I have one of those. They did an ultrasound and decided it was nothing --- a cyst. Nonetheless, in subsequent CT scans, the radiologist has always noted its size (which has been unchanged), so I guess they're still faintly suspicious of it.
stage 2a rectal cancer 11/05 at age 63
LAR 12/05 with adjuvant radiation+5FU,leucovorin 1-2/06
NED for 9 years, cured