Clinical and pathological aspects of microscopic thymoma with myasthenia gravis and review of published reports

Background: Microscopic thymomas, defined as epithelial proliferations smaller than 1 mm in diameter, characteristically occur in patients with myasthenia gravis without macroscopic thymic epithelial tumors. However, some clinical and pathological aspects of this entity are still unclear. Methods: This retrospective study includes five consecutive patients who had undergone extended thymectomy for myasthenia gravis at our institution from April 2007 to March 2016 and in whom microscopic thymomas were diagnosed by histopathological examination of the resected specimens. During the same period, we performed 32 extended transsternal thymothymectomies/thymectomies in patients with myasthenia gravis, including the above five cases. We here review 18 cases of microscopic thymoma, including our five cases and 13 previously reported cases. Results: The incidence of previously undiagnosed microscopic thymoma in patients undergoing thymectomy for myasthenia gravis in our institution is 15.2%. Serum preoperative anti-acetylcholine receptor antibody (anti-AchR Ab) titers were abnormally high in all of our five cases h (74.4 +/- 53.3 nmol/L) and decreased significantly after surgery (11.7 +/- 13.5 nmol/L, P=0.037). We divided our cases into the following three groups: microscopic thymoma group (Group M), thymoma group (Group T) and non-thymic tumor group (Group N). The mean preoperative anti-AchR Ab titers of these groups were 74.4, 26.5, and 368 nmol/L, respectively. All these values decreased postoperatively. The mean anti-AchR Ab titer was significantly higher in Group M than in Group T (P=0.034). All five cases in Group M were found by post-operative pathological examination to have multifocal type A thymomas. Conclusions: Microscopic thymomas tend to be multifocal type A thymomas. Anti-AchR Ab titers decreased significantly in all groups. It is very important to both perform complete extended thymectomies in patients with myasthenia gravis and pathological examination of thin slices of thymic tissue to maximize detection of microscopic thymomas.