What are the RECIST (Response Evaluation Criteria in Solid Tumors) criteria for assessing response of solid tumors?

RECIST Criteria for Solid Tumor Response Assessment

RECIST (Response Evaluation Criteria in Solid Tumors) uses unidimensional measurement of target lesions to standardize tumor response assessment, defining complete response as disappearance of all lesions, partial response as ≥30% decrease in sum of diameters, stable disease as insufficient change, and progressive disease as ≥20% increase or new lesions. 1, 2

Core Response Categories

RECIST version 1.1 defines four response categories based on changes in tumor burden 1:

• Complete Response (CR): Complete disappearance of all measurable and assessable tumor lesions 1, 2
• Partial Response (PR): ≥30% decrease in the sum of longest diameters of all target lesions compared to baseline 2
• Stable Disease (SD): Insufficient shrinkage to qualify as PR and insufficient increase to qualify as PD 2
• Progressive Disease (PD): ≥20% increase in sum of longest diameters from the smallest measurement (nadir), OR appearance of new lesions, OR unequivocal progression of non-target lesions 2

Measurement Methodology

RECIST simplified the older WHO bidimensional approach by using only the longest diameter of each lesion 1, 2:

• Target lesions: Maximum of 5 lesions total (2 per organ) are selected and measured serially 1
• Measurable lesions: Must be ≥10 mm in diameter with conventional imaging (≥15 mm for lymph nodes in short axis) 1, 2
• Non-target lesions: All other disease is recorded but not measured; must be ≥10 mm in short axis for nodal disease 1
• Response confirmation: Requires minimum 4-week interval between assessments 2

The unidimensional approach is more linearly related to cell kill than bidimensional products and shows excellent concordance (kappa statistic 0.91) with WHO criteria 3.

Key Differences from WHO Criteria

RECIST differs from the older WHO/SWOG criteria in several important ways 1:

• Uses unidimensional (longest diameter only) instead of bidimensional measurement
• Limits target lesions to maximum 5 per patient (2 per organ) versus 10 total in WHO
• Different cut-offs: PR requires ≥30% decrease (vs ≥50% in WHO); PD requires ≥20% increase (vs ≥25% in WHO) 1
• Lymph node CR defined as short axis ≤10 mm 1

Special Considerations for Immunotherapy

For immunotherapy trials, iRECIST was developed to account for atypical response patterns including pseudoprogression 1:

• Unconfirmed progression (iUPD): Initial progression requires confirmation at next assessment before declaring confirmed progression (iCPD) 1
• New lesions: Do not automatically indicate progression; must show increase in size (≥5 mm) or number at subsequent scan to confirm iCPD 1
• Clinical stability: Considered when deciding whether to continue treatment after iUPD 1
• Patients can achieve response (iPR, iCR) after initial iUPD, unlike standard RECIST 1

The ASCO-SITC guidelines recommend reporting both standard RECIST and immune-specific criteria in parallel during the validation period 1.

Critical Pitfalls and Limitations

Several measurement challenges can lead to inaccurate response assessment 2:

• Bone lesions: Pure osteolytic or osteoblastic lesions are non-measurable; bone scan "flare" can mimic progression 2
• Small tumors: Lesions <2 cm, irregular shapes, or ill-defined margins are difficult to measure accurately 2
• Treatment effects: Tumor necrosis, fibrosis, or fragmentation complicate assessment 2
• PET/CT: Lacks validated RECIST standards; can only assess progression via new abnormal sites 2
• Imaging consistency: Must use same modality at consistent intervals for serial assessments 2

Clinical Application

RECIST measurements predict overall survival in lung cancer, with each 1% increase in response rate improving median survival by 0.07-0.26 months 1. The criteria guide treatment decisions including:

• Switching chemotherapy regimens when response is inadequate 2
• Using pathologic complete response (pCR) as surrogate endpoint in neoadjuvant breast cancer 2
• Integrating with clinical symptoms, physical examination, and laboratory tests for comprehensive assessment 2

Response assessment frequency should be based on treatment type and disease biology, with immediate reassessment for new or worsening symptoms regardless of scheduled timing 2.