Learning from what went right: a Safety-II application of the SIDER protocol to a case of occult breast cancer

OBJECTIVES: To analyze how diagnostic success was achieved in a challenging case of occult breast cancer using the SIDER protocol, a structured Safety-II framework for reflecting on successful diagnostic processes. CASE PRESENTATION: A 56-year-old woman developed progressive pain extending from the right upper limb to the right periscapular region 8 weeks before presentation, followed by axillary pain, cutaneous changes, and lymphadenopathy. Cervical radiography, cervical magnetic resonance imaging, breast ultrasonography, and computed tomography did not identify a primary breast lesion. At presentation, the clinical team retained breast malignancy in the differential diagnosis because of the patient's sex, progressive symptoms, axillary lymphadenopathy, and unexplained clinical trajectory. Diagnostic uncertainty was explicitly discussed with the patient, supporting continued follow-up. Two weeks later, positron emission tomography-computed tomography showed intense uptake from the right neck to the supraclavicular and axillary regions. Axillary lymph node biopsy demonstrated metastatic carcinoma with immunohistochemical findings consistent with breast origin, establishing the diagnosis of occult breast cancer. CONCLUSIONS: Applying the SIDER protocol showed that diagnostic progress depended on three reproducible processes: maintaining a low-probability but high-impact diagnosis despite nondiagnostic imaging, preserving patient engagement under uncertainty, and enabling diagnostic re-entry through cross-specialty collaboration. This case extends the use of Safety-II reflection by demonstrating how structured analysis of diagnostic success can generate practical lessons for complex, evolving presentations.