Texas Subscriber Answer: If the patient had a cyst of the tendon sheath, you should report 20612 (Aspiration and/or injection of ganglion cyst[s] any location).
If, however, the patient did not have a tendon sheath cyst, it's likely that none of these codes is appropriate for the peroneal sheath aspiration, and no other codes in CPT specifically describe this service. In that situation, you should report 20999 (Unlisted procedure, musculoskeletal system, general).
Because 20612 refers to the aspiration of a ganglion cyst, you cannot report this code for peroneal sheath aspirations in the absence of a tendon sheath cyst. In addition, both 20605 (Arthrocentesis, aspiration and/or injection; intermediate joint or bursa [e.g., temporomandibular, acromioclavicular, wrist, elbow or ankle, olecranon bursa]) and 20610 (... major joint or bursa [e.g., shoulder, hip, knee joint, subacromial bursa]) describe bursa or joint aspiration, so you cannot equate peroneal sheath aspirations to these codes.
Although 20550 (Injection[s]; single tendon sheath, or ligament, aponeurosis [e.g., plantar "fascia"}) mentions injection of the tendon sheath, the descriptor does not refer to aspiration, so this code also fails to describe your physician's procedure.