RECIST — Criteria for Measuring Tumour Response

RECIST provides an objective, consistent framework for measuring changes in tumour burden over time using radiological imaging. Under the current standard, RECIST 1.1, clinicians identify up to five representative 'target lesions' (and a maximum of two per organ) at baseline. These target lesions are measured across their longest diameter and recorded.

Subsequent follow-up scans compare the sum of these measurements against baseline values. RECIST categorises treatment response into four defined tiers: Complete Response (CR, total disappearance of all lesions), Partial Response (PR, at least a 30 percent reduction in total diameter), Progressive Disease (PD, at least a 20 percent increase in diameter or the appearance of new lesions), and Stable Disease (SD, insufficient change to qualify as PR or PD).

Knowing how your cancer behaves under RECIST guidelines helps your oncology team make crucial clinical decisions. If your scans show a Complete or Partial Response, it confirms your current therapy is succeeding and should generally be maintained. If RECIST categorises your scans as Progressive Disease, it gives your oncologist clear, objective rationale to discontinue an ineffective treatment and switch to an alternative therapy or trial, avoiding unnecessary drug side effects.

RECIST assessments rely on serial anatomical imaging, predominantly contrast-enhanced computed tomography (CT) or magnetic resonance imaging (MRI). Scans are performed at regular intervals, typically every six to twelve weeks. Radiologists use computer-assisted measurement tools to record the single longest diameter of measurable target lesions. Functional imaging like PET scans can support assessment, particularly in modified frameworks such as iRECIST for immune checkpoint inhibitors.

RECIST applies exclusively to solid tumours. It is routinely utilised in non-small cell lung cancer, metastatic colorectal cancer, advanced breast cancer, renal cell carcinoma, and malignant melanoma. It is not suitable for haematological malignancies like leukaemia or lymphoma, nor does it typically measure non-measurable diseases such as diffuse peritoneal carcinomatosis or bone-only metastases lacking measurable soft-tissue masses.

RECIST results directly steer treatment continuity. A designation of Partial Response or Stable Disease typically justifies continuing systemic therapies such as targeted inhibitors, chemotherapy, or hormonal regimens. In contrast, an objective assessment of Progressive Disease prompts the termination of the current protocol and triggers discussions regarding next-line therapeutic agents, experimental clinical trials, or a refocus toward palliative supportive measures.