Testicular Cyst. A benign testicular cyst is viewed as a well-defined, spherical, uniformly anechoic mass (arrow) within the testis. Care must be taken to differentiate simple testicular cysts from cystic necrosis within testicular tumors.

http://www.msdlatinamerica.com/ebooks/FundamentalsofDiagnosticRadiology/sid454050.html