HOW CAN YOU TELL IF A CYST IS CANCER BY AN ULTRASOUNDS?

MY SON FOUND A SMALL KNOT IN HIS TESTICAL. HE HAD AN ULTRASOUND DONE. THE DR. TOLD HIM IT WAS A CYSTS, AND WHEN HE RETURN FROM IRAQ, HE COULD HAVE IT REMOVED. THE DR. ALSO TOLD HIM IT WASNT CANCER. MY QUESTION IS HOW CAN YOU TELL IF ITS CANCER OR NOT BY JUST A ULTRASOUND? DONT YOU HAVE TO HAVE A SAMPLE TEST DONE TO KNOW IF ITS CANCER? THANK YOU!!!

i dont know this for sure so u might want to double check my answer.. with an ultra sound they can see if the cyst is filled with a harmless fluid or if it is solid ... if it is solid that doesnt mean its cancer but other test probably should be done to be more sure