What is the definition of a partial response to chemotherapy according to RECIST criteria?

Definition of Partial Response to Chemotherapy

According to RECIST criteria, a partial response (PR) is defined as a ≥30% decrease in the sum of the longest diameters of all target lesions compared to baseline measurements. 1, 2, 3

Core RECIST Measurement Principles

The RECIST system uses unidimensional measurements (longest diameter only) rather than the older WHO bidimensional approach, which simplifies tumor assessment while maintaining accuracy. 1, 2

Baseline Requirements

• Measurable lesions must be ≥2 cm with conventional imaging techniques or ≥1 cm with spiral CT scan to qualify as target lesions. 1, 2
• A maximum of 5 target lesions total (no more than 2 per organ) are selected at baseline and measured serially throughout treatment. 3
• The sum of the longest diameters of all selected target lesions establishes the baseline measurement against which response is calculated. 1, 2

Confirmation Requirements

• Response must be confirmed with a second assessment performed at a minimum interval of 4 weeks after the initial documentation of PR. 1, 2
• The same imaging modality should be used consistently for all follow-up assessments to ensure measurement reliability. 2

Comparison with Historical WHO Criteria

Understanding the evolution helps contextualize current practice:

• WHO criteria required a 50% decrease in bidimensional tumor burden (product of perpendicular diameters) to qualify as partial response. 1
• RECIST lowered the threshold to 30% but uses unidimensional measurements, which has been validated to provide equivalent response rates while reducing measurement variability. 1
• This change was based on retrospective analysis of >4000 patients demonstrating that unidimensional versus bidimensional measurements did not alter response rates in clinical trials. 1

Complete Response Categories for Context

To understand where PR fits in the response spectrum:

• Complete Response (CR): Complete disappearance of all target and non-target lesions. 1, 2, 3
• Partial Response (PR): ≥30% decrease in sum of target lesion diameters. 1, 2, 3
• Stable Disease (SD): Insufficient shrinkage for PR and insufficient growth for progressive disease. 2, 3
• Progressive Disease (PD): ≥20% increase in sum of diameters from nadir, or new lesions. 1, 2, 3

Critical Pitfalls to Avoid

Measurement Errors

• Do not measure lesions <2 cm (or <1 cm on spiral CT) as target lesions, as they fall below the threshold for reliable serial measurement. 1, 2
• Avoid mixing imaging modalities during serial assessments, as this introduces measurement variability that can falsely suggest response or progression. 2

Interpretation Errors

• PR requires assessment of target lesions only—the final overall response must also consider non-target lesions and new lesions, which could downgrade an apparent PR to stable disease or progression. 1
• Bone lesions are generally non-measurable unless they have a soft tissue component ≥2 cm; pure lytic or blastic bone disease should be classified as non-target. 2
• Tumor necrosis can be misleading: RECIST measures anatomic size regardless of viability, which may underestimate response to locoregional therapies that cause extensive necrosis without immediate size reduction. 4

Confirmation Pitfalls

• Single assessment is insufficient—a 30% reduction must be confirmed on a repeat scan ≥4 weeks later to qualify as PR, preventing false-positive responses from measurement error or transient changes. 1, 2
• Lymph nodes require special consideration: A lymph node must be ≥15 mm in short axis at baseline to be measurable as a target lesion, and must reduce to <10 mm in short axis to qualify as CR. 3

Special Considerations for Immunotherapy

For patients receiving immunotherapy, standard RECIST may be inadequate:

• iRECIST criteria should be used in immunotherapy trials, as they account for pseudoprogression—initial apparent progression followed by response. 1, 3
• Unconfirmed progression (iUPD) must be verified at the next assessment before declaring true progression, as patients may still achieve immune-related partial response (iPR) after initial size increase. 1, 3
• During the validation period, report both standard RECIST and iRECIST in parallel for immunotherapy trials. 3

Clinical Significance

Partial response correlates with improved survival outcomes:

• In advanced non-small cell lung cancer, each 1% increase in response rate measured by RECIST is associated with a 0.07-0.26 month increase in median overall survival. 1, 3
• Depth of response matters: Greater tumor shrinkage within the PR category (e.g., 75% reduction vs. 30% reduction) is independently associated with longer overall survival. 5