Grossly, type B3 thymomas are solid tumors with poorly-circumscribed margins. They frequently invade mediastinal fat or contiguous organs such as lung, pleura, or pericardium. The cut surface is grey or tan and there may be areas of hemorrhage and necrosis. Microscopically, type B3 thymomas have lobular architecture with coarse or delicate fibrous bands dividing the tumor into irregular lobules. At low power, the tumor has a pink appearance due to predominance of neoplastic epithelial cells arranged in solid sheets. The epithelial cells often show perivascular palisading. A small number of non-neoplastic lymphocytes, usually immature T-cells, are sprinkled in the background.