Carcinomas arising from salivary glands, such as the parotid, submandibular, sublingual, and minor salivary glands, are relatively uncommon (1-4). A large number of clinical statistics have been reported for salivary gland carcinoma (5-9). The treatment of salivary gland cancer generally involves surgery and postoperative radiotherapy (5,6,10). However, these are often insufficient for high-grade disease, and recurrence or metastasis often occurs, leading to a poor prognosis (4,11). The prognosis depends on the site of origin, histopathology, and staging (1,12) of the carcinoma. However, the small number of cases and variety of pathological presentations make it difficult to study a coherent number of cases on a consistent basis. Furthermore, the treatment of patients with distant metastases, which are more difficult to treat, varies between centres. There have been few studies on distant metastases in salivary adenocarcinoma, and few reports on their treatment and prognosis. In the present study, we investigated the prognosis of patients with distant metastases from salivary gland carcinoma.

The prognosis of salivary gland carcinoma with distant metastases is poor, and the variety of histopathological types often makes it difficult to find a well-established treatment. In the present study, unlike previous reports, salivary duct carcinoma, rather than adenocarcinoma or adenoid cystic carcinoma, was the most common type (1,13,14). This may be because of our focus on cases with distant metastases. Similar to previous reports, we observed that the carcinoma occurred predominantly in males, and the parotid gland was the most common site of occurrence (15,16).

In this study, the 5-year and 10-year survival rates after the appearance of distant metastases were 43.5 and 14.5%, respectively. The 5-year survival rate is not low for head and neck cancer after the appearance of distant metastases, but the 10-year survival rate is low at 14.5%, which may a feature of salivary gland carcinoma. Even if the patients survive for more than 5 years, the mortality rate at 10 years would be 70%, suggesting that long-term observation is necessary.

As shown in Table VI, sex was not predominantly associated with prognosis. No details available on hormone regulations of the salivary gland carcinogenesis. This is a subject for further study. There were no significant differences in survival according to sex, age, site of origin, number of metastatic organs, stage, or whether metastases were resectable. This means that once distant metastases develop, the prognosis is similar regardless of the number of metastatic organs or the stage of the disease. However, in other words, even if the disease has spread to multiple organs and the original stage is IV, there is still the possibility of long-term prognosis depending on treatment. In the present study, we were not able to conduct a comprehensive study because at present there is no unified treatment plan based on the histopathological type of the carcinoma, but we hope to study a larger number of cases and establish better treatment protocols in the future. The only difference was that patients with resectable metastases tended to have a longer survival than those with unresectable metastases (P=0.060). In other words, in patients with distant metastases that are resectable, resection should be performed whenever possible. Molecular-targeted therapy and immunotherapy are promising strategies for refractory carcinoma such as salivary gland carcinoma. Two patients treated with trastuzumab and one with nivolumab are alive, but the number of patients is still small and has not been fully investigated.

In this study, we investigated salivary gland carcinoma with distant metastasis. There are few reports focusing on distant metastases, and we believe that this is a valuable report in this regard. Although the treatment could not be studied in depth due to aforementioned reasons, we were able to evaluate the long-term prognosis and prognostic factors among the included patients in detail. In the future, studies with a larger number of cases should be performed, and the treatment protocols should be standardised.