An ultrasound report states that I have diffuse lipomatosis on the outer side of my ankle, how can I treat this?

Thank you for sharing your excellent question and I am sorry to hear of your pain and discomfort.  Depending on the clinical appearance and examination of your ankle you may be a candidate for either liposuction or direct excision of the fat.  There is no guarantee that your pain will abate but speak with an ASPS board certified plastic surgeon to discuss. 

You need to see a suitably qualified and experienced specialist to advise you.  They would need to take a history from you and examine you in person. Lipomatosis may be a condition with overgrowth of fat ( e.g. lipodystrophy) or may simply be some excess normal fat. Fat can be painful but with increasing pain and swelling I think you should be assessed for another cause of the pain. These causes could be pain from the bones, joints, spine (nerve entrapment) or due to  lack of blood supply to the foot amongst other causes. The likelihood of these different causes will vary with age, gender, smoking history, other medical conditions etc. First stop may best be a family doctor who could advise as to which specialist would be most appropriate.Excess fat can be reduced surgically most typically by liposuction. However, This is unlikely to treat pain and a proper diagnosis as to the cause of the pain and swelling should be made first.Best wishes