ultrasound findings?

After the discovery of a golfball size lump  deep within my lower left back I was given an ultrasound.   The test showed no sign of a cyst or fatty deposit. The tech said she saw nothing on her screen, but could definitely feel it with the wand.

What else would cause this? and why would it not present on an ultrasound?

[SassyLassie]

It could be a calcified nodule. These are usually benign and be seen on a Cat Scan. If it looks suspicious, then they might do a needle biopsy on it. These are done on an out-patient basis. If the doctor does not want to pursue this farther, I would get a second opinion and ask for the CT scan.