The term "carcinoid" is a misnomer: the evidence based on local invasion

Background: Since Oberndorfer proposed the term "carcinoid" in 1907, over 100 years have passed. This attractive term was initially proposed for 6 cases of his own experience with 12 submucosal lesions in the small intestine. Oberndorfer summarized the characteristic features of these lesions as follows: (1) small in size and often multiple, (2) histologically undifferentiated with a suggestion of gland-formation, (3) well-defined without any tendency to infiltrate the surroundings, (4) no metastases, and (5) apparently slow-growing reaching no significant size with a seemingly harmless nature. Review: This article stresses the malignant nature of "carcinoid" on the basis of local invasion prior to metastases in the first two sessions, (1) with Oberndorfer's original diagram, and (2) with an experimental observation on extraglandular microcarcinoid in a form of "budding". Next, (3) a statistical comparison between a carcinoid group and a non-carcinoid ordinary carcinoma group is introduced on metastasis rates at an early stage with two prescribed factors of the depth of invasion restricted within the submucosa (sm-lesion) and a small tumor size category of 1 cm to 2 cm: the carcinoid group exhibited metastasis rates higher than those in the ordinary carcinoma group when calculated in the stomach and rectum. In the author's experience, "carcinoids" are malignant not only in the gastrointestinal tract but also in the other sites on the basis of local invasion. Lastly, (4) discussion on the terminology of "carcinoid" as a misnomer is carried out. Adequate terms referring to the entity of this malignant tumor group are discussed. One of the most adequate and brief terms for "carcinoid" that is included now in neuroendocrine tumor group would be "endocrinocarcinoma" as per the author's proposal, followed by NEC (neuroendocrinocarcinoma) or GEC (gut endocrinocarcinoma). Conclusion: The term "carcinoid" is a misnomer that can be confirmed on the basis of local invasion prior to metastases. "No metastases without local invasion" is not of a negligible importance.