The Importance of Molecular Imaging Response Criteria: Rationale and Overview.

Assessment of therapeutic response in oncology is fundamental for evaluating treatment efficacy, guiding clinical decision-making, and determining prognosis. Historically, response evaluation relied primarily on anatomical imaging criteria derived from tumor size changes on computed tomography (CT), most notably the Response Evaluation Criteria in Solid Tumors (RECIST) and its adaptations. However, advances in molecular imaging, particularly positron emission tomography (PET) using [18F]-2-fluoro-2-deoxyglucose ([18F]FDG) and other radiopharmaceuticals, have introduced new biomarkers capable of detecting tumor response earlier than structural changes. Molecular imaging-based criteria, including EORTC, PERCIST, immune-adapted PET frameworks, and disease-specific systems such as Lugano, RANO, ENETS SSTR-PeRForm and RECIP, provide quantitative assessment of tumor burden and may better reflect biologic response to therapy. This review discusses the rationale behind molecular imaging response criteria, outlines the principles of CT-based and PET-based evaluation systems, and highlights the clinical relevance of integrating metabolic imaging into tumor response.