregates in Warthin’s tumour. (Hematoxylin and Eosin, x 100)

At a focus, 2 cm away from the above lesion, a second tumour was noted. Noting this, gross examination was revisited and the second tumour at a distance of 1 cm from the above first tumour, as described above, was observed. This tumour was well circumscribed, well encapsulated and measured 1 x 1 cm. Cut surface was solid homogenously brown without areas of haemorrhage or necrosis. At microscopy the tumour showed neoplastic cells in glandular and papillary pattern and lined by terminal layer of oncocytic columnar epithelium and surrounded by abluminal layer of basal flattened cells. Oncoytic cells has eosinophilic granular cytoplasm. Surrounding stroma showed dense lymphoid aggregates, at places forming prominent germinal centres. (Figure 6B)

All 21 lymph nodes examined showed reactive hyperplasia and were free from tumour.

Hence, a final diagnosis of Hybrid tumour - Epithelial myoepithelial carcinoma with Warthin’s tumour was made.

Discussion

Salivary gland tumours occasionally display a variety of histologic patterns, including the possibility of two different tumours developing in the same topographical region. A hybrid tumour is one that has two distinct tumours with different morphologies present in the same organ. These tumours can develop in one or both salivary glands and can progress synchronously or metachronously. ^[8-10] The reported range is from 0.2% to 0.7% in various case series and the occurrence of hybrid tumours in the salivary gland is quite rare. In the recent years, due to advancement in the imaging modalities, the incidence has been shown to have increased up to 3.4%. ^[7,8] Clinically, these tumours typically present as a solitary mass. Multiple masses can occasionally be found, and this, combined with other signs of pain or neuralgia, should raise the possibility of hybrid tumours. ^[6] Radiology is helpful for indicating the presence of this uncommon entity before surgery.

The likelihood of a hybrid tumour was neither considered clinically nor detected on radiological investigations in the current report. At radiography, the small second nodule was mistaken for an intraparotid lymph node. We aspirated from four separate places inside the tumour in an effort to ensure universal sampling. This would have resulted in the needle hitting both the tumours, which reflected in the smears and confirmed upon evaluation of the histology slides and review of cytology smears. Had the radiologist commented on existence of two separate tumours, a guided FNAC from each lesion separately would have avoided the initial discrepancy. The presence of dirty background, mucin like material, variability in cell types on the smear and lymphocytes propelled us to consider a diagnosis of mucoepidermoid carcinoma (MEC). In addition to this, mucoepidermoid carcinoma is more common as compared to epithelial myoepithelial carcinoma (EMC) and that the cytology smears of mucoepidermoid carcinoma can show variety of cells, including the lymphocytes and oncocytes. ^[2,11,12] The diagnostic conundrum is generally made more difficult by the presence of Warthin-like mucoepidermoid carcinoma, oncocytic variants of MEC and mucoepidermoid carcinoma originating in Warthin's tumour. Accurate diagnosis of EMC at FNAC is imperative in view of the recurrence and metastasis associated with it, requiring post-surgical intervention in the form of radiotherapy and or chemotherapy. ^[12] Literature on cytology of EMC is limited to few case reports and case series. In a case series by Arora SK et al ^[13], all the four cases were missed at cytology, three being misdiagnosed as pleomorphic adenoma (PA) and the other as epithelial myoepithelioma, post-surgically. ^[13] In the present case, the eosinophilic matrix material attached to the cell clusters was misinterpreted as mucin and chondromyxoid fibrillary stroma of MEC and PA respectively. Other tumours of salivary gland like adenoid cystic carcinoma, basal cell adenoma, polymorphous low-grade adenocarcinoma also shows abundant extracellular stromal matrix material. Hence it is important to accurately identify the nature of extracellular matrix material in FNAC smears of salivary gland and link it to a particular tumour in an appropriate context.

Hybrid tumour must be differentiated from other entities like collision tumour, biphasic tumour, malignant transformation in a benign tumour and a de-differentiated malignancy. ^[8] Collision tumour is the term used to describe a situation when two tumours occur in differing topographic area, invade and fuse with each other. ^[1,2] Biphasically differentiated tumours possess regular, repetitive mixture of two cellular patterns. One important example of a biphasically differentiated tumour is epithelial-myoepithelial carcinoma. Carcinoma ex pleomorphic adenoma, characterized by the development of malignant change in a pre-existing pleomorphic adenoma represents malignant transformation in a benign tumour. Likewise, when a low-grade malignancy differentiates over time to a high-grade malignancy, the term dedifferentiated malignancy is adopted. ^[14]

In hybrid tumours of salivary gland, the lesion should fulfil the following criteria: i) presence of two or more tumours ii) different morphologically as assessed by histopathology, iii) there should be no transition between two tumours, iv) two tumours should occur in same topographic area. ^[1-7,8,15] Fulfilling these criteria, there are only few case reports of hybrid tumours reported to occur in salivary gland. The tumours can be both benign, both malignant or one benign and malignant each. ^[2,3,8,14] Most common association in hybrid tumour has been reported with epithelial myoepithelial carcinoma, adenoid cystic carcinoma and salivary duct carcinoma. ^[1,4,5,8,14] Epithelial myoepithelial carcinoma and Warthin's tumour have only recently been linked in one of the 39 cases of hybrid tumours involving the salivary gland that have been documented overall, that is, by Shah Tong et al in 2021. ^[6] Other tumours have been found to co-exist with EMC, such as: adenoid cystic carcinoma, salivary duct carcinoma, mucoepidermoid carcinoma, basal cell adenocarcinoma and rarely squamous cell carcinoma. ^[8] With Warthin’s tumour, the association has been reported with sebaceous lymphadenoma. ^[15]

The presence of hybrid tumours can lead to misdiagnosis and postoperative recurrence. Hence, awareness of the condition and accurate preoperative diagnosis is important.

In conclusion, despite the many difficulties with the cyto-histopathological findings and the diagnosis of such rarely occurring conditions, we hope that this case report will educate both the pathologist and the surgeon about the existence of this uncommon condition and help them make decisions with the patient's best interests in mind.

Ethics Committee Approval: Obtained from IEC,AIMS, No 27/2023

References

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