The World Health Organization histologic classification system reflects the oncologic behavior of thymoma: a clinical study of 273 patients.
BACKGROUND
Although the histologic classification of thymic epithelial tumors has been confusing and controversial, an agreement on the universal classification system for thymic epithelial tumors was achieved by the World Health Organization (WHO) in 1999. The authors previously reported that the WHO histologic classification system reflects invasiveness and immunologic function of thymic epithelial tumors. In this subsequent study, they examined the prognostic significance of this classification system.


METHODS
Clinical features as well as postoperative survival of patients with thymoma, but not thymic carcinoma, were examined with reference to WHO histologic classification based on an experience with 273 patients over a 44-year period.


RESULTS
There were 18 type A tumors, 77 type AB tumors, 55 type B1 tumors, 97 type B2 tumors, and 26 type B3 tumors. In patients with type A, AB, B1, B2, and B3 tumors, the respective proportions of invasive tumor were 11.1%, 41.6%, 47.3%, 69.1%, and 84.6%; the respective proportions of tumors with involvement of the great vessels were 0%, 3.9%, 7.3%, 17.5%, and 19.2%; and the respective 20-year survival rates were 100%, 87%, 91%, 59%, and 36%. According to the Masaoka staging system, the 20-year survival rates were 89%, 91%, 49%, 0%, and 0% in patients with Stage I, II, III, IVa, and IVb disease, respectively. By multivariate analysis, the Masaoka staging system and the WHO histologic classification system were significant independent prognostic factors, whereas age, gender, association with myasthenia gravis, completeness of resection, or involvement of the great vessels were not significant independent prognostic factors.


CONCLUSIONS
This study showed that histologic appearance reflects the oncologic behavior of thymoma when the WHO classification system is adopted. The WHO classification system may be helpful in clinical practice for the assessment and treatment of patients with thymoma.
Okumura
Meinoshin
M
Division of General Thoracic Surgery, Department of Surgery, Osaka University Graduate School of Medicine, Osaka, Japan. meinosin@surg1.med.osaka-u.ac.jp
Ohta
Mitsunori
M
Tateyama
Hisashi
H
Nakagawa
Katsuhiro
K
Matsumura
Akihide
A
Maeda
Hajime
H
Tada
Hiroto
H
Eimoto
Tadaaki
T
Matsuda
Hikaru
H
Masaoka
Akira
A
D000293:Adolescent; D000328:Adult; D000368:Aged; D002277:Carcinoma / Q000145:classification* / Q000473:pathology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009367:Neoplasm Staging; D011379:Prognosis; D016019:Survival Analysis; D013945:Thymoma / Q000145:classification* / Q000473:pathology*; D013953:Thymus Neoplasms / Q000145:classification* / Q000473:pathology*; D014944:World Health Organization