The description of the mass (soft and non-tender, in the posterior knee) in a young patient suggests that this is a popliteal, or Baker’s, cyst. The article by Dinham reviewed the natural history of 120 cases of popliteal cysts in children and reported that, of seventy untreated cysts, fifty-one disappeared spontaneously during a mean period of one year and eight months. The article also noted that those cysts which fail to regress spontaneously may respond to aspiration, but have a high recurrence rate. Given the lack of symptoms and the likelihood of resolution, observation is the first-line treatment for most popliteal cysts.
Dinham JM. Popliteal cysts in children: The case against surgery. JBJS Br: 1975 (57); pp 69-71.
PMID:1117024 (Link to Abstract)