Yes, breast cancer can spread while you are receiving chemotherapy, though it is uncommon. In a study of over 1,400 patients receiving chemotherapy before surgery, about 5% experienced tumor progression during treatment. The vast majority of patients see their tumors shrink or disappear entirely, but a small percentage have cancer that continues to grow or spread despite active treatment.
How Often Progression Happens
Most patients respond well to chemotherapy. In one large analysis, 38% of patients had a complete response, meaning no detectable cancer remained in the breast after treatment. Another 40% had a partial response, with significant tumor shrinkage. About 17% had stable disease, where the tumor neither grew nor shrank meaningfully. Only 5% had progressive disease, where the tumor grew during treatment.
A separate study of nearly 2,000 patients across all breast cancer subtypes found an even lower progression rate of 3%. The factors most strongly linked to progression during chemotherapy were advanced tumor stage at diagnosis, negative estrogen receptor status, and African-American race. While these numbers are reassuring overall, progression during chemo is a serious finding. Patients whose tumors grow during treatment tend to have significantly worse long-term outcomes, which is why oncologists monitor closely and are prepared to change course.
Why Some Cancers Resist Chemotherapy
Cancer cells can develop the ability to pump chemotherapy drugs back out before the drugs do their job. They do this using specialized proteins embedded in their cell membranes that act like tiny ejection systems, grabbing drug molecules and pushing them outside the cell. This means the drug never builds up to a high enough concentration inside the cell to kill it. Some cancer cells produce large amounts of these pump proteins, making them resistant to not just one drug but many at the same time.
Cancer stem cells, a small subpopulation within a tumor, are particularly good at this. These cells tend to be naturally resistant to treatment and can regenerate the tumor even after chemotherapy kills the majority of other cancer cells. Certain signaling pathways inside cancer cells can also activate survival mechanisms that counteract the cell-killing effects of chemotherapy. This is why some tumors initially appear to respond but later start growing again.
Breast Cancer Subtype Matters
Not all breast cancers behave the same way during treatment. Triple-negative breast cancer, a subtype that lacks the three most common receptors targeted by therapy, tends to have the most polarized outcomes. These tumors are more likely to achieve a complete response to chemo, but they are also more likely to progress during treatment compared to other subtypes. It’s an all-or-nothing pattern: the same aggressive biology that makes these tumors vulnerable to chemo also makes them more capable of resistance.
Hormone receptor-positive, HER2-negative breast cancers sit at the other end of the spectrum. These slower-growing tumors rarely achieve a complete response to chemotherapy, but they also rarely progress during it. They tend to land in the partial response or stable disease categories. Your oncologist factors in your specific subtype when choosing a treatment regimen and deciding how aggressively to monitor your response.
How Doctors Detect Progression
Oncologists use imaging at scheduled intervals during treatment to assess whether the tumor is responding. MRI is commonly used to track changes in tumor size and blood supply. PET/CT scans, which detect areas of increased metabolic activity, can also reveal whether cancer cells are still actively growing or have become less active in response to treatment. These scans are typically performed before treatment begins and again during the course of chemotherapy, allowing doctors to compare results.
Clinically, progression is defined as at least a 20% increase in the combined diameter of target tumors, with a minimum absolute increase of 5 millimeters. The appearance of any new lesion also counts as progression, regardless of size changes in existing tumors.
A newer approach involves testing blood samples for fragments of tumor DNA circulating in the bloodstream. This method, sometimes called a liquid biopsy, can detect signs of cancer growth or recurrence months before it becomes visible on standard imaging. One study found this blood-based detection had a median lead time of 10.5 months ahead of clinical or radiologic relapse, with some cases detected up to 38 months earlier. This technology is increasingly being integrated into monitoring strategies, though it is not yet standard at every treatment center.
Signs You Might Notice
If breast cancer spreads to other parts of your body during treatment, the symptoms depend on where it goes. The most common sites are bones, lungs, and the liver.
- Bones: New or sudden joint pain, bone pain that doesn’t resolve, bones that fracture more easily than expected, or numbness and weakness in your arms or legs.
- Lungs: A persistent cough that won’t go away, difficulty breathing, chest pain, or frequent chest infections.
- Liver: Yellowing of the skin or eyes, unexplained itching or rash, stomach pain, loss of appetite, or nausea.
These symptoms can have many causes unrelated to cancer, and chemotherapy itself can produce overlapping side effects like fatigue and nausea. But if you notice new, persistent, or worsening symptoms during treatment, bringing them up with your oncology team allows them to investigate promptly rather than waiting for the next scheduled scan.
What Happens If Cancer Progresses
If imaging or bloodwork confirms that your cancer is growing during chemotherapy, your oncologist will switch your treatment plan. The specific next step depends on your cancer’s biology. For triple-negative breast cancer that progresses after first-line treatment, an antibody-drug conjugate called sacituzumab govitecan is now the preferred second-line option across major international guidelines, supported by strong clinical trial data. For tumors that express certain markers on their surface, immunotherapy combinations may be appropriate. Patients with inherited BRCA gene mutations may benefit from a class of targeted drugs that exploit a vulnerability in their cancer’s DNA repair system.
For other breast cancer subtypes, options include switching to a different chemotherapy drug, adding targeted therapies, or moving to hormonal treatments. The principle is the same: if one approach isn’t working, there are alternative pathways to pursue. Progression during treatment is not the same as running out of options. It means the current strategy needs to change, and oncologists plan for this possibility from the start.