Cheuk Wah, Tsang William Y W, Chan John K C
Department of Pathology, Queen Elizabeth Hospital, Hong Kong, SAR China.
Am J Surg Pathol. 2005 Mar;29(3):415-9. doi: 10.1097/01.pas.0000152130.58492.de.
We report 2 cases of microscopic-sized thymoma, which probably represents the earliest phase of thymoma development. The 2 patients presented with pure red cell aplasia and myasthenia gravis, respectively. The thymectomy specimens did not reveal tumor on gross examination, but histologically each contained small thymomas measuring 5 mm and 7 mm in largest dimension, respectively. One of the tumors was unencapsulated and involved a single lobule only, and the other was encapsulated and comprised two lobules. The tumors consisted of ovoid epithelial cells with pale nuclei and distinct nucleoli, scattered in a background of small lymphocytes. Foci of medullary differentiation and perivascular space were identified in the 2 cases, respectively. The lymphocytes were confirmed to be immature T cells on immunohistochemical studies (CD3+, TdT+). Except for the microscopic size, the morphology of the two tumors conforms to conventional type B1/B2 and type B2 thymoma, respectively. We propose calling such incidental small tumor "microthymoma" to distinguish it from the so-called microscopic thymoma, which is composed of small thymic epithelial nests and probably more appropriately termed "nodular hyperplasia" of the thymic epithelium.
我们报告了2例微小尺寸胸腺瘤病例，这可能代表胸腺瘤发展的最早阶段。这2例患者分别表现为纯红细胞再生障碍和重症肌无力。胸腺切除标本大体检查未发现肿瘤，但组织学检查显示，每例均含有最大直径分别为5毫米和7毫米的小胸腺瘤。其中1例肿瘤无包膜，仅累及单个小叶，另一例有包膜，由两个小叶组成。肿瘤由核淡染、核仁明显的卵圆形上皮细胞组成，散在于小淋巴细胞背景中。在这2例病例中分别发现了髓质分化灶和血管周围间隙。免疫组化研究证实淋巴细胞为未成熟T细胞（CD3 +、TdT +）。除了微小尺寸外，这两个肿瘤的形态分别符合传统的B1/B2型和B2型胸腺瘤。我们建议将这种偶然发现的小肿瘤称为“微胸腺瘤”，以区别于所谓的微小胸腺瘤，后者由小胸腺上皮巢组成，可能更恰当地称为胸腺上皮的“结节性增生”。
