http://ebooksmedicine.net children younger than 16 years of age. Whatever the histologic pattern, parotid gland neoplasms produce swelling in front of and below the ear. Benign tumors may be present for months to several years before coming to clinical attention, while cancers more often come to attention promptly, probably because of their more rapid growth. However, there are no reliable criteria to differentiate benign from malignant lesions on clinical grounds, and histopathologic evaluation is essential.
Pleomorphic adenomas are benign tumors that consist of a mixture of ductal (epithelial) and myoepithelial cells, so they exhibit both epithelial and mesenchymal differentiation. Epithelial elements are dispersed throughout the matrix, which may contain variable mixtures of myxoid, hyaline, chondroid (cartilaginous), and even osseous tissue. In some pleomorphic adenomas, the epithelial elements predominate; in others, they are present only in widely dispersed foci. This histologic diversity has given rise to the alternative, albeit less preferred, name mixed tumor.
Pleomorphic adenomas represent about 60% of tumors in the parotid, are less common in the submandibular glands, and are relatively rare in the minor salivary glands. They present as slow-growing, painless, mobile discrete masses. They recur if incompletely excised. Recurrence rates approach 25% after simple enucleation of the tumor, but are only 4% after wider resection. In both settings, recurrence stems from a failure to recognize minute extensions of tumor into surrounding soft tissues.
Carcinoma arising in a pleomorphic adenoma is referred to variously as a carcinoma ex pleomorphic adenoma or malignant mixed tumor. The incidence of malignant transformation increases with time from 2% of tumors present for less than 5 years to almost 10% for those present for more than 15 years. The cancer usually takes the form of an adenocarcinoma or undifferentiated carcinoma. Unfortunately, these are among the most aggressive malignant neoplasms of salivary glands, with mortality rates of 30% to 50% at 5 years.
MORPHOLOGYPleomorphicadenomastypicallymanifestasrounded,well-demarcatedmassesrarelyexceeding6cminthegreatestdimension.Althoughtheyareencapsulated,insomelocations(particularlythepalate),thecapsuleisnotfullydeveloped,andexpansilegrowthproducesprotrusionsintothesurroundingtissues.Thecutsurfaceisgray-whiteandtypicallycontainsmyxoidandbluetranslucentchondroid(cartilage-like)areas.Themoststrikinghistologicfeatureistheircharacteristicheterogeneity.Epithelialelementsresemblingductalormyoepithelialcellsarearrangedinducts,acini,irregulartubules,strands,orevensheets.Thesetypicallyaredispersedwithinamesenchyme-likebackgroundofloosemyxoidtissuecontainingislandsofchondroidand,rarely,fociofbone(
Fig.15.6 ).Sometimestheepithelialcellsformwell-developedductslinedbycuboidaltocolumnarcellswithanunderlyinglayerofdeeplychromatic,smallmyoepithelialcells.Inotherinstancestheremaybestrandsorsheetsofmyoepithelialcells.Islandsofwell-differentiatedsquamousepitheliumalsomaybepresent.Inmostcases,noepithelialdysplasiaormitoticactivityisevident.Nodifferenceinbiologicbehaviorhasbeenobservedbetweenthetumorscomposedlargelyofepithelialelementsandthosecomposedlargelyofmesenchymalelements.