What does new asymmetrical density findings on a mammogram report mean?

Breast tissue: Is mostly fat, especially as a woman ages. More dense areas are potentially of concern. Here the breast radilogist is saying they see differences when they compare one side to the other, and that was not the case on a prior study.

Needs Further Eval: Asymmetry implies that there may be an underlying tumor where the breast tissue is more prominent. The next step is to get a diagnostic mammogram +/- ultrasound to see if this is just a "shadow" or an abnormality that warrants a biopsy (these turn out to be shadows most of the time). Please discuss this further with the doctor that ordered your mammogram.

Interval change: There has been a change since the prior mammogram, with an area of relatively denser tissue present in a new area. This needs additional evaluation, probably special mammographic views and maybe ultrasound. At this point this does not mean you have a mass, only a potential abnormality. Most of the time it turns out to be nothing of concern, sometimes due to differences in positioning.