In this radiology lecture, we review the ultrasound appearance of sublingual dermoid cyst and explain floor of mouth anatomy! Key teaching points include: * The floor of the mouth is a horseshoe-shaped area beneath tongue and in between sides of mandible, inferiorly bounded by mylohyoid muscle, and containing sublingual space (SLS) * SLS medial border: Midline genioglossus/geniohyoid muscle complex; SLS inferolateral border: Mylohyoid muscle * Anterior margin of hyoglossus muscle projects into posterior SLS * Sublingual dermoid cyst is a rare, benign cyst with squamous epithelial lining and contains skin appendages * Dermoid and epidermoid cysts are in same family, terminology often used interchangeably, although epidermoid cysts less common and tend to contain fluid contents only * Dermoid cyst mean age of presentation late teens to twenties, average age 30 * Presents as a slowly enlarging neck mass, may cause dysphagia * Often round or oval in shape and homogeneously hypoechoic with punctate echogenic foci * May have pathognomonic “sack of marbles” appearance * Relationship to mylohyoid is key for surgical planning: Intraoral resection for sublingual (above mylohyoid) location, extraoral approach for submental/submandibular (below mylohyoid) location * Most cysts are midline * DDx: Suprahyoid thyroglossal duct cyst, ranula (simple and diving), abscess and lymphangioma To learn more about the Samsung RS85 Prestige ultrasound system, please visit: https://www.bostonimaging.com/rs85-prestige-ultrasound-system-4 Click the YouTube Community tab or follow on social media for bonus teaching material posted throughout the week! Spotify: https://spoti.fi/462r0F2 Instagram: https://www.instagram.com/Radquarters/ Facebook: https://www.facebook.com/Radquarters/ X (Twitter): https://twitter.com/Radquarters Reddit: https://www.reddit.com/user/radiologistHQ/