In a histologic re‐examination and reclassification of a series of 1678 tumors of the parotid gland, 63 (3.7%) exhibited the structures characteristic of mucoepidermoid carcinoma. A long‐term follow‐up study showed that mucoepidermoid carcinoma differs from acinic cell and adenoid cystic carcinoma in that the determinate survival rate does not fall markedly when the follow‐up exceeds 5 years. This implies that the prognosis based on the determinate survival rate can be evaluated after a 5‐year follow‐up study. Histologically, the 63 tumors have been divided into two subgroups, one consisting of 20 tumors exhibiting invasive growth (high grade malignant) and the other of 43 without invasive growth (low grade malignant). The follow‐up study showed the difference between the prognosis in the two groups. In order to improve the poor prognosis in high grade malignant mucoepidermoid carcinoma, the authors recommend that parotidectomy should be routinely combined with radical neck dissection in high grade malignant tumors.

In a histological re‐examination and re‐classification of a series of 1,678 tumors of the parotid gland 37 (2.2%) exhibited the structures characteristic of acinic cell carcinoma. A long‐term follow‐up study showed that the prognosis quoad vitam in this type of tumor is poorer than previously has been inferred from short‐term follow‐ups. Thus the determinate survival rate fell from 89% at 5‐year follow‐up to 56% after 20‐year follow‐up. The prognosis is nevertheless much better than in adenoid cystic carcinoma of the parotid gland, as disclosed by a comparative study. This difference is evident after 5‐year follow‐up and becomes still more marked with time. Acinic cell carcinoma cannot be divided into a benign and a malignant variant on the basis of various histological criteria, such as infiltrative growth and high incidence of mitesis. This is because even the histologically apparently benign variant may have a clinically malignant course. Metastasis was observed in 7 cases, in 4 of them to regional lymph nodes. Since acinic cell carcinoma has proved to be a malignant, metastatic type of tumor, the authors recommend that parotidectomy be combined with neck dissection.

In a histologic re‐examination and re‐classification of parotid tumors in 1,678 patients treated by surgery, 13 cases of oncocytoma were found. Five patients had bilateral tumors. The cell picture in oncocytoma is very characteristic, made up of oncocyte‐like cells with eosinophilically granulated abundant cytoplasm, and often also of clear cells. Metastases or death in oncocytoma did not occur in any case, but local “recurrence” was seen in 4 of the 13 cases. Oncocytoma often shows multinodular growth, and oncocytic foci are very often found in the surrounding glandular tissue. Of the 5 patients who had bilateral tumors, 2 had synchronous tumors. The high frequency of multinodular and bilateral growth observed in the present series supports earlier theories that oncocytoma is not a real neoplasm but a nodular hyperplasia. The main differential diagnoses are benign mixed tumor with oncocytic foci and acinic cell carcinoma.

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