Non-Hodgkin's lymphomas can be slow-growing (low-grade) or rapidly growing (high-grade) cancer. For most patients, the cause is unknown, but lymphomas may develop in people with suppressed immune systems as a result of organ transplantation, for instance. The tumors are graded according to their level of malignancy (aggressiveness)-- low-grade, intermediate-grade or high-grade. Burkitt's tumor is an example of a high-grade lymphoma. Non-Hodgkin's tumors occur more frequently than Hodgkin's lymphoma. Usually, non-Hodgkin's lymphomas affect people older than 50. High-risk groups include organ transplant recipients and immunosuppressed people. The incidence is 3 in 10,000 people.
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