Castleman disease (CD) is a cytokine-driven lymphoproliferative disorder with characteristic lymph node histopathology. This standard operating procedure (SOP) provides a method for grading the key histologic features of Castleman disease in excisional lymph node biopsies. It is intended for pathologists, clinicians, trainees and patients as a teaching guide. The procedure can be applied using a conventional light microscope or via digital pathology, using hematoxylin & eosin (H&E) stained slides. Core needle biopsies are not adequate for full histologic evaluation. An intact/excisional lymph node is required for accurate grading of architecture and features.
Before grading, scan the entire slide at low magnification to get oriented to the lymph node architecture. Identify where follicles are located and the overall pattern (e.g., nodularity of follicles, interfollicular regions, etc.). Then proceed with the stepwise evaluation below.
Identify if any lymphoid follicles contain two or more germinal centers within a single mantle zone. Twinned germinal centers appear as dual pale centers within one follicle, often enveloped by a shared mantle zone.
Grading (Ideally evaluate at least 10 follicles):| Grade | Description |
|---|---|
| 0 | No twinned follicles |
| 1 | ~10% to <25% of follicles show twinning |
| 2 | 25% to <50% |
| 3 | ≥50% of follicles are twinned |
These are abnormally small, “burned out” germinal centers (approximately <15 lymphocytes across in diameter). They often appear depleted of lymphocytes and may show hyalinization (pink collagenous deposition) in the center. Mantle zones around regressed centers are typically expanded and form concentric rings (“onion-skin” pattern) of small lymphocytes. Sometimes a sclerotic penetrating arteriole is seen coursing through the regressed center at right angle, creating the classic “lollipop” appearance (a vessel with a hyalinized coat in the middle of an atrophic follicle). Regressed germinal centers are a hallmark of the hyaline-vascular (hypervascular) variant of CD.
Grading (Ideally evaluate at least 10 follicles):| Grade | Description |
|---|---|
| 0 | Germinal centers are normal (0% regressed) |
| 1 | 10% to <25% of follicles are regressed |
| 2 | 25% to <50% |
| 3 | ≥50% of follicles in the node are regressed/atrophic |
These are the opposite of regressed. Hyperplastic GCs are enlarged, hypercellular germinal centers with abundant follicular lymphocytes and active appearance (many tingible body macrophages, high mitotic activity). Follicular hyperplasia is often accompanied by a prominent mantle zone but not onion-skinned (mantle may be thinner or irregular because the germinal center is expanded). This feature is typically prominent in the plasma cell variant of CD or reactive nodes. On low power, hyperplastic GCs appear as numerous large pale nodules (approximately >30 lymphocytes across in diameter).
Grading (Ideally evaluate at least 10 follicles):| Grade | Description |
|---|---|
| 0 | None or very few small follicles (0% large GCs) |
| 1 | 10% to <25% of follicles are large/hyperplastic |
| 2 | 25% to <50% |
| 3 | ≥50% of follicles show hyperplasia |
| Grade | Description |
|---|---|
| 0 | Interfollicular vasculature is normal (no obvious increase) |
| 1 | Mild increase (few more vessels than usual, in 10% to <25% of interfollicular surface area) |
| 2 | Moderate (definite increase in vessel number in 25% to <50% of surface area) |
| 3 | Marked hypervascularity (easy to appreciate, ≥50% of interfollicular regions packed with proliferating vessels). A grade 3 vascularity often correlates with easily seeing the piercing vessels at 4x scan (the lollipop sign). |
Evaluate the number of plasma cells in the interfollicular regions. Plasma cells are distinctive with their eccentric nuclei, perinuclear halo, and abundant cytoplasm. Count the number of plasma cells in ideally at least 10 high-power fields (HPFs) across the node.
| Grade | Description |
|---|---|
| 0 | No significant plasmacytosis (only rare plasma cells scattered, essentially 0 to 10% of interfollicular area involved) |
| 1 | Mild (plasma cells present but sparse – 10% to 25% of cellularity) |
| 2 | Moderate (notable plasma cells 25% to <50% of interfollicular cellularity) |
| 3 | Marked plasmacytosis (large confluent sheets of plasma cells replacing ≥50% of interfollicular regions) |
Follicular dendritic cells are stromal cells present in germinal centers that form a meshwork to support B-cells and T-cells. In Castleman disease, especially the hyaline-vascular type, FDCs can be prominent and dysplastic in the regressed germinal centers. A regressed GC may consist almost entirely of FDCs with very few lymphocytes. At high power, FDC prominence is recognized by the presence of numerous spindle to ovoid nuclei in the germinal center and a pale eosinophilic stroma in GC. Immunostains like CD21 can highlight this meshwork, but are not a part of grading.
| Grade | Description |
|---|---|
| 0 | FDCs are not particularly conspicuous (germinal centers have normal cellular makeup, you cannot readily identify FDCs on H&E) |
| 1 | Slight prominence (<25% of cellularity of GC) |
| 2 | Moderate (FDC meshwork obvious in many follicles 25% to <50%) |
| 3 | Marked FDC prominence (≥50% of cells are FDCs) |
This grading SOP can be executed via traditional microscopy or digital slides:
For convenience, the below summarizes the histologic features, definitions, and scoring criteria:
| Feature | Description | Grade 0 | Grade 1 | Grade 2 | Grade 3 |
|---|---|---|---|---|---|
| Follicular Twinning | >1 germinal center in one follicle (shared mantle zone, low power) | None (0%) | ~10% to <25% follicles twinned | 25% to <50% | ≥50% follicles twinned |
| Regressed Germinal Centers | Atrophic, lymphocyte-depleted GCs ± hyaline, onion-skin mantle | None (0%) | 10% to <25% follicles regressed | 25% to <50% | ≥50% follicles regressed |
| Hyperplastic Germinal Centers | Enlarged, reactive-appearing GCs | None or very few (0%) | 10% to <25% follicles hyperplastic | 25% to <50% | ≥50% follicles hyperplastic |
| Interfollicular Vascularity | Proliferation of blood vessels in interfollicular areas | Normal | Mild (10% to <25%) | Moderate (25% to <50%) | Marked (≥50%) |
| Plasmacytosis | Excess plasma cells in interfollicular zones | None (0% to 10%) | Mild (10% to 25%) | Moderate (25% to <50%) | Marked (≥50%) |
| FDC Prominence | Expansion of follicular dendritic cell networks (often filling regressed GCs) | Normal meshwork | Slight (<25%) | Moderate (25% to <50%) | Marked (≥50%) |
Each feature should be independently scored as grade 0, 1, 2, or 3 as defined. Do not sum the scores to make a diagnosis. Current diagnostic criteria for Castleman disease do not use a total score cutoff. Instead, the presence of high-grade changes in either spectrum (vascular/FDC or plasma cell) helps classify the histopathological variant. In fact, the World Health Organization (WHO) classification for idiopathic multicentric CD (iMCD) requires at least a grade 2 or 3 in either regressed GCs or plasmacytosis (or both), along with supportive clinical findings, to diagnose CD.
Remember that no single histologic feature is pathognomonic for Castleman disease: all features must be interpreted together. As research progresses, the hope is to quantify these changes for prognostic or diagnostic indices, but at present the grading is primarily for teaching and documentation. Always mention in the bottom line or comment of your report that an excisional biopsy was used and note the dominant histologic pattern and grade (ie, “Castleman disease, hypervascular type, with marked regressed germinal centers (grade 3), hypervascularity (grade 3), and minimal plasmacytosis (grade 0-1)”). This conveys the key findings to clinicians.