Most people on immunotherapy get their first clear answer about whether treatment is working around three months in, when imaging scans can reliably show whether tumors are shrinking, stable, or growing. But between that first infusion and that first scan, there are several earlier signals you and your care team can track, from blood markers to side effects to how you physically feel.
When Scans Happen and What They Show
The standard timeline for a first follow-up scan is roughly 12 weeks after starting treatment. Your oncologist will typically order a CT or PET/CT scan at that point to compare tumor size against your baseline images. Some newer treatment protocols move this up to three to six weeks, and research in advanced melanoma has shown that PET/CT scans can detect meaningful metabolic changes in tumors as early as one week after a first dose of pembrolizumab.
What your doctor is looking for on those scans falls into a few categories. A complete response means no visible cancer remains. A partial response means measurable shrinkage. Stable disease means the tumor hasn’t grown or shrunk significantly. And progressive disease means growth or new tumors. With immunotherapy, doctors use a modified scoring system called iRECIST that accounts for the unique way these drugs work. Unlike chemotherapy, where any growth is bad news, immunotherapy allows for a “wait and confirm” approach: if a scan shows possible progression, clinicians can rescan a few weeks later before making a final call.
Why Tumors Sometimes Grow Before They Shrink
One of the most confusing aspects of immunotherapy is pseudoprogression, where tumors appear to get larger on imaging before eventually shrinking. This happens because immune cells flood into the tumor, temporarily swelling it. On a scan, this influx of immune cells looks identical to actual cancer growth.
Pseudoprogression is real, but it’s less common than many people hope. In melanoma, where it’s most frequently documented, it occurs in roughly 10% to 25% of patients. In non-small cell lung cancer, rates fall between 6% and 17%. For other cancers, it’s rarer still: less than 6% in colorectal or pancreatic cancer, about 1.8% in head and neck cancers, and extremely rare in metastatic bladder cancer. So while pseudoprogression is an important reason not to panic over a single scan showing growth, the odds are that apparent progression is real progression. Your oncologist will use repeat imaging, usually four to eight weeks later, to distinguish the two.
Blood Markers That Offer Early Clues
You don’t have to wait for imaging to get early signals. Two blood-based markers are increasingly useful for tracking immunotherapy response well before the first scan.
Circulating tumor DNA (ctDNA) is genetic material that tumors shed into the bloodstream. When immunotherapy is working, those levels drop. A study in head and neck cancer patients found that a greater than 50% reduction in ctDNA from baseline to the first follow-up blood draw (at a median of 23 days after starting treatment) strongly predicted longer progression-free survival. Not all cancer centers offer ctDNA monitoring routinely yet, but it’s worth asking about.
Lactate dehydrogenase (LDH) is a standard blood test your oncologist likely already orders. LDH rises when there’s significant tumor activity in the body. In melanoma patients treated with pembrolizumab, those who responded to treatment showed a median LDH drop of about 15.6% after six weeks, while patients whose cancer was progressing saw LDH climb by about 6.2%. Similar patterns appeared in bladder cancer patients on immunotherapy: responding patients had a median LDH decline of nearly 11%, while non-responders saw levels rise. An LDH increase of 25% or more was strongly associated with poor outcomes. If your LDH is trending down over the first few treatment cycles, that’s a genuinely encouraging sign.
Side Effects Can Be a Positive Signal
This one surprises many patients: developing certain side effects from immunotherapy is actually associated with better outcomes. Because these drugs work by activating your immune system against cancer, an immune system that’s revved up enough to cause side effects may also be revved up enough to attack the tumor effectively.
In a study of non-small cell lung cancer patients on checkpoint inhibitors, 68% of those who developed significant immune-related side effects responded to treatment, compared to just 20% of those with mild or no side effects. One-year survival was highest in the group with more pronounced side effects (68.6%), compared to 62.4% for mild side effects and 42.6% for those with none. Skin reactions like rashes have shown a particularly strong correlation with treatment response. Thyroid changes, both overactive and underactive thyroid, are also common immune-related effects that suggest robust immune activation.
This doesn’t mean you should hope for severe side effects, and it certainly doesn’t mean that a lack of side effects guarantees failure. Plenty of people respond well to immunotherapy without noticeable immune-related reactions. But if you’re dealing with a new rash, thyroid issues, or other inflammatory symptoms, it may be a sign that your immune system is doing exactly what the treatment intended.
Physical Changes You Might Notice
Imaging and lab work provide objective data, but your own experience matters too. When immunotherapy is working, many patients notice gradual improvements in symptoms caused by the cancer itself. If a lung tumor was causing a persistent cough, that cough may ease. If cancer-related fatigue was overwhelming, energy levels may slowly improve. Pain from a tumor pressing on surrounding tissue may lessen as the tumor responds.
These changes tend to be subtle and gradual rather than dramatic. Immunotherapy generally works more slowly than chemotherapy, so symptom relief often takes weeks to months. Some patients feel worse before they feel better, particularly if immune-related side effects kick in early. Keeping a simple daily log of your energy, appetite, pain levels, and any new symptoms gives your care team useful context that complements what scans and blood work reveal.
What a Durable Response Looks Like
One of immunotherapy’s most remarkable features is the potential for long-lasting responses. A “durable response” is formally defined as a complete or partial response that begins within the first 12 months of treatment and lasts at least six months continuously. That six-month threshold was chosen because most non-immunotherapy treatments for cancers like melanoma rarely maintain responses beyond that window.
If your scans show continued shrinkage or stability past the six-month mark, you’re in a favorable category. Many patients with durable responses go on to maintain them for years, and some eventually stop treatment while their cancer remains controlled. The combination of checkpoint inhibitors has produced four-year survival rates of 61% in melanoma patients with normal LDH levels, though outcomes vary significantly by cancer type and individual factors.
The overall picture of whether immunotherapy is working comes together over time, not from any single data point. A declining LDH, stable or shrinking tumors on imaging, immune-related side effects, and improving cancer symptoms collectively build the clearest picture. Your oncologist synthesizes all of these signals, and the first three months of treatment are when the most important early answers emerge.