A 17-year-old female presents to your office with a mass on her calf that has been present for 3 years. It does not cause pain and has not been growing in size overall but does enlarge towards the end of the day. Physical examination demonstrates a 2cm non-tender mass near the Achilles tendon without skin discoloration. The patient has a normal neurovascular examination of the extremity, and no other masses are found elsewhere. Advanced imaging is shown in Figures A-C. A biopsy was previously performed by an outside surgeon. What does the biopsy most likely demonstrate and what is the best treatment at this time?