The LNT Model and ALARA in Diagnostic Imaging: A Critical Review of Scientific Evidence in the Low-Dose Range.

Background: The Linear No-Threshold (LNT) model and the As Low As Reasonably Achievable (ALARA) principle form the cornerstone of contemporary radiation protection. Although these frameworks were designed as conservative regulatory tools, their extrapolation to the low-dose and dose-rate (LDDR) range commonly encountered in diagnostic imaging remains scientifically contentious. Objective: This review critically examines the scientific basis for applying the LNT model and the ALARA principle in the LDDR range (1-50 mGy) routinely encountered in diagnostic imaging, with the aim of evaluating whether current regulatory practices are proportionate to demonstrated risks. Methods: We conducted a narrative review of peer-reviewed literature published between 1995 and 2025, retrieved from PubMed, Scopus, and Web of Science using the search terms "LNT model," "low-dose radiation," "ALARA," "radiation hormesis," "adaptive response," and "diagnostic reference levels." Priority was given to systematic reviews, large-cohort epidemiological studies, and consensus statements from professional organizations (ICRP, UNSCEAR, AAPM, HPS, IOMP). Studies were grouped thematically into radiobiological evidence, epidemiological evidence, and clinical/policy implications. We acknowledge that this is a non-systematic review and that selection bias cannot be excluded. Results: Multiple lines of radiobiological evidence-including DNA repair fidelity at low doses, adaptive cellular responses, apoptotic clearance of damaged cells, and immune surveillance-suggest that the dose-response relationship for stochastic effects in the LDDR range is unlikely to be linear. Large-scale epidemiological studies, including atomic bomb survivor reanalyses, occupational cohorts, and high-background radiation populations, generally fail to demonstrate statistically significant excess cancer risk below approximately 100 mGy, although confidence intervals remain wide. Case examples in scoliosis screening, mammography, and low-dose CT lung cancer screening illustrate scenarios in which overly conservative ALARA implementation may delay or deter clinically beneficial imaging. Conclusions: Within the LDDR range, the available evidence does not support a linear dose-response relationship, and the indiscriminate application of ALARA based on hypothetical risks may produce net harm through deferred imaging, radiophobia, and inefficient resource allocation. We propose that radiation protection in medical imaging shift toward an evidence-informed framework that prioritizes image quality optimization, individualized risk assessment, and the adoption of complementary principles such as ASARA (As Safe As Reasonably Achievable) and AHARA (As High As Reasonably Achievable benefits). A linear-with-threshold (LT) model and the adverse outcome pathway (AOP) framework warrant further investigation as alternatives to LNT for regulatory purposes.