Follicular lymphoma most often looks like painless, swollen lymph nodes, typically in the neck or armpits. The swelling is firm, rubbery, and not tender to the touch. Many people discover it by chance during a routine exam or while feeling a lump that doesn’t hurt and doesn’t go away. Because the swelling is painless and grows slowly, it can be present for months or even years before anyone notices.
How Swollen Lymph Nodes Feel and Where They Appear
The most common locations for noticeable swelling are the neck and armpits. Lymph nodes in the abdomen and, less commonly, the groin can also be involved, though abdominal nodes aren’t something you’d feel from the outside. The affected nodes are typically firm and movable under the skin, without the tenderness you’d expect from an infection.
Size varies widely. Nodes may range from just over a centimeter to 6 cm or larger. When a mass exceeds 7 cm in its widest dimension, it’s considered “bulky disease,” which can sometimes create visible fullness or pressure symptoms in the area. But many people have nodes in the 1 to 3 cm range, roughly the size of a grape to a walnut, that they might not notice unless they happen to press on the right spot.
One hallmark of follicular lymphoma is that swollen nodes may wax and wane on their own, sometimes shrinking temporarily before growing again. This can be misleading and delay diagnosis.
Skin Signs in Cutaneous Cases
A less common form, primary cutaneous follicle center lymphoma, shows up directly on the skin rather than in lymph nodes. It appears as firm, painless bumps or raised lesions, most often on the scalp, forehead, neck, or upper torso. The color depends on your skin tone: on lighter skin, they tend to look red or pink; on darker skin, they may appear purple or a deeper shade that contrasts with surrounding skin.
You might have a single bump or a small cluster grouped together. These lesions grow slowly, often over months to years, and they don’t itch or hurt in most cases. This cutaneous form carries an excellent prognosis and behaves very differently from follicular lymphoma found in lymph nodes.
What It Looks Like on Imaging Scans
Most people with follicular lymphoma will get a PET-CT scan as part of their workup, and this is where the disease often reveals more than what’s visible or palpable from the outside. On PET-CT, the lymphoma shows up as clusters of “lit up” lymph nodes that absorb more of the radioactive tracer than normal tissue. These bright spots appear both above and below the diaphragm in most patients, which is one reason the disease is often diagnosed at an advanced stage.
The brightness on PET scans is measured with a number called SUVmax. In typical follicular lymphoma, these values average around 6 to 8, which is noticeably lower than aggressive lymphomas. The spleen, bones, and gastrointestinal tract can also light up, revealing disease that wasn’t suspected clinically. One limitation: PET scans miss bone marrow involvement about half the time, so a separate biopsy is often needed to check.
CT scans alone show enlarged nodes as round or oval soft-tissue masses, sometimes clustered together in chains along major blood vessels. They typically enhance mildly with contrast dye and lack the necrotic (dead-tissue) centers more common in aggressive cancers.
What Pathologists See Under the Microscope
The defining feature of follicular lymphoma is its growth pattern. Under a microscope, the normal architecture of the lymph node is replaced by tightly packed, similarly sized round structures called neoplastic follicles. These sit in a “back-to-back” arrangement, crowding out the normal tissue. In a healthy lymph node, follicles have distinct surrounding zones and vary in size. In follicular lymphoma, those surrounding zones are thin or absent, and the follicles look monotonously uniform.
Inside these follicles are two main cell types. The smaller ones have angular, irregularly shaped nuclei and very little surrounding material. The larger ones are roughly three times the size of a normal white blood cell, with rounder nuclei and visible internal structures. The ratio between these two cell types determines the grade of the disease, which matters for treatment decisions.
How Grading Changes the Microscopic Picture
Pathologists grade follicular lymphoma from 1 to 3 by counting how many large cells appear in a single microscopic field:
- Grade 1: 0 to 5 large cells per field. The tissue looks dominated by small, angular cells. This is the most common and slowest-growing pattern.
- Grade 2: 6 to 15 large cells per field. Still predominantly small cells, but with a noticeable increase in larger ones.
- Grade 3A: More than 15 large cells per field, but small cells are still present. The follicular pattern is maintained.
- Grade 3B: More than 15 large cells per field, and the small cells are essentially absent. This looks and behaves more like an aggressive lymphoma and is now classified separately as follicular large B-cell lymphoma in the latest WHO system.
Grades 1 and 2 are grouped together clinically because they behave similarly. Grade 3B is treated more like diffuse large B-cell lymphoma due to its aggressive nature.
Protein Markers That Confirm the Diagnosis
Under the microscope, follicular lymphoma can sometimes resemble other types of lymphoma. Pathologists use special stains to detect proteins on the surface of the cancer cells. Follicular lymphoma cells are positive for a set of proteins associated with the germinal center, the part of the lymph node where healthy B-cells normally mature. When all four of these germinal center markers light up on a stained tissue slide, the diagnosis is confirmed with high confidence.
About 85% of cases also carry a specific genetic rearrangement that causes overproduction of a survival protein, which is why the cancer cells accumulate over time. This rearrangement is visible with specialized lab testing and is one of the most consistent genetic signatures across all cancers.
Warning Signs of Transformation
Follicular lymphoma can transform into a faster-growing cancer, most commonly diffuse large B-cell lymphoma. This transformation changes both how the disease looks and how it behaves. Under the microscope, the orderly follicular pattern breaks down and is replaced by sheets of large cells growing without structure. The organized, repetitive follicles give way to a chaotic, diffuse pattern with a high rate of cell division.
From the outside, transformation often announces itself through rapid, localized growth of one node or node cluster that seems out of proportion to the rest of the disease. Other signs include new pain at a lymph node site, drenching night sweats, unexplained fevers, significant weight loss, or swelling in unusual locations like the liver or bones. On PET scans, a transformed site often shows dramatically higher tracer uptake, with SUVmax values jumping from the typical 6 to 8 range up to 20 or higher. A value above 10 at any single site raises significant concern for aggressive transformation.
Not every flare in symptoms means transformation has occurred, but any sudden change in the pattern of disease, especially rapid growth of a single area, warrants a new biopsy to check.