SLL leukemia, properly called small lymphocytic lymphoma, is a slow-growing cancer of B lymphocytes that is the same disease as chronic lymphocytic leukemia (CLL). The difference is simply where the cancer cells collect: in SLL they gather mainly in the lymph nodes and spleen, while in CLL they fill the blood and bone marrow. Many people with SLL live for years without needing treatment, and when therapy is needed, modern targeted drugs work well.
Because the two share one biology, the World Health Organization groups them as a single disease, often written CLL/SLL. In this overview I explain how SLL is defined, what causes symptoms, how it is diagnosed, and how treatment decisions are made. It is part of the wider family of hematologic cancers.
What Is SLL Leukemia?
SLL arises from mature B lymphocytes, the white blood cells that make antibodies. A single B cell acquires genetic changes and multiplies into a large population of identical (clonal) cells. These cells look small and normal under the microscope but live far too long and do not function properly.
The dividing line between SLL and CLL is a blood count. CLL is diagnosed when there are at least 5 x 10^9 clonal B cells per liter of blood. SLL is diagnosed when there are fewer than that in the blood but the same cells are enlarging lymph nodes or the spleen.
| Feature | SLL | CLL |
|---|---|---|
| Main location | Lymph nodes and spleen | Blood and bone marrow |
| Clonal B cells in blood | Below 5 x 10^9/L | 5 x 10^9/L or more |
| Usual diagnostic test | Lymph node biopsy | Blood flow cytometry |
| Staging system | Lugano (Ann Arbor-based) | Rai or Binet |
| Treatment | Same drugs and criteria as CLL | Same drugs and criteria as SLL |
SLL can later evolve into a CLL picture as cells spill into the blood, and the reverse is also possible. Neither change alters the underlying disease.
Causes and Risk Factors
No single cause of SLL is known. The strongest risk factors are older age, since most patients are diagnosed over 60, and a family history of CLL, SLL, or other lymphomas. Men are affected more often than women.
Some studies link farming chemicals such as herbicides to a higher risk, and in the United States exposure to Agent Orange is recognized as a risk factor for CLL/SLL. It is not caused by radiation, lifestyle, or diet in any clear way, and it cannot be caught.
Genetic features that shape the disease
Genetic changes inside the cancer cells do not usually explain why SLL starts, but they strongly influence how it behaves. Loss of part of chromosome 17 (del(17p)) or TP53 mutations predict a more aggressive course and poor response to chemotherapy. Loss of 11q, which includes the ATM gene, is also linked to bulkier disease. By contrast, mutated IGHV status and an isolated 13q deletion are associated with a slower course.
Signs and Symptoms
Many people with SLL have no symptoms and are diagnosed after a doctor notices enlarged lymph nodes. When symptoms do occur, they commonly include:
- Painless swollen lymph nodes in the neck, armpits, or groin
- A sense of fullness under the left ribs from an enlarged spleen
- Fatigue
- B symptoms: fevers, drenching night sweats, and unintentional weight loss
- Frequent infections, because the immune system is weakened
Some patients develop autoimmune problems, where the immune system attacks their own red cells or platelets, causing anemia or bruising.
Diagnosis and Testing
Biopsy and flow cytometry
SLL is usually confirmed with a lymph node biopsy, ideally removing a whole node. The cells have a characteristic marker pattern on flow cytometry or tissue staining: they carry CD5, CD19, and CD23, with weak CD20 and weak surface immunoglobulin. This pattern helps separate SLL from look-alikes such as mantle cell lymphoma, which is also CD5 positive but usually CD23 negative.
Staging and baseline tests
A complete blood count, kidney and liver tests, and a CT scan of the neck, chest, abdomen, and pelvis show the extent of disease. A bone marrow biopsy is not always required but may help explain low blood counts. Before treatment, the cancer cells are tested for del(17p), TP53 mutation, and IGHV status because these guide drug choice.
Treatment Options
Watch and wait
Early treatment of symptom-free SLL does not help people live longer, so most start with active monitoring. This means regular check-ups and blood tests. Patients often find this hard emotionally, but it spares them drug side effects for as long as possible.
When treatment starts
Therapy is started for troublesome symptoms, bulky or rapidly growing nodes, a large painful spleen, falling hemoglobin or platelets, or autoimmune complications that do not respond to standard treatment.
Modern therapy
- BTK inhibitors (ibrutinib, acalabrutinib, zanubrutinib): daily tablets taken continuously
- Venetoclax, a BCL-2 inhibitor, combined with the antibody obinutuzumab for a fixed course
- Chemoimmunotherapy such as fludarabine, cyclophosphamide, and rituximab, now used less often and only in selected fit patients
- CAR T-cell therapy or stem cell transplant for disease that relapses after several lines
Patients with del(17p) or TP53 mutations are generally treated with targeted drugs rather than chemotherapy. For how SLL fits among other blood cancers, see our leukemia guide.
When to See a Doctor
See your doctor if a lymph node stays enlarged for more than a few weeks, or if you have night sweats, fevers, or weight loss without explanation. If you already have SLL, report new rapidly growing nodes, marked fatigue, or high fevers promptly. Rarely, SLL transforms into an aggressive lymphoma, called Richter transformation, and these changes can be the first sign.
Frequently Asked Questions
Is SLL a leukemia or a lymphoma?
Technically it is a lymphoma, but it is the same disease as CLL. Doctors use the label that matches where most of the cancer cells are found.
Is SLL curable?
Current treatments usually control SLL for long periods rather than cure it. Many people live with it for many years, often with good quality of life.
Why am I not being treated right away?
For symptom-free SLL, starting treatment early has not been shown to extend life. Waiting until treatment is needed avoids side effects without harming the outcome.
Does SLL raise my infection risk?
Yes. SLL weakens antibody production, so vaccines (avoiding live vaccines) and prompt treatment of infections are important.
Key Takeaways
- SLL and CLL are one disease; SLL describes cells mainly in lymph nodes rather than blood.
- Diagnosis rests on a biopsy with a CD5, CD19, CD23 marker pattern.
- Many patients are safely monitored for years before treatment.
- BTK and BCL-2 inhibitors have largely replaced chemotherapy.
