Can Past Cancer Treatment or Chemical Exposure Cause CMML?

Many people diagnosed with chronic myelomonocytic leukemia (CMML) wonder if a past event caused their illness. While most cases of CMML have no identifiable cause, previous cancer treatments like chemotherapy and radiation can increase the risk of developing the disease. When CMML develops after these treatments, it is classified as therapy-related CMML (t-CMML) ^[1].

It is important to understand that having a history of cancer treatment or chemical exposure does not definitively prove it was the single cause of your leukemia. Furthermore, you should not regret receiving past cancer treatments—those therapies were necessary at the time, and the benefits of treating a prior cancer outweigh the possible long-term risks.

Therapy-Related CMML (t-CMML)

In large observational studies of patients already diagnosed with CMML, researchers found that about 9% to 11% had a prior history of chemotherapy, radiation therapy, or both ^[1][2]. In these specific study groups, the median time between their initial cancer treatment and their CMML diagnosis was roughly 6.5 to 7 years ^[1][3]. However, these are population estimates, and the timeline can vary widely from person to person.

Certain classes of chemotherapy drugs are known to be associated with therapy-related blood cancers in general (known as therapy-related myeloid neoplasms). These include alkylating agents (drugs that damage the DNA of cancer cells) and topoisomerase-II inhibitors (drugs that interfere with enzymes needed for cell division) ^[4][5]. The actual risk depends heavily on the specific drug, cumulative dose, combination of treatments, radiation, and individual patient factors.

Even when t-CMML develops, the prior treatment might not be the whole story. As we age, it is common to develop early mutations in blood-forming stem cells—a condition called clonal hematopoiesis ^[6][7]. A person may have had these cells before ever receiving cancer therapy, and the treatment simply added stress to the bone marrow. It is usually impossible to know retrospectively if you had clonal hematopoiesis before your cancer treatment, and developing it is no one’s fault.

Occupational and Chemical Exposures

While there is strong evidence linking some industrial chemicals (like heavy benzene exposure) to other types of leukemia, the evidence linking everyday or workplace chemicals specifically to CMML is much weaker and often inconsistent.

There are isolated case reports of CMML developing after substantial or repeated exposure to very specific industrial materials ^[8], but a single case report cannot prove that the substance causes the disease. For most people, routine environmental or occupational exposures cannot be proven as the cause of their CMML. If you have a history of substantial, documented industrial exposures, you can discuss this with your doctor or an occupational medicine specialist.

The Role of Your Treatment Records

If you have a history of prior cancer therapy, it is helpful for your hematologist to review your past medical records. Studies suggest that, as a group, t-CMML cases may be associated with more adverse genetic features compared to de novo CMML (CMML that happens on its own without prior qualifying therapy) ^[1][3]. However, individual prognosis depends on your specific blood counts, overall health, and current genetic testing, not just whether you had prior treatment.

Your care team will look for:

• The exact chemotherapy drugs and cumulative doses you received.
• The type and location of any radiation therapy.
• The dates of your past treatments.

While obtaining these records provides important context for your doctor’s risk assessment and classification, do not worry if some old records are missing. Your current CMML diagnosis and treatment plan are based primarily on your current bone marrow biopsy, blood counts, and molecular tests.