Presentation
Routine mammograms almost 4 years apart. This case only makes sense if you see both images.
Patient Data

The spiculated asymmetry is visible but almost hidden.

The lesion is clearly visible now with long spiculated runners.
Case Discussion
Not all cancers grow quickly in the breast. In a very general sense, the postmenopausal cancers tend to grow more slowly than the premenopausal ones, all other factors being equal. Which is why some feel that we should screen annually above 40 and perhaps less frequently after 70...
As a general rule small, dense cancers with long runners turn out to be the tubular variety of IDC.
If it looks like a cancer it is still a cancer, even if it grows slowly. Do not assume that because a lesion has more or less stable volume it is innocuous.