Objective: To study the clinicopathologic relevance of a thymic epithelial tumor (TET) grading standard with the WHO classification.
Methods: A grading system for TET was proposed based on the application of WHO histological typing of thymic tumors and analyzed in relation to clinical therapy results and follow-up data of 200 TET cases.
Results: In this series, 8 patients (4.0%) belonged to type A, 68 (34.0%) were type AB, 17 (8.5%) were type B1, 39 (19.5%) were type B2, 27 (13.5%) were type B3 and 36 (18.0%) were type C. The remaining 5 cases were rare thymomas. The overall postoperative survival data showed highly significant differences among the histological subtypes (P < 0.001). Type A & type AB thymomas showed excellent prognosis, none of these patients died of tumor; in type B1, only 1 case (5.9%) died at 22 months postoperatively. Types B2, B3 and C thymomas shared the bad, worse and worst prognosis. Ninety-six patients (48.0%) were in stage I, 26 (13.0%) in stage II, 65 (32.5%) in stage III and 13 (6.5%) in stage IV. Clinical stage is also highly significant in predicting survival (P < 0.001). It was found that tumor histology could predict survival expectancies well in stage I and stage II cases. It was also found that type B2, B3 and C thymomas had a statistically significant worse prognosis than type A, AB and B1 thymomas (P < 0.001). According to the histology, clinical data, biological behavior and prognosis, it is proposed that thymomas be divided into 4 grades: grade I, II, III and IV. Follow-up is the best strategy for grade I & II patients after radical surgery. In this series, the 30 patients (15.0%) presenting clinical signs of myasthenia gravis were mostly in type B2 and B3 groups (P < 0.01).
Conclusions: The WHO classification for TET provides good pathological definitions and criteria for diagnosis, which can independently predict the invasiveness and prognosis of TET. TET grading is of use in unifying pathological and clinical findings, in selection of proper therapy and in predicting prognosis.