There is uncertainty in the management of cancer-associated isolated splanchnic vein thrombosis (SpVT).
To describe the natural history of SpVT by cancer type and thrombus composition and to review anticoagulation (AC) practices and associated rates of usual-site venous thromboembolism (VTE), major and clinically relevant nonmajor bleeding (MB/CRNMB), recanalization/progression, and mortality.
We performed a retrospective cohort study in patients with SpVT at two cancer care centers in Houston, TX. We estimated the incidence of usual-site VTE and MB/CRNMB at 6 months using competing risk method and examined venous patency in a subset of patients with repeat imaging. We assessed associations with mortality using Cox regression.
Among 15,342 patients with incident cancer diagnosis in 2011-2020, we identified 298 with isolated SpVT. Patients with hepatocellular carcinoma (HCC) (n=146) had the highest prevalence of SpVT (20%), lowest rates of AC (2%), and no differences in usual-site VTE (4.2%) vs. those without SpVT (5.2%) at 6 months, though tumor thrombus was associated with worse overall survival. In patients with non-HCC cancers and bland SpVT (n=114), AC (n=37) was more common in those with non-upper GI cancers and fewer comorbidities. AC was associated with more recanalization (44% vs. 15%, p=0.041) but no differences in usual-site VTE, MB/CRNMB, or mortality at 6 months.
Cancer-associated isolated SpVT is a common but heterogeneous thrombotic disease that is treated differently from usual-site VTE. Tumor thrombus is a negative prognostic factor. Initiation of AC in bland thrombi requires judicious consideration of thrombotic and bleeding risk.

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