[Appendiceal Tumors, Up to Date].

Appendiceal tumors are relatively rare and their classification remains controversial. In particular, for tumors specific to the appendix, such as appendiceal mucinous neoplasm and goblet cell adenocarcinoma, understanding the disease has become difficult due to transition of terminology and complex grading systems. There has been little large-scale data on appendiceal tumors in Japan, but a full picture is gradually emerging on account of project research by the Japanese Society for Cancer of Colon and Rectum. Now, we explain the overview of appendiceal mucinous neoplasm and goblet cell adenocarcinoma, as well as their histological grading and staging system, including the latest findings, and describe their impact on patient prognosis and treatment. Both of these tumors frequently cause clinical problems due to peritoneal dissemination. In Europe and the United States, the combination of cytoreductive surgery(CRS)and hyperthermic intraperitoneal chemotherapy(HIPEC)is recommended, but in Japan, HIPEC is not widely used, so there is not enough data available. There are still many unknowns about appendiceal tumors, so it is important to clarify the concept of the disease and accumulate accurate data. In addition, the histological classification of appendiceal tumors in this article conforms to Japanese Classification of Colorectal, Appendiceal, and Anal Carcinoma, 9th edition and the WHO Classification, 5th edition, although new editions of both are scheduled to be published soon. The outlook for the 10th edition of the Japanese Classification is briefly mentioned in the text, but please note that the classification introduced here may be subject to change.