An incidental finding on a CT lingual to the mandibular anterior teeth. What is your diagnosis?

While we may be concerned about the presence of a tumor or neoplasm (and rightly so from the single image we have), there's actually no lesion here.  What we are seeing is a foreign object or more specifically, a lemonhead (a type of hard candy).

Foreign objects, especially comestible items like gum, chewing tobacco, and candy, can cause a surprisingly difficult diagnostic challenge. While patients are asked to removed all items from their mouth, patients may not comply and these objects are then captured in the radiographs.  Foreign objects can mimic the appearance of intraoral pathology and can lead to misdiagnosis, unnecessary radiation, and avoidable patient distress.  Compare this lemonhead to the second included image of a solitary fibrous tumor and you can see how these can easily be confused.  The best way to prevent these situations is to train your staff to watch for patients, especially children or teenagers, who are likely to be chewing gum or candy.

This case is from the paper 'What's in Your Mouth? The CT Appearance of Comestible Intraoral Foreign Bodies' by M. McDermott, B.F. Branstetter, E.J. Escott.