Fat Necrosis 12 Months Post-op, DCIS Bilateral Mastectomy?

Is this near your mastectomy scar?  Could do an excisional biopsy for definitive diagnosis.

Fat necrosis occurs when a fat lobule dies, from either traumatic injury or because its blood supply was cut off. When this happens, it first swells, then becomes tough and enlarged, then eventually starts to shrink and dissolve away. This occurs over months to about a year. As this process evolves, sometimes lumps are felt that then shrink down, becoming harder over time and "suddenly" noticeable, even though surgery was months previously. This happens because as the fat lobule shrinks, the dead cells absorb calcium leaving a safe little "stone" that can has a telltale appearance on ultrasound and x-ray (mammograms). These coarse, chunky calcium deposits tend to look much different from tiny, fine microcalcifications that are associated with abnormal cell overgrowth inside milk ducts and glands that is a red flag signal for possible disease. It should be easy to tell with an exam or an ultrasound what this is, and very easy to take a tiny pinch to be sure if there is any question. We do avoid mammograms in patients who have had mastectomies, with very few exceptions. If this has happened near a lumpectomy or breast reduction site, a mammogram can help with additional information.

Given your breast cancer history, if you are feeling a mass, then this warrants medical attention.  Please see your reconstructive plastic surgeon and/or your oncologist for an evaluation.