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The purpose of this article is to review the status and limitations of anatomic tumor response metrics including the World Health Organization (WHO) criteria, the Response Evaluation Criteria in Solid Tumors (RECIST), and RECIST 1.1. This article also re- views qualitative and quantitative approaches to metabolic tu- mor response assessment with 18F-FDG PET and proposes a draft framework for PET Response Criteria in Solid Tumors (PERCIST), version 1.0. Methods: PubMed searches, including searches for the terms RECIST, positron, WHO, FDG, cancer (in- cluding specific types), treatment response, region of interest, and derivative references, were performed. Abstracts and arti- cles judged most relevant to the goals of this report were reviewed with emphasis on limitations and strengths of the ana- tomic and PET approaches to treatment response assessment. On the basis of these data and the authors’ experience, draft cri- teria were formulated for PET tumor response to treatment.

Results: Approximately 3,000 potentially relevant references were screened. Anatomic imaging alone using standard WHO, RECIST, and RECIST 1.1 criteria is widely applied but still has limitations in response assessments. For example, despite effec- tive treatment, changes in tumor size can be minimal in tumors such as lymphomas, sarcoma, hepatomas, mesothelioma, and gastrointestinal stromal tumor. CT tumor density, contrast en- hancement, or MRI characteristics appear more informative than size but are not yet routinely applied. RECIST criteria may show progression of tumor more slowly than WHO criteria. RECIST 1.1 criteria (assessing a maximum of 5 tumor foci, vs. 10 in RECIST) result in a higher complete response rate than the original RECIST criteria, at least in lymph nodes. Variability appears greater in assessing progression than in assessing re- sponse.

Qualitative and quantitative approaches to 18F-FDG PET response assessment have been applied and require a con- sistent PET methodology to allow quantitative assessments. Statistically significant changes in tumor standardized uptake value (SUV) occur in careful test–retest studies of high-SUV tu- mors, with a change of 20% in SUV of a region 1 cm or larger in diameter; however, medically relevant beneficial changes are often associated with a 30% or greater decline. The more exten- sive the therapy, the greater the decline in SUV with most effective treatments. Important components of the proposed PERCIST criteria include assessing normal reference tissue values in a 3-cm-diameter region of interest in the liver, using a consistent PET protocol, using a fixed small region of interest about 1 cm3 in volume (1.2-cm diameter) in the most active region of metaboli- cally active tumors to minimize statistical variability, assessing tumor size, treating SUV lean measurements in the 1 (up to 5 op- tional) most metabolically active tumor focus as a continuous variable, requiring a 30% decline in SUV for ‘‘response,’’ and de- ferring to RECIST 1.1 in cases that do not have 18F-FDG avidity or are technically unsuitable. Criteria to define progression of tu- mor-absent new lesions are uncertain but are proposed. Con- clusion: Anatomic imaging alone using standard WHO, RECIST, and RECIST 1.1 criteria have limitations, particularly in assessing the activity of newer cancer therapies that stabilize disease, whereas 18F-FDG PET appears particularly valuable in such cases. The proposed PERCIST 1.0 criteria should serve as a starting point for use in clinical trials and in structured quan- titative clinical reporting. Undoubtedly, subsequent revisions and enhancements will be required as validation studies are un- dertaken in varying diseases and treatments.

Key Words: molecular imaging; oncology; PET/CT; anatomic imaging; RECIST; response criteria; SUV; treatment monitoring

J Nucl Med 2009; 50:122S–150S
DOI: 10.2967/jnumed.108.057307


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