Ⅰ. INTRODUCTION
Mucoepidermoid carcinoma(MEC) is one of the most common salivary gland malignancies. MEC comprises 2.8 - 15.5% of all salivary gland tumors,1) and accounts for 30 – 40% of all malignancies of the salivary glands.2,3) The World Health Organization(WHO) defines salivary mucoepidermoid carcinoma as “a malignant glandular epithelial neoplasm characterized by mucous, intermediate and epidermoid cells, with columnar, clear cell and oncocytoid features”.4)
As its name implies, the MEC is composed of a mixture of mucus-secreting cells and epidermoid cells.5) MEC demonstrates highly variable clinical behavior, ranging from slow growing to locally aggressive and highly metastatic tumors.6) Histologically, mucoepidermoid carcinomas have been categorized into one of three grades : low, intermediate, or high grade.7) This classification based on the relative proportion of cell types.1) low grade tumors show a relatively high proportion of mucous cells, prominent cyst formation, minimal cellular atypia.8) High grade tumors consist of islands of squamous and intermediate cells, which can demonstrate considerable pleomorphism.9) Mucus-producing cells are rare, and the tumor sometimes can be difficult to distinguish from squamous cell carcinoma. Intermediate-grade tumors show features that fall between those of the low-grade and high- grade neoplasms. All three major cell types are present, but the intermediate cells usually predominate.6,9)
Parotid gland is the predominant site of MEC.10) But it can also occur in the submandibular and minor salivary glands. Although the MEC is the most common salivary gland tumor in both adults and children, MEC of sublingual gland accounts for only 1% of epithelial salivary tumors.11)
In this report, we present a rare case of 47-year-old man with MEC of the sublingual gland and discuss the histological types and management of MEC.
Ⅱ. CASE REPORT
A 47-year old man was presented with the chief complaint of swelling in the floor of the mouth. His past medical, dental, social, and family history was unremarkable.
On extraoral physical examination, there were no signs of increased volume or facial asymmetry. On intraoral examination, a solitary, well-defined ovoid swelling was noticed on the left side of the floor of mouth(Fig. 1). The overlying mucosa was erythematous and the surface was smooth. He did not have any symptoms of pain during eating, drinking, or the opening of the mouth.
A panoramic radiograph revealed a permanent set of the dentition without any osseous lesion(Fig. 2). For the diagnostic purpose, incisional biopsy through intraoral approach was done. Part of the lesion sized 2.8 x 1.3cm was removed and sent for histopathological examination. The lesion was diagnosed as intermediate-grade MEC.
Based on the result of histopathological examination, enhanced CT and MRI imaging were taken(Fig. 2). The examinations show about 32x15x27mm sized lesion in left sublingual space with no definite mandible invasion. No lymphadenopathy was noticed and all major organs were unremarkable.
Because of the malignancy, complete excision was planned after the diagnosis. 2 weeks after the biopsy, complete excision of tumor was performed under general anesthesia. The mass was excised and sent for histopathological examination( Fig. 3). The borders were mostly well-circumscribed with no inflammatory changes in the adjacent tissue, and no suspicious lymph nodes or destructive bone lesions were noticed.
Histopathological examination confirmed the lesion as intermediate grade MEC of sublingual gland(Fig. 4). Following the surgery, the healing was uneventful and a regular follow up of the patient is ongoing. At the time of follow up at 6 months, all margins were free of tumor and no lymph node involvement was present. Postoperative radiotherapy was performed with a dose of 60 Gy. And the patient was followed up in every 3months after the therapy.
Follow up of the patient for a period of 1 year did not reveal any clinical evidence of recurrence or newly developed lesion. Although long-term regular check-up is necessary, the result was stable and the patient was satisfied with the result of the treatment.
Ⅲ. DISCUSSION
The majority of salivary gland neoplasms are benign and since clinical presentation of MEC often takes on a benign appearance of a soft tissue mass, they are frequently misdiagnosed clinically and radiographically as a cyst or benign tumor.12) It makes timely and accurate diagnosis of salivary gland malignancy challenging. However, the malignant characteristics of these neoplasms call for efficient and effective care. Therefore, clinicians must be aware of the differential diagnosis.
Sublingual gland is rare site for the tumors of the major salivary glands. It accounts for less than 0.5% of all cases.13) MECs are rarely found in sublingual gland and difficult to distinguish from a submandibular gland or a minor salivary gland tumor in the floor of the mouth. In a study by Granic et al., sublingual/submandibular MECs had poor prognosis compared with parotid or other minor salivary gland MECs.14) Hence, it is important to diagnose the tumor of sublingual gland in its early stages.
The prognosis of major salivary MEC depends on the grade and clinical stage, site of the tumor. Above all, histologic grade is the most important prognostic factor. Low-grade tumors are reported as less invasive and patients with lowgrade tumors generally have a good prognosis.8) They invade locally and recur in about 15% of cases with a 5-year survival rate over 90%.15) High-grade tumors are associated with lower 5-, 10-, and 15-year survival rates (0%-22%) compared with low-grade tumors (90% - 100%).15,16,17) The prognosis for those with intermediate-grade tumors is slightly worse than that for low-grade tumors.6)
We should consider clinical, imaging and pathological features together with management. Most cases of MEC are managed surgically, with treatment dependent on the disease stage and the presence of distant metastases. Radical surgical resection is widely accepted treatment for MEC of the salivary gland.18) The prognosis of radical treatment is better than conservative treatment like enucleation or curettage. Local recurrence after conservative treatment was 40% in contrast to 13% of which after radical treatment.9) Low-grade tumors are show good 5-year survival rates with local excision. For low-grade tumors, only a modest margin of surrounding tissue can be removed, but high-grade are generally treated with wider margin resection. High-grade tumors are generally acknowledged to require additional adjuvant radiotherapy and neck dissection to control the disease.9)
Postoperative radiotherapy for MEC patients with surgical margin has been reported to decrease local failure.18,19,20) Radical surgery followed by postoperative radiotherapy for salivary gland malignancies has improved local control, but it is difficult to control salivary gland cancer by radiotherapy alone.21)
In conclusion, MEC of the sublingual gland is a rare malignancy in salivary gland, and can be confused with cystic lesion or benign conditions. This case highlights the importance of diagnosis for optimal patient management and the histopathological examination played a critical role to confirm the diagnosis. Clinicians should consider mucoepidermoid carcinoma in the differential diagnosis of salivary gland neoplasm. Surgical excision with clear margins and additional radiotherapy seems to be an effective treatment option for intermediate grade mucoepidermoid carcinoma of the sublingual salivary gland.
