Factors that affect PERCIST-defined test-retest comparability: an exploration of feasibility in routine clinical practice

PET/CT imaged with 18F-FDG is widely utilized for the clinical management of a variety of malignancies.1–4 Because neoadjuvant treatment is currently used as the de facto standard treatment in an increasing number of cancers, a procedure to accurately evaluate the treatment response quantitatively, especially in the early phase, is imperative, so that response-adaptive treatment may be tailored to the individual.5–10 In view of this, PET/CT, with its ability to functionally detect metabolic changes before anatomical response occurs, is a feasible method to evaluate early treatment response.11 However, PET/CT-based functional quantitative parameters are dependent on multiple factors, such as blood glucose level, injected dose, and uptake time, and so on.12,13 As a result, fundamental standardization is needed to ensure the necessary reproducibility so as to obtain comparable quantitative parameters between scans.14,15 To date, 2 criteria have been recommended for quantitative PET/CT-based response evaluation, the European Organization for Research and Treatment of Cancer Recommendations criteria16 and the PET Response Criteria in Solid Tumors 1.0 (PERCIST) criteria.17 Both of them require standardization of technical factors, that is, injected dose, uptake time, scan time, and glucose level, to assure comparability for the assessment of SUV normalized by the lean body mass (SUL) values at designated lesions. For any trial, the comparability should be verified before response evaluation. To achieve this comparability, ideally, there should be zero discrepancy in these parameters between scans. However, this may not be easily implemented or controllable in clinical practice.18 Practically, a case with change in mean SUL in the reference volume of interests (VOIs) placed in the liver (3-cm sphere in right lobe, when liver is disease free) or descending aorta (1 × 2 cm cylinder, when liver is involved) of less than 20% (and 0.3 SUL mean units) at baseline and follow-up scans is still considered reasonably comparable, rendering minor discrepancies in the technical parameters acceptable for the response assessment.17 In light of this, we aim to evaluate the effect of the discrepancies of these factors on comparability. Thus, the extent of discrepancy that can be allowed to produce adequate comparability can be determined in a clinical practice.