Atypical type A thymoma: a narrative review of a distinct and progressive variant of low-risk thymoma.

Background and objective: Atypical type A thymoma is a rare subtype of thymoma recently recognized for its more progressive clinical course and biological behavior, in contrast to the conventionally indolent course and favorable prognosis of conventional type A thymoma. Histologically, atypical type A thymoma retains the spindle or oval epithelial cell morphology of the conventional type, but is characterized by atypical features such as increased mitotic counts, hypercellularity, and focal necrosis, which are generally absent in the conventional type. This review integrates the latest knowledge on clinicopathological features, diagnostic criteria, and treatment.

Methods: A literature search was performed using PubMed for articles in English published between January 1990 and July 2025. We included case reports, case series, and original studies with comprehensive clinicopathological and survival data for atypical type A thymomas.

Key content and findings: We identified 67 cases of atypical type A thymoma in 16 case reports/series and two original reports that specifically mentioned detailed information for each case. Recurrence was observed in 27.3% of patients, predominantly manifesting as distant metastasis to sites such as the lungs, the rib/chest wall, and brain, a rate notably higher than that observed in conventional type A and AB thymomas. In contrast, the 5-year overall survival (OS) rate was 91.9%, indicating a prognosis closer to that of conventional type A and AB thymomas than to type B2-B3 thymomas. Its proliferative capacity, indicated by a median Ki-67 index of 10-14.5%, is closer to that of type B3 thymoma or thymic carcinoma (TC).

Conclusions: Since current studies on metastatic type A thymomas often mix conventional and atypical cases, further case accumulation and the establishment of strict, standardized pathological criteria are crucial for a clearer understanding of this rare but potentially progressive tumor and for refining long-term follow-up and treatment strategies. Moreover, the relationship between metastatic and atypical type A thymomas needs to be clarified.