An anechoic cyst is a type of cyst, a fluid-filled sac that can form in different parts of the body, which appears homogenous and does not reflect sound waves (echogenic) on an ultrasound scan. This means it appears as a dark area on the scan. Anechoic cysts are mostly non-cancerous (benign) but in some cases, they may indicate a more serious underlying condition. The characteristics and location of the cyst, along with additional diagnostic tests, help in determining its nature and whether any treatment is necessary.