How do they know if cancer treatment is working? What do partial response, stable disease, and progression mean?
The assessment of anticancer treatment efficacy is most commonly performed using standardised imaging examinations (e.g., CT, MRI, PET-CT) based on the internationally accepted RECIST (Response Evaluation Criteria in Solid Tumors) criteria system, supplemented by clinical symptoms, tumour markers, or liquid biopsy (ctDNA). The classic response categories are based on changes in the size of measured target lesions:
- Partial Response (PR): The sum of the longest diameters of the target tumour lesions decreases by at least 30% compared to baseline, without the appearance of new lesions or unequivocal progression of other lesions. - Stable Disease (SD): The size of the tumour has neither shrunk sufficiently to qualify for partial response (did not reach -30%) nor increased sufficiently to qualify as progression. The disease is essentially unchanged or in equilibrium. - Progression or Progressive Disease (PD): The sum of the longest diameters increases by at least 20% (and generally by at least 5 mm in absolute terms) compared to the smallest tumour size recorded during therapy (nadir), or an unequivocal new tumour metastasis/lesion appears.
In the case of immunotherapies, these standard categories are refined by specialised criteria (e.g., iRECIST), which are also able to account for the transient inflammatory tumour enlargement (pseudoprogression) that can occur at the beginning of treatment.
In addition to standard morphological size measurement, newer therapies (such as targeted agents, immunotherapy, or locoregional treatments) may also employ supplementary criteria (mRECIST, PERCIST) and molecular monitoring, because tumour cell death or functional inhibition often precedes morphological shrinkage.
References
- Automated RECIST tumor response classification through prompt-guided large language models. PMID 42204251
- Assessing the reliability of Response Evaluation Criteria In Solid Tumors (RECIST): a systematic review of the factors contributing to inter-observer variability. PMID 42380551
- Utility of iRECIST for evaluating treatment efficacy in patients receiving immune checkpoint inhibitors. PMID 41514317
- From RECIST to PERCIST: Evolving Considerations for PET Response Criteria in Solid Tumors PMID 19403881
- PMID 19403881