Discordance between fine-needle aspiration cytology and histopathology in patients with mucoepidermoid carcinoma of parotid gland

Fine-needle aspiration cytology (FNAC) is an important diagnostic tool used preoperatively for the diagnosis of a parotid lump.^1,2 It has good accuracy and helps to plan the extent of parotid surgery.² The procedure itself is less invasive and yields good result in experienced and skilful hands.² The opposing school of thought warns against the use of FNAC as it has a high false positive rate.^3,4

Parotid gland tumours are a heterogeneous group. They comprise 3% of all head and neck cancers and 0.6% of all tumours of the human body.⁵ Mucoepidermoid carcinoma comprises 5–10% of all salivary gland tumours and is the most common malignant tumour of salivary glands.⁶ The majority of mucoepidermoid carcinomas are found in the parotid gland.⁶ Mucoepidermoid carcinoma poses a diagnostic challenge on FNAC with high false negative rate.^3,4 Diagnosis of mucoepidermoid carcinoma is made by the presence of mucous, intermediate and squamous cells in the background.⁷ Low-grade mucoepidermoid carcinoma presents as a cystic lesion with difficulty in obtaining cells for smear, which leads to underdiagnosis.^8,9 High-grade mucoepidermoid carcinoma mimics squamous cell carcinoma, which results in overdiagnosis of the lesion.⁶ The objective of this study was to evaluate the discordance between cytology/FNAC and histopathology in patients with mucoepidermoid carcinoma.

A cross-sectional study was conducted at department of surgery in a tertiary care hospital, from 1 January 2010 to 31 December 2014. Institutional ethical review committee exemption was obtained before commencement of the study. Adult patients with FNAC or histopathology suggestive of mucoepidermoid carcinoma were identified by the hospital information management system. Patients with a history of receiving prior treatment or those with missing records were excluded from our study. A total of 16 patients fulfilled our eligibility criteria.

FNAC was carried out using a 22 gauge needle attached to a 5cc or 10cc syringe. All FNAC was done with the guidance of ultrasound in the radiology suite. At least two passes were made in the tumour. The collected specimen was expressed on to a glass slide and prepared with Papanicolaou or Giemsa stain.

Descriptive statistics were used to summarise patient demographic details. Mean and standard deviations (SD) were reported for normally distributed continuous variables. Frequency and percentages were reported for categorical variables. FNAC when compared with histology (gold standard) was classified into true positive (presence of mucoepidermoid carcinoma correctly diagnosed on FNAC), true negative (absence of mucoepidermoid carcinoma correctly diagnosed on FNAC), false positive (FNAC incorrectly diagnosed mucoepidermoid carcinoma), false negative (FNAC failed to diagnose mucoepidermoid carcinoma).

There were 16 cases with mucoepidermoid carcinoma of the parotid gland diagnosed on FNAC or histopathology. Of these, 10 were male and 6 female. The age ranged from 13 years to 70 years with a mean of 44.8 years (SD 17.9 years). All 16 patients presented with swelling in their parotid glands, with one patient recently developing facial weakness involving the facial nerve (Table 1).

Most of our patients (12; 75%) had ultrasound for evaluation of parotid swelling. Four patients had magnetic resonance imaging (MRI), including the patient with facial nerve weakness. The tumour was limited to the superficial lobe of the parotid in 15 patients. The patient with facial weakness had a tumour extending to the deep lobe, sparing the mandible.

Eight patients were diagnosed with mucoepidermoid carcinoma on cytology, four patients had an inconclusive result and four were diagnosed with pleomorphic adenoma (Table 2). When compared with histopathological findings, seven cytological samples were true positive (ie correctly diagnosed as mucoepidermoid carcinoma by FNAC), eight cytological specimens were false negative (ie could not pick up mucoepidermoid carcinoma on FNAC). One case was false positive on cytology (ie diagnosed mucoepidermoid carcinoma on FNAC but was reported to be Warthin’s tumour on histopathology) and none were true negative (Table 3).

Mucoepidermoid carcinoma is the most common malignancy of the salivary gland.⁶ The majority are seen in the parotid gland.⁶ High-grade neoplasms are easily recognised and appropriate medical treatment is initiated. On the other hand, low-grade tumours are relatively difficult to diagnose as malignancy, which leads to treatment delay.^6,10 In our current series, seven cases were accurately diagnosed as mucoepidermoid carcinoma on FNAC. Eight cases were false negative and we reported one case of false positive mucoepidermoid carcinoma.

The simultaneous presence of epidermoid (squamoid) cells, intermediate cells and mucous cells in the presence of mucinous background constitute a definitive diagnosis of mucoepidermoid carcinoma.⁷ This is difficult to obtain on cytology, especially in the case of low-grade mucoepidermoid carcinoma, owing to the varying degree of cystic fluid.^8,9 They may be confused or misinterpreted with other benign lesions or mucus cyst.^2,7 Three of our cases were labelled inconclusive on cytology, which later were diagnosed as low-grade mucoepidermoid carcinoma on histopathology. This was because of a lack of adequate cellularity to make a definitive diagnosis.

Case number one was a false positive; a 70-year-old man who had cytology suggestive of low-grade mucoepidermoid carcinoma but histopathology revealed Warthin’s tumour. A similar scenario is also reported by other authors; this could be because of bland cytological characteristics, paucicellularity and the non-representative nature of the aspirate.¹ Two patients with intermediate-grade mucoepidermoid carcinoma; one was reported as inconclusive and the other was pleomorphic on cytology. It is an accepted pitfall, as reported by Kotwal et al,¹¹ where 75% of the lesions were observed to be reported as pleomorphic adenoma. The presence of metaplastic squamous and goblet cells adds to the diagnostic dilemma.¹² Other authors have also suggested that metaplastic squamous cells may cause this error.^2,13

Three cases (5, 6 and 8) were underdiagnosed as pleomorphic adenoma on cytology and were reported as high-grade mucoepidermoid carcinoma on histopathology. It has been shown that the trauma caused by fine-needle aspiration in parotid tissue leads to metaplastic squamoid cells.¹² These cells are commonly seen in pleomorphic adenoma, together with chondromyxoid material, thus posing a challenge for the cytopathologist.¹⁴ To our knowledge, no previous case of high-grade mucoepidermoid carcinoma has been underreported to be pleomorphic adenoma. We were unable to review the slides as cytology was performed outside our institute; this is a limitation of our study.

Grading of mucoepidermoid carcinoma is done based on proportions of various cell types, the ratio of solid to cystic component and degree of cytomorphological atypia.² Histological grading is used as a prognostic indicator.¹⁴ Some authors recommend using a two-category system (low and high) as it is more reproducible than a three-grade system.⁶

Our study is limited by a small sample size. Sensitivity, specificity, positive and negative predictive value of a small study sample cannot be generalised, so these parameters were not reported. The accuracy of cytology in our study was less than 50%, making it unreliable for a definite diagnosis of mucoepidermoid carcinoma. However, we do suggest the routine use of FNAC for parotid lesions.

FNAC should not be the only tool to rely on in the diagnosis of mucoepidermoid carcinoma. Presence of all three cell types make a definitive diagnosis that can be missed on FNAC. In the case of inconclusive results, either a repeat FNAC or core biopsy should be offered. We recommend a high index of clinical suspicion in cases with features of malignancy given the poor yield of cytology.