Lymphoma is a cancer that originates in the lymphatic system, the network of tissues and organs that helps rid the body of toxins and waste. The definitive diagnosis of this disease relies on histology, the microscopic examination of tissue. Through histology, doctors observe the cancerous cells, providing the foundational information needed to classify the lymphoma and determine a course of action.
The Pathologist’s Role in Diagnosis
A pathologist, a physician specializing in disease diagnosis by analyzing tissue, processes and interprets a sample from a biopsy. The tissue is preserved, thinly sliced, and mounted onto glass slides. These sections are then treated with stains, like hematoxylin and eosin (H&E), which color different cell parts to make them visible under a microscope.
During the initial review, the pathologist focuses on the tissue’s overall structure, or architecture. A healthy lymph node has a well-defined organization, which the pathologist checks for disruption by a uniform population of abnormal cells. The effacement of this normal architecture provides strong clues about the presence of lymphoma and sets the stage for a more detailed investigation.
Hodgkin Lymphoma Under the Microscope
The diagnosis of Hodgkin lymphoma depends on identifying the Reed-Sternberg cell, the hallmark of the disease. Under the microscope, Reed-Sternberg cells are very large, often measuring between 30 and 50 micrometers. They have a large amount of cytoplasm and either multiple nuclei or a single nucleus that is bilobed.
This bilobed nucleus often gives the cell an “owl’s eye” appearance, with two prominent, pink-staining nucleoli. While these are the primary cancer cells, they are sparse, making up only about 1% of the cells within the affected lymph node. The majority of the tissue is composed of a mixture of non-cancerous inflammatory cells, such as lymphocytes, eosinophils, and histiocytes. The composition of this surrounding cellular environment helps pathologists classify the disease into different subtypes.
Non-Hodgkin Lymphoma Cellular Patterns
In contrast to Hodgkin lymphoma, Non-Hodgkin lymphomas (NHLs) are a diverse group of cancers defined by the appearance of the malignant lymphocytes. Pathologists distinguish between NHL subtypes by analyzing two main features. These are the growth pattern of the cancer cells and the cell of origin, meaning whether the lymphoma arises from B-cells or T-cells.
The growth pattern is categorized as either follicular or diffuse. A follicular pattern appears as abnormal, nodular clusters of cancer cells that mimic the natural follicles of a lymph node. In contrast, a diffuse pattern is characterized by a widespread sheet of cancerous lymphocytes that has effaced the normal lymph node structure.
The cell of origin is another factor in classification, as most NHLs are of B-cell origin, with T-cell lymphomas being less common. Pathologists get clues about the cell type based on the size, shape, and nuclear features of the malignant cells. For example, some lymphomas are composed of small lymphocytes, while others consist of large, rapidly dividing cells. This morphological assessment provides initial evidence before more advanced testing confirms the lineage.
Advanced Histological Techniques
After the initial assessment, pathologists use advanced methods like immunohistochemistry (IHC) to refine the diagnosis. IHC is a technique that identifies proteins, known as markers, on or inside cells. This method uses manufactured antibodies designed to bind to specific protein antigens. When an antibody finds its target protein on the lymphoma cells, a chemical reaction creates a visible color change, “tagging” the cells.
This molecular tagging is important for diagnosing and subtyping lymphomas. For instance, IHC can determine if a lymphoma is from B-cells by staining for markers like CD20, or from T-cells by staining for CD3. Specific patterns of marker expression can pinpoint the subtype of lymphoma. Reed-Sternberg cells in classical Hodgkin lymphoma are characteristically positive for CD30 and CD15, while being negative for CD20, providing a unique signature for the final pathology report.
How Histology Guides Treatment Decisions
The information in the final histology report is the foundation for an oncologist’s treatment plan. The distinction between Hodgkin and Non-Hodgkin lymphoma is a primary consideration, as the therapeutic approaches for these two categories are different. For example, chemotherapy regimens for Hodgkin lymphoma, such as ABVD, differ from the R-CHOP regimen used for diffuse large B-cell lymphoma, a type of NHL.
The specific subtype of lymphoma further refines treatment choices. The identification of the CD20 marker on B-cell lymphomas allows for targeted therapies like rituximab, a monoclonal antibody that targets CD20-positive cells. The histological analysis also provides information on the lymphoma’s grade, or how quickly the cells are proliferating, which influences the intensity of the therapy. The pathologist’s examination provides a detailed roadmap that guides the clinical team toward the most effective treatment strategy.