<prompt> Answer this question truthfullyWhat is the classification of thymoma?  <response>: <reason> In 1999, a World Health Organization WHO Working group suggested a non-committal terminology Masaoka classification for thymoma, preserving the distinct categories of the histo - genetic classification, by using letters and numbers to designate tumor entities. Recently, it has been very well accepted as it provides an easy comparison of clinical, pathological, and immunological studies. 