Keytruda doesn’t work like chemotherapy, where the drug directly kills cancer cells within days. Instead, it reactivates your immune system to fight the cancer itself, a process that typically takes weeks to months before measurable tumor shrinkage appears on imaging. Most oncologists schedule the first scan to check for a response somewhere between 6 and 12 weeks after treatment begins, depending on the type of cancer being treated.
How Keytruda Works in Your Body
Cancer cells survive partly by hiding from your immune system. They display a protein on their surface that acts like a “stand down” signal to your T cells, the immune cells responsible for finding and destroying threats. When a T cell encounters this signal, it treats the cancer cell as harmless and moves on.
Keytruda blocks that signal. It attaches to a receptor on T cells called PD-1, preventing cancer cells from flipping the off switch. Once that brake is released, your T cells can recognize the cancer as a threat, multiply, travel to the tumor, and begin attacking it. This chain of events, from immune activation to meaningful tumor damage, is why Keytruda takes longer to show results than treatments that poison cancer cells directly. Your immune system needs time to ramp up, infiltrate the tumor, and kill enough cancer cells for the difference to show on a scan.
When Your First Scan Happens
The timeline for your first imaging check depends on your cancer type. For lung cancer, the first scan is often scheduled at 6 weeks after starting treatment. For melanoma, it’s typically at 12 weeks. Bladder cancer patients usually get their first response check at 9 weeks. Hodgkin lymphoma and some other blood cancers follow a 12-week schedule.
After that initial check, scans continue at regular intervals, generally every 6 to 12 weeks for the first year and less frequently after that. These follow-up scans matter because Keytruda’s effects can build over time. Some people show no change at their first scan but have clear tumor shrinkage by the second or third. A single scan rarely tells the whole story with immunotherapy.
What “Working” Looks Like
Response to Keytruda falls into a few categories, and understanding them helps set realistic expectations. A complete response, where the tumor disappears entirely on imaging, is possible but uncommon in most cancer types. A partial response, where the tumor shrinks significantly but doesn’t vanish, is more typical among people who respond. Stable disease, where the tumor stops growing but doesn’t shrink much, can also be a meaningful win. Stopping a cancer from progressing buys time and often translates into longer survival.
Not everyone responds. Across different cancers, response rates to Keytruda vary widely, from roughly 15% to over 40% depending on the cancer type, whether it’s combined with chemotherapy, and how strongly the tumor expresses the PD-L1 protein that Keytruda targets. Your oncologist may have tested your tumor for PD-L1 levels before starting treatment, which gives a rough sense of how likely you are to benefit.
Why Tumors Sometimes Appear to Grow First
One of the more unsettling aspects of immunotherapy is a phenomenon called pseudoprogression. This is when a tumor appears to grow larger or new spots appear on a scan, even though the treatment is actually working. It happens because immune cells flood into the tumor, causing it to swell temporarily before the cancer cells inside are destroyed.
Pooled data across immunotherapy trials estimates that pseudoprogression occurs in about 15% of patients whose scans initially look worse. It tends to show up at the first follow-up scan after starting treatment. This is one reason oncologists don’t immediately stop Keytruda after a single scan showing growth. They’ll often continue treatment and rescan a few weeks later to see if the apparent progression reverses. If you’re feeling better physically but your scan looks worse, pseudoprogression is one possible explanation worth discussing with your care team.
That said, the majority of early tumor growth on scans does reflect true progression. Your oncologist weighs the imaging results alongside your symptoms, blood work, and overall condition to decide whether to continue Keytruda or switch strategies.
Signs You May Notice Before a Scan
Imaging is the gold standard for measuring response, but some people notice changes in their body before their first scheduled scan. Reduced pain, improved energy, decreased coughing (in lung cancer), or shrinking of visible or palpable tumors can all signal that the drug is working. These aren’t reliable on their own, but they provide useful context.
Conversely, some people feel worse before they feel better. Keytruda’s immune activation can cause side effects like fatigue, skin rashes, joint pain, or inflammation in organs like the thyroid, lungs, or intestines. These side effects are distinct from cancer progression. In some cases, experiencing immune-related side effects has been loosely associated with a better treatment response, though this isn’t consistent enough to use as a predictor.
How Long Treatment Continues
If Keytruda is working, treatment typically continues for up to two years. Some patients whose cancer responds well may stop after two years and remain in remission. Others may need to restart if the cancer returns after stopping. For patients who aren’t responding, the decision to stop usually comes within the first few months, after two or three scans confirm that the cancer is progressing despite treatment.
The first three to four months are the most uncertain period. You’re waiting for your immune system to do its job, managing side effects, and living scan to scan. For many people, the picture becomes clearer by the 12-week mark, though late responses extending beyond that window do occur. Patience during this early phase is genuinely part of how immunotherapy works.