World Health Organization histologic classification: an independent prognostic factor in resected thymomas

The histologic classification of thymoma remained controversial since 1999, when the World Health Organization (WHO) Consensus Committee published a histologic typing system for tumours of thymus. Clinical features, postoperative relapsing rates, and survival of patients with thymoma were evaluated with reference to the WHO histologic classification, based on a series of 178 patients, submitted to surgery between 1988 and 2000. There were 21 type A, 49 type AB, 45 type B1, 50 type B2 and 13 type B3 tumours. The invasiveness of tumours was 23.8%, 51%, 73.3%, 82% and 100% for types A, AB, B1, B2 and B3 tumours, respectively. The frequency of invasion of the great vessels increased according to the tumour type in the order A (0%), AB (4%), B1 (6.6%), B2 (22%), and B3 (23%). The 10-year disease-free survival was 95%, 90%, 85%, 71% and 40% for types A, AB, B1, B2 and B3, respectively. According to the Masaoka staging system, the disease-free survival rates were 94%, 88% and 66% for stages I, II and III, respectively, at 10 years. No stage IVA thymomas reached 10 years follow-up. Overall survival at 10 years were 88% and 25% when complete and incomplete resection were considered. By multivariate analysis, Masaoka staging system, WHO histologic classification and complete resection were significant independent prognostic factors, whereas age- and sex-associated myasthenia gravis were not. The present study demonstrated the World Health Organization histologic classification a good prognostic factor, such as completeness of surgical resection and Masaoka staging system.