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BACKGROUND The aim of the study was to retrospectively evaluate the incidence and outcomes of inferior vena cava (IVC) filter thrombus during catheter-directed thrombolysis (CDT) for acute proximal deep venous thrombosis (DVT). METHODS From October 2006 to June 2015, patients diagnosed with acute proximal DVT and received CDT after a retrievable IVC filter was placed were included. The incidence, treatment, and outcomes of IVC filter thrombus during CDT were recorded and analyzed. RESULTS A total of 189 patients (91 women, 98 men; mean age, 57.6 ± 9.8 years; range, 24-85 years) were included in this study. Among the 189 cases, the DVTs involved popliteal iliofemoral veins in 54 patients, iliofemoral veins in 113 patients, and iliac veins in 22 patients, of which 18 patients had thrombus extended into the IVC. Of the 189 patients, a total of 8 (4.2%, 8 of 189) patients were identified with IVC filter thrombus during CDT. The IVC filter thrombus was detected on a median of 2 days (range, 2-4 days) of CDT therapy, including small-size (n = 6) and large-size (n = 2) filter thrombus. Of the 8 patients, CDTs were performed with a mean 7.6 ± 1.1 days (range, 6-11 days) after the presence of symptoms for the treatment of proximal DVT, and all the IVC filter thrombi were lysed during CDT for the proximal DVT. All the IVC filters were removed successfully with a mean of 12.8 ± 0.93 days from placement. There were no procedure- or thrombolysis-related major complications, and no symptomatic pulmonary embolism breakthrough was seen in any of the patients after the filter placement. CONCLUSIONS IVC filter thrombus during CDT for the acute proximal DVT is uncommon, and all of them did not need any additional treatment.