Thymomas and thymic carcinomas. A retrospective investigation with histological reclassification.

The morphological heterogeneity of thymomas has caused much confusion respecting their classification. Recently Kirchner & Müller-Hermelink (4) proposed a histological subclassification which has been claimed to represent an independent prognostic factor: medullary and mixed thymomas are benign; organoid and cortical type as well as well-differentiated thymic carcinomas are low-grade malignant tumors, which have the capacity to recur and spread, even if they are clinically benign. High-grade malignant thymomas are always malignant. We present the clinicopathological data on 10 clinically benign and 14 clinically malignant thymomas. Having reclassified the thymomas, we found four low-grade malignant examples among the clinically benign thymomas. It is important to identify this group of patients as they are at risk of tumor recurrence. However, further investigation is needed to support the validity of this subclassification system.