I had about 10 FNAB's all under untrasound guidance, which makes them more accurate, and some were as small as 3mm x 7mm, and done by a pathologist. Lymph fluid is sticky, so the term can mean differently to others. As far as cancer being immobile, that's not always true. When a cancer is small it can be mobile, and when it gets larger it can attach to structures making it immobile, I had both at times. The doctors also go by other factors such as the shape, thickness, but the only way to tell is through a biopsy.

Good luck.