Thank you for the query.
scans can be T1 or T2 weighted (two methods of obtaining images). In T1-weighted scans water is darker and the fat is whiter. In T2-weighted images water is whiter and the fat is darker. So not knowing what type of images were yours it is hard to answer your questions.
However according to what radiologist
has told you I assume that the lump is a fluid accumulation. If so, it can be a hamatoma for example due to some trauma.
Such lump can be also a lipoma
or other benign lump. If your orthopedist wont give you the answer what exactly it is, I suggest you to have simple ultrasound
of this lump and depending of the time of it removal or incision will cure it.
Hope this will help.