When oncologists talk about how a cancer is responding to treatment, they use a standardised vocabulary to describe what is happening on imaging. Understanding these terms helps you interpret scan results and conversations with your clinical team more clearly.
The RECIST categories
Response Evaluation Criteria in Solid Tumours (RECIST) is the internationally used framework for assessing how much a tumour has changed in response to treatment. Scans are compared against the baseline measurement taken before treatment started. The categories are:
- Complete response (CR): All target lesions have disappeared. No evidence of tumour detectable on imaging. A complete response is the best possible outcome - though it does not mean the cancer is cured, it means current imaging cannot detect it.
- Partial response (PR): Target lesions have shrunk by at least 30% in their longest diameter. The cancer has responded meaningfully to treatment but is not fully resolved.
- Stable disease (SD): The cancer has neither shrunk enough to meet the criteria for partial response nor grown enough to meet the criteria for progressive disease. Stable disease can be a positive outcome, particularly in patients who were previously progressing rapidly.
- Progressive disease (PD): Target lesions have grown by at least 20% or new lesions have appeared. The cancer is not controlled by the current treatment.
Immunotherapy and pseudoprogression
Pembrolizumab adds a complication to standard response assessment: pseudoprogression. In a small proportion of patients, the tumour appears larger on early scans as a result of immune cell infiltration rather than actual tumour growth. This can look like progressive disease on imaging while the patient is in fact responding. Guidelines for immunotherapy incorporate iRECIST - modified criteria that allow clinicians to continue treatment while confirming whether apparent progression is real or pseudoprogression.
When and how response is assessed
Response assessment typically involves CT scanning - usually at baseline and then every two to three months during treatment. PET-CT is used in some tumour types. The scan results are reviewed by a radiologist alongside your oncologist, and the assessment informs decisions about continuing or changing treatment.
Want to understand the monitoring schedule for home treatment?
Scan schedules and how response is communicated are covered on the Welcome Call. Start with the eligibility check.
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