Drug trials are notoriously expensive and time consuming.   The purpose of drug evaluations is to determine whether a treatment has enough merit to be used by larger numbers of patients – in other words, whether or not it is effective. The most widely used standard for determining the effectiveness of new oncology drugs is a set of rules called Response Evaluation Criteria in Solid Tumors, or RECIST.

RECIST was jointly created by the National Cancer Institute of the United States, the National Cancer Institute of Canada Clinical Trials Group and the European Organisation for Research and Treatment of Cancer. It is designed to provide a clear set of criteria to evaluate the progression, stabilization or responsiveness of tumors. Based on RECIST results, a drug may be chosen to move through additional clinical trial phases.

Target Lesions versus Non-Target Lesions

A lesion is a localized change in a tissue or an organ. Tumors are types of lesions. In the terminology of RECIST, “lesion” is generally used instead of “tumor.” It is important to understand the difference between a target lesion and a non-target lesion. Target lesions are lesions that have been specifically measured. Non-target lesions are lesions whose presences have been noted, but whose measurements have not been taken.

Evaluation Criteria for Target Lesions

At the beginning of an evaluation, selected lesions are measured to provide bases for comparison. Response assessment and evaluation criteria for target lesions are as follows:

• Complete Response, or CR – Signifies that all target lesions have disappeared during the course of treatment.
• Partial Response, or PR – Signifies that decreases of at least 30% have been noted in the lesion that has the largest diameter, or LD.
• Stable Disease, or SD – Signifies that there has been no significant decrease or increase in the size of target lesions, based on the smallest sum LD.
• Progressive Disease, or PD – Signifies that there has been an increase of at least 20% in the sum of the LD of targeted lesions.

Evaluation Criteria for Non-Target Lesions

The criteria for non-target lesions are similar to those of target lesions:

• Complete Response, or CR – Signifies the disappearance of all non-target lesions.
• Non-Complete Response or Non-Progressive Disease – Signifies the continued presence of one or more non-target lesions.
• Progressive Disease, or PD – Signifies that appearance of at least one new lesion, or the increasing size of at least one existing non-target lesion.

Ideally a drug trial will return results like CR or PR. Responses of SD or PD may indicate that a drug is not an effective treatment for cancer. Although RECIST has its detractors, it continues to be an important set of evaluation rules in the medical and pharmaceutical communities.

Differences from WHO criteria

The World Health Organization has evaluation criteria that differ somewhat. In the WHO system, Partial Response means the largest lesion has decreased in size (diameter) by 50 percent or more, and Progressive Disease means the largest lesion has increased in diameter by 35 percent or that new lesions are visible.