It happens quite frequently. Usually the situation involves an original film, like a mammogram or breast ultrasound, that demonstrates a "mass." A consultation is ordered and then a biopsy using imaging guidance is scheduled. The patient goes back to the imaging center for her biopsy, a new image is generated to locate the original "mass" and, for goodness sakes, the "mass" is gone. What happened? While it is possible that a mass resolved - sometimes cysts can come and go - more often what has happened is that the original "mass" seen was not really a mass at all - the image was "over-read" and when the image is taken again either the mass is not seen at all or, and this is quite often the case, the interpretation is different ---- there wasn't really a "mass" to begin with and there is no "mass" now. Hope that helps.