OBJECTIVES: To estimate an upper bound of memory-based re-identification risk for chest radiographs by testing radiologists under conditions that favor recognition. MATERIALS AND METHODS: In this prospective, multicenter, web-based reader study, radiologists from 38 centers completed two reading phases. In Phase 1, each reader interpreted ten chest radiographs. After a minimum interval of 24 h, Phase 2 included six follow-up target examinations and six new non-target examinations (50% target prevalence). After each Phase-2 examination, readers indicated whether they remembered the patient. Following a positive response, they were asked separately whether they remembered the Phase-1 pseudonym and/or case position. Thirty-three readers with fully classifiable Phase-2 data contributed 396 Phase-2 examinations to the predefined primary analysis. RESULTS: Readers answered "remember" in 139 of 396 examinations (35.1%). Sensitivity for repeated target examinations was 50.0% (99/198), whereas 20.2% of new non-target examinations were nevertheless judged as remembered (40/198). Explicit identifiers were attempted in 23 of 396 examinations (5.8%). At least one explicit identifier, defined as the Phase-1 pseudonym and/or case position, was correct in five of 396 examinations (1.3%). In a low-prevalence model with one known patient per dataset, the positive predictive value of a "remember" response decreased from 2.44% in datasets of 100 radiographs to 0.25% in datasets of 1000 radiographs. CONCLUSIONS: Even in a design that favored memory, correct recall of explicit identifiers was rare, whereas false-positive recognition remained common. These findings support treating radiologists' memory as a limited, upper-bound component of re-identification risk, rather than assuming that familiarity routinely translates into identification. CRITICAL RELEVANCE STATEMENT: Even under conditions deliberately favoring memory, radiologists rarely converted familiarity with prior chest radiographs into correct explicit identifiers; in low-prevalence datasets, false-positive recognition dominated the practical meaning of a "remember" judgment. KEY POINTS: How often does a radiologist's feeling of familiarity with a previously seen chest radiograph translate into correct recall of a usable explicit identifier? In a deliberately memory-favoring design, sensitivity was 50.0%, but 20.2% of new examinations were false positives and only 1.3% yielded a correct explicit identifier.
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