Cytopathology in focus—Lymph node cytopathology: WHO system for reporting

Timothy M. Bell, DO
Diane D. Davey, MD

May 2024—Fine-needle aspiration has been an important diagnostic tool for the workup of patients with abnormal enlargement of lymph nodes and related organs, including the spleen and thymus. More specifically, benign conditions, metastatic neoplasms, and recurrent malignancies frequently can be diagnosed by FNA alone when appropriate ancillary techniques are used in conjunction with clinical-pathologic correlation. Consequently, the WHO Reporting System for Lymph Node, Spleen, and Thymus Cytopathology represents a collaboration of the WHO, the International Agency for Research on Cancer, and the International Academy of Cytology.^1,2 This terminology and atlas aim to standardize the reporting of lymph node FNA to include the integration of ancillary studies, imaging findings, and clinical assessment to enable more uniform and optimal patient management. The WHO lymph node reporting system categories are based on the previous Sydney system introduced by the International Academy of Pathology, which was published in 2020.³

The five major reporting categories are defined in Table 1, with examples provided. While the risk of malignancy, especially for the insufficient and atypical categories, varies considerably, a recently published meta-analysis of the Sydney system provides practice-driven data, as shown.⁴ A detailed section on sampling techniques and ancillary testing (immunophenotyping, molecular, other) is included, as well as a discussion of the risk of malignancy and management recommendations, which always includes clinical correlation. The standardized report should include demographic information, type of specimen, clinical and imaging information, and a diagnostic summary. The diagnostic summary first lists the diagnostic category, followed by detailed diagnosis, macroscopic description (slides, fluid, etc.), details of any onsite evaluation (if applicable), and microscopic description with ancillary test results, and concludes with applicable comments. For suspicious entities, the report should always include a comment regarding the entities under consideration to help guide additional biopsy or testing procedures.

Examples of benign entities, which are described and illustrated, include infectious and inflammatory conditions. Infectious and inflammatory processes within lymph nodes can induce a wide range of tissue changes, including suppurative, granulomatous, histiocytic, plasmacytic, and necrotic patterns of injury. Some cases may pose a diagnostic challenge when they lack unequivocal benign features and show some changes that may be seen in a neoplastic process. Ancillary studies can be helpful in these scenarios. Special staining techniques from a cell block using Grocott methenamine silver (GMS) and Ziehl-Neelsen/acid-fast bacilli stains may help in identifying infectious agents (Fig. 1). Additional ancillary testing including polymerase chain reaction and microbial cell culture may be beneficial in challenging infectious workups. In some cases, the differential of lymphoma versus follicular hyperplasia can be difficult, and flow cytometry may be helpful in discerning a lymphoproliferative disorder from a benign process. If cytologic features are concerning for lymphoma, the case should be placed in the atypical category. Cases concerning for malignancy but without sufficient abnormal features and/or material for ancillary studies are placed in the suspicious category. Both categories require additional patient correlation and additional samples as indicated.

In the malignant category, metastatic carcinoma is one of the most common entities; clinical correlation and ancillary studies are useful in determining the primary site of origin. FNA is frequently used for diagnosis of recurrent malignancies, including lymphoma. Primary diagnoses of lymphoma and hematologic neoplasms can be particularly challenging using FNA, and cytopathologists should use a multiparameter approach with ancillary testing including flow cytometry, immunohistochemical stains, molecular testing, and hematopathology consultations as appropriate (Fig. 2). According to published guidelines for the laboratory workup of lymphoma published by the CAP and ASCP, FNA cytomorphology alone without ancillary testing should not be done to achieve a definitive diagnosis of lymphoma.⁵ Also, a surgical biopsy is recommended when feasible in a setting where Hodgkin lymphoma is highly suspected.⁵

Cytopathology services should also be aware that the WHO has a new fifth edition of hematolymphoid tumors online.⁶ Laboratories can refer to this version as well as the WHO lymph node cytopathology reference for definitive diagnosis of lymphomas and other hematopoietic neoplasms. Added to the fifth edition of hematolymphoid tumors are several new entities, which are largely recognized due to new molecular and sequencing techniques. Mature B-cell neoplasms are one of the most common lymphoma categories cytopathologists encounter. The fifth edition includes a major category of “large B-cell lymphomas,” recognizing that diffuse architecture is not always present in this larger category. In addition to the largest category of diffuse large B-cell lymphoma (DLBCL), not otherwise specified, 17 specific large B-cell lymphoma entities are listed. The previously described double- and triple-hit high-grade B-cell lymphomas with specific rearrangements have been redefined and are restricted to cases with MYC and BCL2 rearrangements since these are more homogeneous. Cases with MYC and BCL6 rearrangements (without BCL2 rearrangement) are more heterogeneous and are now included in the broader large B-cell lymphoma category. The reader is referred to the online atlas for additional changes and new entities.

In summary, the WHO Reporting System for Lymph Node, Spleen, and Thymus Cytopathology aims to standardize reporting of specimens, with the goal of promoting optimal patient care, including appropriate patient follow-up, testing, and management.

1. Andrew Field: The WHO system for reporting lymph node cytopathology. Accessed Feb. 12, 2024. https://​www.​youtu​be.com/watch?v=​x6ft5​2vsts8
2. Nigam JS, Bharti JN, Pradeep I, Rath A. Upcoming World Health Organization system for reporting lymph node cytopathology—a preliminary outline. Cytopathology. 2024;35(2):321–323.
3. Al-Abbadi MA, Barroca H, Bode-Lesniewska B, et al. A proposal for the performance, classification, and reporting of lymph node fine-needle aspiration cytopathology: the Sydney system. Acta Cytol. 2020;64(4):306­–322.
4. Ahuja S, Aziz Khan A, Ahuja R, Ahuja P, Zaheer S. Systematic review and meta-analysis of the diagnostic accuracy of the Sydney system for reporting lymph node fine-needle aspiration biopsy in diagnosing malignancy. Acta Cytol. 2024;68(1):13–25.
5. Kroft SH, Sever CE, Bagg A, et al. Laboratory workup of lymphoma in adults: guideline from the American Society for Clinical Pathology and the College of American Pathologists. Arch Pathol Lab Med. 2021;145(3):269–290.
6. WHO Classification of Tumours Editorial Board. Haematolymphoid Tumours. World Health Organization; 2023. WHO Classification of Tumours; 5th ed.

Dr. Bell is a PGY-3 AP/CP resident at Cleveland Clinic. Dr. Davey is professor emerita, University of Central Florida, and pathologist, Orlando VA Medical Center. Both are members of the CAP Cytopathology Committee.