OBJECTIVETo discuss the pathological and biological characteristic and the clinic-pathological relevance of combined thymoma (AB + B).

METHODSAccording to the WHO Histological Typing of Thymic Tumors, the clinical features and pathological characteristics of AB + B thymomas were analyzed in a series of 141 thymic epithelial tumors (TET) collected in Shanghai Chest Hospital and analyzed statistically with clinical therapy and follow-up data.

RESULTSSeven cases of type A (5.0%), 48 cases of type AB (34.0%), 14 cases of combined thymoma (AB + B) (9.9%), 9 cases of type B1 (6.4%), 22 cases of type B2 (15.6%), 21 cases of type B3 (14.9%) and 20 cases of type C (14.2%) were classified. The overall postoperative survival data showed highly significant differences amongst the histological subtypes. Combined thymoma (AB + B) showed distinctive histopathological characteristics, their prognostic properties were between A, AB and B1-B3, the differences between them being highly significant (log rank: chi(2) = 48.18, P < 0.001). Seventy-three patients (51.8%) were in stage I, 16 (11.3%) were in stage II, 44 (31.2%) were in stage III, and 8 (5.7%) were in stage IV. The association between histological subtype and invasive stage was significant (chi(2) = 71.3, P < 0.001). Combined thymoma (AB + B) was grouped to grade II thymoma. Thymoma grading has a positive relationship with therapy and prognosis (log rank: chi(2) = 57.49, P < 0.001).

CONCLUSIONSIt is proposed that combined thymoma (AB + B) may be named as the borderline thymoma. It has a distinctive histopathological definition and criteria and may be grouped as a subtype of thymoma. Its pathological and biological properties are between benign and malignant thymomas, and have significant differences.