How Do We Know If Chemotherapy Is Working?

If you're undergoing cancer treatment, your doctor may order a set of tests and scans to determine how your cancer is reacting to it. This standardized testing is called RECIST, an acronym for "response evaluation criteria in solid tumors."

RECIST is a standard system to measure how cancer responds to different treatments, including chemotherapy, immunotherapy, and radiation therapy. It can tell the doctors if your treatments are effective or if they should try another approach.

If a tumor can be measured, the doctors assign scores to the patient's response to treatment, depending on how much the tumor has changed in size. Common terminology used to describe a cancer's response to treatment in the RECIST system includes complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD).

The RECIST system has its drawbacks. It works only when there is at least one tumor and that tumor can be measured using traditional imaging technologies, like with computed tomography (CT) scans, X-rays, or magnetic resonance imaging (MRI). That means it can't be used for blood cancers, for example. 

This article will explain how the RECIST system measures how cancer responds to treatment, including chemotherapy, and discuss the different terminology and categories of responses.

RECIST Categories 

When you get your RECIST results, you’ll usually see an entry for the target lesion (tumors that are specifically monitored to track disease progression). This is the tumor that has been measured and imaged for the RECIST scans. Non-target tumors are also analyzed with the scans to determine if they’ve grown larger in number or size.

The classic RECIST categories used for the target lesion are complete response (CR), partial response (PR), stable disease (NR/SD), and progressive disease (PD). 

Target Lesions

The complete response classification means the original tumor can no longer be detected on the RECIST scans. It’s the best outcome from treatments and can be the first step to being cured of your cancer.

The partial response category means the original tumor has shrunk by 30% or more. While this is a good finding, it’s also possible this could mean that you need more or different types of treatment to potentially cure your cancer. The next steps forward in your treatment plan will depend on the type of cancer you have and how advanced it is.

If the tumor is still increasing in size even given the treatments, it’s classified as a progressive disease. There are two ways you might receive a PD result on your RECIST scoring. First, the original tumor may have increased in size by 20% or more. Or new tumors may be found in the scan.

Lastly, if the original tumor doesn’t fall into a partial response or progressive disease category, it’s classified as stable disease. It may have shrunk or grown, but not enough to be classified into one of the other categories. 

Stable disease also indicates that there are no new tumors detected and no new metastases (spread to other sites) of the original tumor. Stable disease can be a good sign—it can mean your treatment is working if the tumor was expected to grow. It’s better than progressive disease but not as good as partial response.

Non-Target Lesions

You may also see a RECIST category applied for the non-target lesions, which are tumors that have not been the main target of the RECIST scanning. Their presence has been noted, but they were not measured.

If your non-target tumor is classified as a complete response, it has fully disappeared from the scan and your tumor markers have returned to normal levels. Tumor markers are substances that can be measured that are released by cancer cells or produced by the body in reaction to the cancer.

If you see a category of incomplete response or stable disease for your non-target tumor, that means that either there is still one or more non-target tumors in the body, or the tumor marker levels are still elevated.

If a non-target tumor is classified as progressive disease, it is not responding to treatment and new tumors may have appeared. It may also mean that the existing tumors have undergone “unequivocal progression.” 

Unequivocal progression (UP) is a term used when the cancer is obviously getting worse. It may be used if the target tumors are stable but the non-target tumors are increasing in size or number. It would indicate that the current therapeutic approach isn’t working.

There are two other things that may trigger a UP classification. While the original tumor may have improved or stayed steady, the non-target lesions can have worsened.

It could also mean that other signs visible on imaging have worsened—such as an increase in the fluid in the lungs (pleural effusion) or the spread of the tumor into the lymphatic vessels (lymphangitic disease).

Other Terminology 

Some other terms you may see on your RECIST report might include:

Pseudoprogression

Pseudoprogression (PP) is a term you may see or hear being used if you’re getting immunotherapy with checkpoint inhibitors. This is when a tumor increases in size on imaging studies before shrinking later.

Immunotherapy can have delayed, but long-lasting, effects. Pseudoprogression may occur because the body’s immune response makes the tumor look bigger, or just because the treatment took some time to work. 

Pseudoprogression is rare. Your healthcare team will decide the best course of action in your case.

Recurrence

Recurrence is when cancer comes back after initially responding to treatment. It could be months or years later. 

Chemorefractory 

The term chemorefractory has been used to indicate that a tumor isn’t responding to chemotherapy. Chemorefractory cancer is not shrinking after chemotherapy treatment. 

It can be a characteristic of cancer from the beginning of treatment, or it may be something that develops over time, as the tumor cells mutate and stop being sensitive to the chemo drugs. 

Durable Response

Durable responses are when a treatment’s effects are long-lasting. There isn’t really a standard definition of what this term means by “long-lasting,” but some medical professionals use a timeframe of a year. 

Understanding RECIST Classification 

It’s easy to think that complete response is the best RECIST outcome—and it is—but it's not the only positive outcome of cancer treatment.

As a result of newer, more effective treatments, doctors increasingly measure success in terms of quality of life and symptom-free disease, rather than simply the size of a tumor. In many cases of aggressive or advanced cancers, stable disease is a very desirable outcome.

Metastatic cancers, for example, are very infrequently able to be cured. But treatments for these diseases have led to an increase in progression-free survival (PFS). PFS is a newer metric used to analyze how long a person with cancer survives without their condition getting worse.

Summary

RECIST is a classification system for solid tumors that measures response to treatment. In order to be classified, the tumor must be measurable via imaging. Categories in the RECIST system include complete response (CR), partial response (PR), stable disease (SD), and progressive disease (PD).

A Word From Verywell

RECIST classification are just one way of measuring how your body is reacting to treatments. Seeing that your cancer isn't responding well to treatments may be disheartening, especially if you're experiencing symptoms or side effects of the treatment. 

Multiple treatment options are available for many cancers, so don't despair if your cancer isn't responding to the first treatment you try (or the third). Instead, talk to your medical team about your options, including participating in clinical trials of new therapies. 

Throughout your treatment, you will hear many new terms used by your healthcare team, Have them explain any confusing terms to you. Don't hesitate to ask questions.