How Long Does It Take for Immunotherapy to Work?

Some patients begin responding within weeks, while others may need several months before a clear response becomes apparent. There are also patients whose cancer does not respond to treatment.

This uncertainty can be difficult, particularly when someone feels well but has not yet had their first scan, or when early results show little change.

The important distinction is that immunotherapy can begin affecting immune activity before a measurable change in the cancer becomes visible.

How Quickly Does Immunotherapy Start Working?

Immunotherapy works by helping the immune system recognise and attack cancer cells. Commonly used treatments called immune checkpoint inhibitors block signals that cancer cells can use to suppress immune responses.

This does not mean the tumour will begin shrinking immediately after the first infusion.

An effective immune response involves several biological steps, and the time required varies between patients. For some, measurable tumour shrinkage appears relatively early. For others, the response becomes clearer over subsequent treatment cycles.

The aim of a personalised immunotherapy treatment plan is to match treatment to the cancer's characteristics while establishing how its effectiveness and safety will be monitored.

It is also important to distinguish between the treatment becoming biologically active and evidence that the cancer is responding. These are not necessarily visible at the same time.

The First Scan May Take Several Weeks

For patients receiving immunotherapy for measurable advanced cancer, the first response scan is often arranged around six to twelve weeks after treatment begins, although the timing depends on the cancer, drug and treatment protocol.

The purpose is to compare the cancer's appearance with scans taken before treatment.

Doctors look for changes in tumour size, new lesions, and other findings that may indicate whether the disease is responding or progressing.

A first scan does not always provide a complete answer. Some patients have clear tumour shrinkage, while others show little change and need further monitoring.

Scan timing is therefore planned around the treatment being given, rather than assuming everyone should respond within the same number of weeks.

What Counts as a Response to Immunotherapy?

A successful response does not always mean that every tumour disappears.

In advanced cancer, the aim may be to shrink the disease, prevent further growth or maintain control for as long as possible.

Scan results are commonly described using several categories:

• Complete response: No detectable evidence of the measured cancer remains on imaging, although this does not necessarily mean cure.
• Partial response: The measured cancer has decreased sufficiently to meet established response criteria.
• Stable disease: The cancer has neither decreased enough to qualify as a partial response nor increased enough to meet progression criteria.
• Progressive disease: Findings suggest that the cancer is growing or new disease has appeared.

Stable disease can still represent a meaningful treatment benefit, particularly when cancer was previously growing.

However, stability alone does not prove that immunotherapy is responsible. The significance depends on the cancer's previous behaviour, treatment goals and the wider clinical picture.

Why Some Patients Respond Earlier Than Others

There is no single characteristic that can predict exactly when immunotherapy will work.

Several factors influence the likelihood and pattern of response, including the type of cancer, its molecular characteristics, immune environment, previous treatment and the patient's general health.

In certain lung cancers, PD-L1 expression can help guide immunotherapy decisions, although it does not reliably predict the timing or certainty of an individual response.

Similarly, the presence of a particular biomarker does not guarantee that treatment will work.

The factors that make someone a suitable candidate for immunotherapy are therefore broader than any single blood test or tumour result.

Treatment response also differs between cancers. Immunotherapy has established roles in melanoma and certain types of lung cancer, but the expected benefit and recommended treatment schedule are not identical.

Can Immunotherapy Work Even If a Tumour Initially Looks Larger?

Occasionally, yes.

A phenomenon called pseudoprogression can occur when a tumour initially appears larger on imaging before subsequently shrinking.

This may be related to immune cells entering the tumour and associated inflammation. However, pseudoprogression is uncommon, and apparent growth is more often a sign of genuine disease progression.

Doctors therefore cannot assume that an enlarging tumour represents a positive immune response.

If a patient remains clinically stable and the circumstances support it, further imaging may sometimes help distinguish pseudoprogression from actual progression.

Continuing immunotherapy after apparent progression is not appropriate for everyone. The decision requires careful assessment of symptoms, scan findings and the risk of delaying a more effective treatment.

Feeling Better Does Not Always Mean Treatment Is Working

Some patients notice improvements in symptoms before a scan confirms a response.

For example, reduced breathlessness, less pain or improved appetite may suggest that the cancer is becoming better controlled.

However, symptoms are not a reliable measure of tumour response on their own.

Someone may feel better because of supportive medication or recovery from another medical problem. Equally, a patient may experience fatigue or other treatment-related symptoms even while the cancer is responding.

This is particularly relevant because immunotherapy side effects can develop independently of whether the treatment is controlling the cancer.

A lack of side effects does not mean immunotherapy is ineffective, and experiencing side effects does not prove that it is working.

How Long Can an Immunotherapy Response Last?

One of the important features of checkpoint inhibitor immunotherapy is that some patients experience durable responses.

For selected patients, cancer control may continue for months or years. In some circumstances, responses can persist after treatment has stopped.

However, this is not guaranteed. Some cancers initially respond and later begin growing again, while others never achieve a meaningful response.

The duration of treatment also varies. Some treatment protocols have a planned course, while others continue until disease progression, unacceptable toxicity or another clinically appropriate stopping point.

In advanced melanoma, immunotherapy has produced long-lasting responses in a proportion of patients, although outcomes vary considerably.

It is therefore important to distinguish between how quickly treatment begins working, how long it is administered and how long its benefits may last.

What Happens If Immunotherapy Is Not Working?

If scans indicate that the cancer is progressing, the oncology team will review whether the findings represent genuine progression and whether continuing treatment remains appropriate.

The next step depends on the cancer type, previous treatment, molecular findings, current symptoms and general health.

In some cases, another systemic treatment may be considered. For others, radiotherapy or a different approach may be appropriate for particular areas of disease.

For patients with lung cancer, the wider role of systemic cancer treatment includes selecting treatments according to tumour biology and reviewing their effectiveness over time.

If the scan findings are difficult to interpret or the recommended next step is unclear, a second opinion can provide an independent assessment of the treatment response and available alternatives.

The aim is not to continue an ineffective treatment indefinitely, but to make decisions based on the most reliable clinical information available.

Monitoring Matters Throughout Treatment

Regular monitoring remains important even after a favourable response has been established.

Scans help assess the cancer, while blood tests and clinical reviews can identify potential complications associated with immune system activation.

Patients should report new or worsening symptoms rather than waiting for the next scheduled appointment. Significant diarrhoea, increasing breathlessness, severe skin reactions or unusual fatigue may require assessment for immune-related side effects.

These complications can occur even when immunotherapy is controlling the cancer.

A private oncology consultant can help review how the response to immunotherapy, treatment-related symptoms and the patient's overall condition influence ongoing treatment decisions.

The most useful assessment considers both how effectively the cancer is being controlled and whether treatment remains appropriate and tolerable.

About Dr James Wilson

Immunotherapy has created important treatment opportunities for patients with certain cancers, but assessing its effectiveness requires more than looking for immediate tumour shrinkage.

Dr James Wilson is a consultant clinical oncologist in full-time private practice in Central London, specialising in lung cancer, melanoma and advanced radiotherapy. His practice includes systemic cancer treatments such as immunotherapy, with an emphasis on clear explanations, careful response assessment and individualised treatment decisions.