Supplementary Material for: Pulmonary Tumor Thrombotic Microangiopathy in a Patient with Rapid Progressive Triple-Negative Breast Cancer

<b><i>Introduction:</i></b> Pulmonary tumor thrombotic microangiopathy (PTTM) is a rare complication of metastatic carcinoma, which occurs in patients with pulmonary arterial hypertension, and is mostly fatal. Circulating tumor cell clusters have been recognized as critical factors during breast cancer progression. <b><i>Case Presentation:</i></b> An 80-year-old woman with triple-negative breast cancer was admitted to our hospital with progressive dyspnea and lower back pain. Breast cancer treatment included mastectomy, neoadjuvant and adjuvant chemotherapy as well as adjuvant radiotherapy, receiving her last cycle of radiotherapy 8 days before death. At admission, D-dimers were strongly elevated and platelets were low. NT-pro-BNP was moderately elevated. A CT scan of the chest did not show pulmonary embolism but revealed interlobular septal thickening, centrilobular consolidation, and distension of the pulmonary arteries. Moreover, new skeletal and most likely lymphatic metastasis was described. Treatment with oxygen and oral glucocorticoids was initiated, assuming radiotherapy-induced pneumonitis. Due to low expression of PD-L1 and her markedly bad performance status, tumor-specific therapy was not possible, and the treatment regimen was changed to best supportive care. The patient died 8 days after admission. Autopsy revealed numerous events consistent with tumor emboli in the pulmonary vessels, suggesting PTTM. <b><i>Conclusion:</i></b> PTTM is a rare and mostly fatal complication in malignant breast cancer. As an early detection is difficult, further investigation is needed. Circulating tumor cluster cells may be one way to detect PTTM early and improve patients’ survival.