What muscle do you think it is in Xue?
It looks too superficial to be within the joint itself so I favour a mass outside the joint.
I agree it has characteristics consistent with a haemangioma with some venous lakes contained within it.
I agree this is the most likely diagnosis,
However you are still dealing with an unusual mass within a muscle (or between the muscles) so something sinister like a sarcoma still needs to be excluded.
I always think anything intramuscular that is not a standard post traumatic muscle tear needs cross sectional imaging workup just to be sure we are not missing a sarcoma.
I apply this to intramuscular lipomas was well.
Better be safe than sorry.
Steve