Colloid cyst of the third ventricle: a rare case of spontaneous regression. Illustrative case.

backgroundColloid cysts (CCs) are benign lesions commonly located in the third ventricle, near the foramen of Monro. They constitute about 0.5%-1% of all intracranial tumors. As benign lesions, CCs can be found incidentally, but they can also present with neurological deterioration and are even associated with sudden death, which might be explained as a result of obstructive hydrocephalus. Thus, larger and symptomatic CCs are often considered for surgery. OBSERVATIONS: The following case presents a rare and curious instance of a spontaneously regressing CC. This represents the 11th case ever reported about an extremely rare occurrence: the spontaneous regression of a previously diagnosed CC in the third ventricle. LESSONS: Carefully selected asymptomatic CCs can be monitored through regular imaging studies and neurological examinations. If regression of the cyst is observed, physicians should not be surprised, as this event, although rare, has been documented and published in 11 cases, including this one. https://thejns.org/doi/10.3171/CASE24498.