Fine-Needle Aspiration Biopsy Diagnosis of T-Cell Non-Hodgkin Lymphomas: Morphologic Features, Ancillary Testing, and Diagnostic Pitfalls

BACKGROUND: T-cell and NK-cell lymphomas are a heterogeneous group of mature lymphoid neoplasms that usually pursue an aggressive clinical course and involve both nodal and extranodal sites. These neoplasms have diverse clinical presentations, histopathologic patterns, immunophenotypes, genetic features, and prognoses. SUMMARY: Accurate classification, as outlined in the 5th edition of the WHO Classification of Haematolymphoid Tumours (WHO-HAEM5) and in the WHO Reporting System for Lymph Node, Spleen, and Thymus Cytopathology, requires integration of clinical information and the results of histopathology, immunophenotyping, viral status, and, when indicated, molecular analyses. Fine-needle aspiration biopsy (FNAB) provides a minimally invasive opportunity to sample nodal and extranodal lesions and can support either a definitive diagnosis or a highly suggestive provisional diagnosis in selected T-cell lymphoma subtypes, particularly when combined with immunocytochemistry, flow cytometry (including TRBC1), and molecular clonality testing. However, cytomorphologic overlap with reactive or other neoplastic conditions often limits the ability of FNAB alone to provide full subclassification. In this review, we summarize the cytologic features and ancillary test profiles of T-lymphoblastic lymphoma (T-LBL), systemic anaplastic large cell lymphoma (ALCL), breast implant-associated ALCL (BIA-ALCL), nodal T-follicular helper cell lymphoma-angioimmunoblastic type (nTFHL-AI), hepatosplenic T-cell lymphoma (HSTL), and peripheral T-cell lymphoma-not otherwise specified (PTCL-NOS). For each entity, we emphasize practical diagnostic algorithms in FNAB-based cytology, typical immunophenotypic and molecular patterns, common pitfalls, and situations in which histology - preferably excisional biopsy - remains mandatory. A comparative overview indicates which entities can be definitively subclassified on cytology with appropriate ancillary techniques and provides examples of structured reporting according to the WHO Reporting System. KEY MESSAGES: FNAB, when appropriately triaged for preparation of a cell block, immunocytochemistry, flow cytometry, and molecular studies can provide a definitive or near-definitive diagnosis in some T-cell lymphomas (e.g., T-LBL, systemic ALCL, BIA-ALCL effusions), whereas in other entities it serves mainly to raise strong suspicion and guide tissue biopsy (e.g., nTFHL-AI, PTCL-NOS). Understanding which entities can be reliably recognized by cytologic assessment, which ancillary panels are most informative, and where the limits of cytopathology lie are essential for optimal patient management and for appropriate use of the WHO Reporting System in daily practice.