Knowing whether Kadcyla is working involves a combination of imaging scans, blood tests, and changes in how you feel. There’s no single moment where your doctor declares it effective. Instead, your care team tracks several signals over time, typically reassessing every few treatment cycles to build a picture of how your cancer is responding.
How Doctors Measure Response
The most definitive way to know if Kadcyla is working is through imaging, usually CT scans, PET scans, or MRIs performed every two to four cycles (roughly every 6 to 12 weeks). Your oncologist compares your tumor measurements against a baseline scan taken before treatment started.
Doctors use a standardized system to categorize your response. A partial response means your tumors have shrunk by at least 30% in their combined diameters. A complete response means all detectable tumors have disappeared on imaging. If your tumors stay roughly the same size without growing, that’s classified as stable disease, and it still counts as the drug doing its job. Kadcyla is designed to be given every three weeks until the cancer progresses or side effects become unmanageable, so stable disease can mean months or even years of continued treatment.
The result you don’t want to see is progressive disease, meaning new tumors have appeared or existing ones have grown significantly. That’s the clearest sign Kadcyla has stopped working, and your oncologist will discuss switching to a different treatment.
What You Might Notice Physically
Imaging is the gold standard, but your own body can offer clues between scans. If your cancer was causing specific symptoms before treatment, like pain from a tumor pressing on tissue, a palpable lump, shortness of breath from lung involvement, or bone pain from metastases, improvement in those symptoms often tracks with treatment response. Pain that gradually eases over the first few cycles, a lump that feels smaller, or breathing that gets easier are all encouraging signs.
General well-being matters too. Many people with metastatic breast cancer experience fatigue, loss of appetite, or unexplained weight loss from the disease itself. If those symptoms improve once treatment is underway, it can suggest the cancer burden is shrinking. That said, Kadcyla has its own side effects, including fatigue, nausea, and muscle or joint pain, so it can be hard to separate treatment side effects from cancer symptoms. Your oncologist can help you sort out which is which.
The absence of new symptoms is also meaningful. No new lumps, no new areas of pain, and no dramatic changes in energy or weight are quiet but real indicators that the disease isn’t advancing.
Blood Work and What It Shows
Before every Kadcyla infusion, you’ll have blood drawn. These tests primarily monitor your safety rather than measuring how well the drug is fighting cancer, but they provide useful context.
Your liver enzymes and bilirubin are checked each cycle because Kadcyla can stress the liver. Platelet counts are monitored every cycle as well, since low platelets are one of the most common side effects. Your heart function is assessed through an echocardiogram or similar test roughly every three months, because the trastuzumab component of Kadcyla can affect the heart’s pumping ability.
Some oncologists also track tumor markers through blood tests. These are proteins that certain cancers release into the bloodstream. A declining trend in tumor markers over several cycles can support what imaging is showing, but tumor markers alone aren’t reliable enough to confirm or rule out a response. They can fluctuate for reasons unrelated to treatment, and not all patients have elevated markers to begin with. Your doctor will interpret them alongside scans, not instead of them.
Timing: When You’ll Get Answers
Most oncologists order the first reassessment scan after two to three cycles, which means roughly 6 to 9 weeks into treatment. This is typically the earliest point where you’ll get a clear answer about whether the drug is having an effect. Some responses take longer to show up on imaging, so a scan that shows stable disease early on isn’t a bad result. It may take two or three scan intervals to see a definitive trend.
Between scans, your oncologist is gathering information at every visit. They’ll ask about your symptoms, examine you, and review your blood work. All of these data points feed into the overall assessment. If something concerning pops up, like new pain in a specific area, your doctor may order imaging earlier than planned.
What “Working” Really Means
It’s worth reframing what success looks like with Kadcyla, especially in the metastatic setting. Dramatic tumor shrinkage is the best-case scenario, but keeping the cancer stable for an extended period is a genuine win. Many patients stay on Kadcyla for months with stable disease, maintaining their quality of life while the drug holds the cancer in check.
If you’re receiving Kadcyla as adjuvant therapy (after surgery for early-stage HER2-positive breast cancer), the goal is different. There’s no tumor left to measure on scans. Instead, “working” means the cancer doesn’t come back. Your oncologist will monitor you with regular checkups and periodic scans, but the absence of recurrence is the evidence of success. In this setting, Kadcyla is given for a fixed number of cycles rather than indefinitely.
The clearest signs that Kadcyla is working are tumors shrinking or staying stable on imaging, cancer-related symptoms improving or not worsening, and tumor markers trending downward if they were elevated at the start. No single one of these tells the full story on its own, but together they give your oncologist a reliable picture of how your treatment is going.