The World Health Organization Reporting System for Lymph Node, Spleen, and Thymus Cytopathology: Part 2 - Spleen and Thymus.

[BACKGROUND] The International Academy of Cytology in collaboration with the International Agency for Research on Cancer has developed a standardized World Health Organization Reporting System (WHO System) for the cytopathology of the lymph nodes, spleen, and thymus. Fine-needle aspiration biopsy (FNAB) of the spleen and thymus, performed under ultrasound or computed tomography guidance, constitutes a minimally invasive, well-tolerated, and cost-effective diagnostic technique. Despite a limited number of published studies on the diagnostic accuracy of FNAB in these organs, it remains a valuable tool particularly when combined with rapid on-site evaluation and ancillary techniques, such as immunophenotypic analysis by flow cytometry or immunocytochemistry and molecular testing.

[SUMMARY] The WHO System categorizes thymic and splenic cytopathology into five diagnostic categories: "Inadequate/Insufficient/Nondiagnostic," "Benign," "Atypical," "Suspicious for malignancy," and "Malignant." This standardized approach aims to enhance diagnostic accuracy, provide risk assessment, and facilitate clinical decision-making. However, applicability of FNAB in the thymus and spleen is still debated, particularly concerning sample adequacy, diagnostic accuracy, and potential complications such as hemorrhage, especially for splenic FNAB. Thymic FNAB is primarily used for diagnosing thymic hyperplasia, thymoma, thymic carcinomas, and lymphomas. However, distinguishing these entities and their subtypes is challenging. Splenic FNAB is valuable in evaluating splenomegaly, infections, lymphomas, and other neoplasms, particularly in staging and monitoring disease progression. While using FNAB to evaluate these organs is not yet a widely adopted standard practice, its role in minimizing unnecessary surgical interventions and guiding therapeutic strategies is being increasingly recognized.

[KEY MESSAGES] The WHO System for the thymus and spleen seeks to establish a harmonized, evidence-based framework for cytopathological diagnosis, incorporating key diagnostic criteria, malignancy risk assessment, and standardized reporting protocols. Future research is needed to refine and develop diagnostic role of FNAB, enhance its integration with advanced molecular techniques, and optimize its use in personalized medicine.