The “Mushroom Cap Sign” for Fundal Localized Gallbladder Adenomyomatosis

Background: Localized gallbladder adenomyomatosis is a common benign condition characterized by focal thickening of the gallbladder wall, typically at the fundus. The current descriptive term “cap-like thickening” is accurate but lacks vividness.Observation: We propose the “Mushroom Cap Sign” as a novel sonographic sign for a subset of localized gallbladder adenomyomatosis that exhibits a typical cap-like elevation with visible Rokitansky-Aschoff sinuses (RAS), together resembling a mushroom cap. The anechoic cystic spaces correspond to the spots on a mushroom cap. Conclusion: The “Mushroom Cap Sign” provides a simple, intuitive sonographic sign for recognizing those cases of localized gallbladder adenomyomatosis with a cap-like elevation and internal anechoic cystic spaces. It may serve as a useful teaching tool for trainees.