The mucoepidermoid carcinoma is the second most common tumor of the salivary glands overall, the most common salivary gland malignancy overall, and the most common minor salivary gland malignancy (Auclair and Ellis 1991). During the greater than 60 years since its first description, this neoplasm has generated significant debate regarding the possible existence of a benign variant, the optimal number of grades, and the proper treatment for certain minor salivary gland lesions. The term "mucoepidermoid tumor" was first introduced by Stewart, Foote, and Becker in 1945 in their publication of 45 cases (Stewart, Foote, and Becker 1945). In this report, only two grades were utilized, including relatively favorable (benign) and highly unfavorable (malignant) tumors. The authors indicated that the adjective "benign" was rarely ever applicable in an absolute sense and as used in their report did not imply innocent behavior. It did indicate, however, that the authors had not observed metastasis from these tumors. The designation "malignant" indicated a histologic structure that was associated with the ability to produce regional lymph node and distant metastases. This notwithstanding, the authors explicitly referred to and separated the benign and malignant tumors in their series of 45 cases in this report, of which there were 26 "benign" tumors and 19 "malignant" tumors. In 1953 this grading scheme was modified to include three grades due to the development of metastases related to tumors previously referred to as benign (Foote and Frazell
1953). These investigators accepted all of these tumors as malignant, and clinical and pathologic correlation suggested that separation into low-, intermediate-, and high-grade malignant subgroups might be useful, mainly due to histologically overlapping qualities. The designation of intermediate grade was recognized as behaving more like the low-grade tumors than the high-grade tumors. Interestingly, despite the authors' recognition that all of these tumors were malignant, the designation "mucoepidermoid tumor" persisted throughout their paper. Subsequent studies were undertaken to more objectively determine if a benign variant existed. One such study investigated 23 mucoepi-dermoid carcinomas with a malignant course, such as evidence of local extension of tumor outside the capsule, local recurrences, histologically verified metastases, or death due to the tumor (Eneroth et al. 1972). Fifteen patients showed local recurrences, 13 showed histologically verified metastases, and 22 patients died of their disease. In 7 of the 23 cases the histology revealed highly or moderately differentiated structures, and in 3 of these cases the primary tumor as well as the lymph node metastases were highly differentiated. Six of the 23 patients had tumors in the palate, with 2 of these patients developing recurrences, 1 with lymph node metastases, and 5 of the patients died due to their disease. The authors concluded by stating that well-differentiated metastases in cases with a malignant course contradicted the existence of a benign variety of mucoepidermoid carcinoma, such that all of these neoplasms should be considered cancers (Eneroth et al. 1972).
Of the 712 mucoepidermoid carcinomas occurring in the minor salivary glands in the AFIP registry, 305 (43%) of them were noted in the palate, 93 (13%) in the buccal mucosa, and 58 (8%) in the lip, with 37 specifically designated as the upper lip and 12 specifically designated as the lower lip (Auclair, Ellis, and Gnepp et al. 1991). While the AFIP data is generally recognized as being representative of the incidence of most salivary gland tumors, some authors have identified the mucoepidermoid carcinoma to be more common in minor salivary gland sites than in major salivary gland sites (Plambeck, Friedrich, and Schmelzle 1996).
Histologic grading of mucoepidermoid carcinomas is an important exercise. Histologic grade connotes biologic aggressiveness and prognosis and also provides the surgeon with important information with which to plan surgical treatment (Brandwein, Ivanov, and Wallace et al. 2001; Evans 1984). Mucoepidermoid carcinomas are composed of three cell types: mucous secreting, epidermoid, and intermediate. The intermediate cell is appropriately named because it is likely the progenitor of the two other cells (Batsakis and Luna 1990). Three grading schemes have found general acceptance among pathologists, and differences in biologic behavior could be demonstrated as a function of grade, even though clinical stage has also been considered an important prognosti-cator. Indeed, Brandwein, Ivanov, and Wallace et al. (2001) found that only 5% of low-grade mucoepidermoid carcinomas of the major glands, and only 2.5% of low-grade mucoepidermoid carcinomas of the minor glands, metastasized to regional lymph nodes or resulted in death. Spiro indicated that survival of patients with minor salivary gland carcinoma is significantly influenced by the clinical stage and the histologic grade, but the applicability of grading to survival was limited to patients with mucoepidermoid carcinoma or adenocarcinoma in their study (Spiro, Thaler, and Hicks et al. 1991). They determined that staging was important in all patients regardless of the histologic diagnosis.
The mucoepidermoid carcinoma is the most common salivary gland malignancy in children (Auclair, Ellis, and Gnepp et al. 1991; Luna, Batsakis, and El-Naggar 1991; Ord 1994; Rogerson 1995). Although most of these tumors are noted in the parotid gland, the palate is the second most common site of involvement. Most appear to occur in teenagers, and the majority are low-grade or intermediate-grade histology. Mucoepidermoid carcinoma in children appears to follow a more favorable course with cure rates of 98-100% (Ord 1994).
Surgical treatment of the mucoepidermoid carcinoma of minor salivary gland origin is primarily a function of the anatomic site of the tumor and its histologic grade. Those arising in the palate are not only the most common but also the most variable insofar as surgical treatment is concerned. It is the histologic grade that is of utmost importance when determining treatment in the palate. Large series show that low-grade cancer is most common in this anatomic site (Pires et al. 2007). Incisional biopsy is clearly essential to establish the histopathologic diagnosis, as previously described. Computerized tomograms are essential in planning surgical treatment of palatal mucoepi-
dermoid carcinomas, as they assess the involvement of the underlying palatal bone. When the palatal bone does not appear to be involved by the cancer, a bone-sparing, periosteal sacrificing wide local excision with split thickness sacrifice of the soft palate musculature is the surgical treatment of choice (Figure 10.6). Similar to the surgery for the palatal pleomorphic adenoma, the periosteum serves as the anatomic barrier on the superior aspect of the tumor specimen, and tumor-free peri-
Figure 10.6a. A mass of the palate in a 45-year-old man.
Figure 10.6c. A periosteal sacrificing, bone-sparing wide local excision with split thickness sacrifice of the soft palate is planned with 1 cm mucosal linear margins.
Figure 10.6d. A sharp dissection is performed with a peri-osteal elevator between the periosteum on the superior aspect of the tumor specimen and the overlying palatal bone.
Figure 10.6f. The association of the superior aspect of the tumor and the periosteum is noted histologically.
Figure 10.6g. The remaining tissue bed is temporarily covered with a palatal stent.
osteal frozen and permanent sections should be obtained so as to confirm this concept. When the periosteum has not been invaded by the cancer and all radial soft tissue margins are free of tumor, this surgery has a high frequency of cure.
The designation of an intermediate mucoepi-dermoid carcinoma of the palate may change the recommended surgical treatment of the tumor in this and other anatomic sites, with a more aggressive surgical procedure required for curative intent (Figure 10.7). This is particularly true if the designation of intermediate grade is made by the pathologist based on the worst microscopic pattern observed in the tumor. For example, a mucoepi-dermoid carcinoma that is predominantly low grade, but that shows a component of intermediate-grade cancer, will likely be designated intermediate grade. The behavior of such a tumor is likely to be low grade in nature. This scenario is different from a cancer that is designated intermediate grade that shows a predominantly intermediate-grade pattern with intermixed low-grade cancer. The surgeon may wish to offer more aggressive surgical therapy in the form of a partial maxillectomy for the mucoepidermoid carcinoma of the palate that is predominantly intermediate grade on microscopic sections. While rare, a high-grade mucoepidermoid carcinoma of the palate would require a partial maxillectomy, and prophylactic surgical treatment of the neck in the case of an N0 neck, or a therapeutic neck dissection in the case of an N+ neck. Postoperative radiation
Figure 10.6h. Mucosalization of the exposed bone and soft palate musculature is noted at 9 months postoperatively. This surgery provided curative care for this patient's tumor. Reprinted with permission from Carlson ER, Schimmele SR. 1998. The management of minor salivary gland tumors of the oral cavity. In: Surgical Management of Salivary Gland Disease, The Atlas of the Oral and Maxillofacial Surgery Clinics of North America 6. Philadelphia: W.B. Saunders, pp. 75-98.
therapy would also be administered in such circumstances.
Mucoepidermoid carcinoma of the buccal mucosa is the second most common minor salivary gland site affected. In contrast to benign neoplasms of this anatomic site, a mucosal-sacrificing tumor surgery is required, with attention to the sacrifice of surrounding submucosal anatomic barriers. The same is true of the lip (Figure 10.8).
Survival of patients with mucoepidermoid carcinomas of the minor salivary glands is clearly related to grade. Five-year survival rates have been estimated at 90% and 15-year survival rates have been estimated at 82% for low-grade mucoepider-moid carcinomas (Ord 1994).
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