Response Evaluation Criteria in Solid Tumours (RECIST) are designed to provide a standardised and relatively simple method for assessing the treatment effect on tumours in protocolled studies. However, RECIST criteria are also used as a tool to evaluate the treatment efficacy in most oncology patients on standard treatment (chemotherapy, targeted therapy, immunotherapy, endocrine therapy). RECIST criteria were adopted in 2000 and revised in 2009. The current standard is RECIST 1.1.¹

• Measurable disease
• Non-measurable disease
• Target lesions
• Non-target lesions

• CT or MRI scan
• Clinical
• Chest X-ray

It is important that the same modality is used when evaluating the effect of a treatment.

Tumours – measured on longest diameter where minimum size is:

• ≥10 mm on CT or MRI scan
• ≥10 mm on clinical examination – visible/cutaneous/subcutaneous metastases
• ≥20 mm on chest x-ray

Lymph nodes – measured on the shortest link, where the minimum size is:

• ≥15 mm of lymph node – pathologically enlarged lymph node

Other lesions:

• Small tumours/lesions <10 mm
• 10 mm < lymph nodes <15 mm

Non-measurable lesions:

• Ascites, pleural and pericardial fluid, leptomeningeal disease, inflammatory breast cancer, lymphangitis carcinomatosis of the lungs and skin, abdominal masses or organomegaly, cystic lesions, sclerosing bone metastases

Based on the baseline examination (most often CT scan), measurable tumours/lymph nodes are defined and target lesions to be measured/compared from examination to examination are selected with regard to assessing the efficacy of the treatment. They must be:

• Representative of all involved organs
• Selected based on size and suitability for repeated measurements
• In the case of several measurable lesions, select up to five representative lesions in total, but no more than two lesions per organ.

The diameter of each target lesion will be added to a number – baseline sum – which will be used later to measure response.

The remaining lesions, both non-measurable lesions and the remainder of measurable lesions, should be noted:

• Measurable tumours that are not selected as target lesions
• Too small tumours/lymph nodes
• Non-measurable lesions (bone metastases, pleural effusion, ascites, peritoneal carcinoma, etc.)
• Previously irradiated lesions

When evaluating response, there may be 4 outcomes:

• CR – complete response
• PR– partial response
• PD – progressive disease/progression
• SD – stable disease

• Shrinkage of all target lesions and all pathologically enlarged lymph nodes is reduced to <10 mm in the shortest link
• Loss of all non-target lesions
• No appearance of new lesions

All of the above points must be met.

• At least 30% reduction in the total sum of target lesions vs. baseline sum
• Not concurrent unequivocal progression of non-target lesions
• No appearance of new lesions

All of the above points must be met.

• At least 20% increase in the total sum of target lesions vs. where the sum was lowest (nadir sum) and an absolute increase of at least 5 mm from the nadir sum
• New lesion
• Unequivocal progression of non-target lesions
• Clinical progression

Only one of the above points needs to be met in order to call it progressive disease.

• Neither PR nor PD
• No new lesions

The above options are shown here in a table to provide a better overview of the overall response in accordance with RECIST 1.1.

Example of assessment:

Immunotherapy uses the body’s own immune system to target the cancer (see the section on oncological treatment immunotherapy).  In clinical studies with immunotherapy, a different/unusual response was observed in relation to what had previously been seen with other oncology treatments. I.e. initial progression in accordance with RECIST 1.1 with increasing size of target lesions and appearance of new lesions, but subsequent positive and long-lasting response. Based on this, the RECIST working group decided to create a guideline with modified RECIST criteria for clinical studies with immunotherapy (iRECIST) to ensure uniform design and data collection².

iRECIST is based on RECIST 1.1, but more concepts have been added to the response evaluation:

• iCR – Immune complete response
• iPR – Immune partial response
• iUPD – immune unconfirmed progressive disease
• iCPD – Immune confirmed progressive disease
• iSD – immune stable disease
• New lesions are subcategorised to:
  • Target lesions
  • Non-target lesions

The main differences between RECIST 1.1 and iRECIST:

Further information on iRECIST and the management of new lesions and iUPD can be found in “iRECIST: guidelines for response criteria for use in trials testing immunotherapeutics”, Seymour et al. Lancet Oncol 2017².

1. Eisenhauer EA et al. New response evaluation criteria in solid tumours: Revised RECIST guideline (version 1.1). EJC 2019)
2. Seymour et al. iRECIST: guidelines for response criteria for use in trials testing immunotherapeutics, Lancet Oncol 2017.