Ⅰ. INTRODUCTION
Salivary gland lesions usually present as enlargement of the glands, which need to be differentiated between a tumor, an inflammatory process, or a lymphadenopathy. Although open surgical biopsy provides information on the exact nature of the swelling, it is not advocated as a standard diagnostic procedure due to complications such as incisional contamination, facial nerve injury, and fistula formation1). Alternatively, fine needle aspiration biopsy (FNAB) is used as the primary choice for differentiation of tumorous and non-tumorous lesions since it is safe, simple, non-invasive, and cost-effective2). FNAB may be beneficial in evaluating ill-defined masses that are clinically suspicious of malignancy and in planning extent of the surgical procedure1). On the other hand, some salivary gland masses are non-surgically treatable, such as lymphoma, and FNAB may help in the diagnosis of such pathologic conditions1). However, evaluation of salivary gland lesions with FNAB is controversial due to various morphological patterns and overlapping characteristics between benign and malignant lesions, making diagnosis between the two difficult occasionally3). According to previous studies, high specificity and sensitivity of FNAB have been reported for benign salivary gland lesions, while sensitivity as low as 29% has been reported in detecting malignancies, which may be attributed to several factors4-9). The purpose of this study is to present a case of mucoepidermoid carcinoam(MEC) of parotid gland diagnosed as pleomorphic adenoma by FNAB and discuss its limitations in the pre-operative evaluation of malignant salivary gland tumors.
Ⅱ. CASE REPORT
A 45-year-old woman visited the Department of Oral and Maxillofacial Surgery at Seoul St. Mary’s Hospital with a chief complaint of lump in the right side of the mandible. The mass was first noticed about 1 year ago as a painless swelling, which became painful during the past few months. Clinical examination revealed firm mass in the right angle of the mandible near the ear lobe and intact facial nerve functions. Computed tomography (CT) showed about 31x 29x31mm sized mass involving the posterior aspect of right masseter muscle and anteroinferior aspect of right parotid gland with nonspecific lymph nodes in level II(Fig. 1).
The mass had relatively well-defined contour with parotid gland and ill-defined margin with masseter muscle, which suggested nodular fasciitis with differential diagnosis of myositis or benign mixed tumor of parotid gland with inflammatory change. In order to rule out the possibility of malignancy, FNAB was carried out. The aspirate smear showed cohesive cluster of epithelial cell and some cells were spindle with bland nuclei which are features of pleomorphic adenoma(Fig.2). However some aspirate smears showed a well-defined cytoplasm and some cells with vacuolated cytoplasm, which are characteristics of MEC(Fig.3).
With the results of pre-operative evaluation, superficial parotidectomy was performed under general anesthesia. The mass showed ill-defined margin with masseter muscle. The histology of the tumor showed numerous mucous cells surrounding cystic structures and sheets of epidermoid cells with focal mucous cells suggesting MEC (H&E; x100) (Fig.4). However, some specimen showed epithelial cells forming ducts and cystic structures with some cells having plasmacytoid appearance that could be diagnosed as pleomorphic adenoma. (Fig.5). It also showed vein and perineural invasion. The final diagnosis was high grade mucoepidermoid carcinoma. Therefore, further resection with right level II selective neck dissection was performed, and lymph node was negative for malignancy. The patient was decided to go under post-operative radiotherapy. Currently the patient is being regularly followed-up, and 1-year post-oper-ative examination and imaging showed no sign of recurrence.
Ⅲ. DISCUSSION
The application of FNAB in salivary gland lesions was first promoted by Eneroth in 1950 and 1960 and used for the diagnosis of salivary tumors for decades, as well as for differentiating neoplastic lesions from non-neoplastic lesions 10),11). However, reported sensitivity of salivary gland FNAB has varied widely between studies. Possible explanations related to the wide range of sensitivity may be due to technical issues, such as experience of the clinician performing the FNAB and the immediate accessibility of expert cytopathology examination to evaluate specimen adequacy 10).
Mucoepidermoid carcinoma (MEC) is the most common malignant tumor of the parotid gland, comprising between 2.8% and 15.5% of all salivary gland tumors and between 12% and 29% of malignant salivary gland tumors12),13). Use of FNAB in the diagnosis of MEC has been challenging, and several reasons for false-negative result have been identified in the previous literatures. The most common reason was sampling error10). Malignant mixed tumors are often composed of both benign and malignant characteristics. Therefore, use of only cytopathology diagnosis in the diagnosis of MEC is challenging due to the heterogeneous cellular population and scant cellularity10). Also, some samples cannot be evaluated because of poor cellularity or quality. In these particular cases, aspirates from multiple sites are necessary for more accurate diagnosis12).
Another reason for false-negative results is due to its cytodiagnosis and cytological typing since majority of the aspirated material is consisted of partly degenerated epithelial cells12). Other possible reason may be due to fluid dilution of tumor cells, inflammatory cells, and debris which may hinder the diagnosis of tumor cells12). In this case report, a diagnosis of pleomorphic adenoma was made with FNAB. This could be associated with the inadequate cellularity of the smears and lack of presence all the cell types (intermediate, squamous and mucin-secreting cells) necessary to diagnose MEC14). These cells are not always clearly identified, especially in low-grade MEC, which is the most challenging neoplasm to diagnose with FNAB14). Low-grade tumors are easy to be misdiagnosed which could lead to delay in treatment or inappropriate pre-operative management 15). Some salivary gland malignancies can only be diagnosed through the presence of capsular or perineural invasion, which is not possible through FNAB2). Therefore, the surgeon should always be aware that MEC has similar cytomorphology with benign lesions, and it is recommended to plan the surgery with the possibility of malignancy and frozen section examination must be performed to check the tumor invasion of the resection interface.
In conclusion, a negative FNAB result should not replace clinical and radiological findings in the management of salivary gland tumors. Due to the heterogeneous characteristics of MEC, multiple representative sampling and awareness of its morphologic complexity is critical to an accurate diagnosis and proper treatment.
