Bland thrombus is a term used to describe any non-malignant venous thrombus (e.g. a "normal" DVT). However the term is most often used in the context of malignancy, to differentiate non-malignant clot from the malignant form, as management of the two may differ; of course they often coexist.
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Pathology
In the patient with malignancy, bland thrombus may form due to abnormalities of Virchow's triad, e.g. venous flow haemodynamic changes secondary to extrinsic compression of a nearby tumour not by direct invasion, usually at the portal vein and IVC in a setting of hepatic, renal or other proximate malignancy. The correct discrimination from tumour thrombus is helpful for therapeutic strategies and prognostication.
Radiographic features
It appears on cross-sectional imaging as a non-enhancing luminal filling defect with no definite continuity with the adjacent tumour tissue. It may be adherent to an intact vein wall with no luminal expansion (although acute bland thrombus may mildly expand the vessel).
Ultrasound
Acute thrombosis may be difficult to detect with grey-scale imaging alone, as the thrombus may be anechoic/hypoechoic. With time, it becomes more echogenic and easier to identify.
Colour Doppler should be able to demonstrate partial or complete flow defect in the affected vein. It can also help to differentiate it from tumour thrombus, which will show internal colour vascularity. Bland thrombus, in comparison, is avascular on colour Doppler 1.
CT
It appears as a hypodense intraluminal structure with no enhancement after contrast. Contrast is necessary to differentiate it from the nearby tumour and delineating it from the surrounding adjacent luminal flow. On unenhanced CT, bland thrombus shows lower density values (mean 33 HU) cf. tumour thrombus (mean 81 HU) 2.
MRI
Acute thrombus appears of high signal intensity on T1 and T2 weighted sequences. Chronic thrombus appears of low T2 signal or appears as a flow voids. In both situations, it shows no enhancement or diffusion restriction 3.
Bland thrombus generally shows non-restricted diffusion. It shows higher ADC values than tumour thrombus 4. Yet, it may demonstrate low ADC values due to the high viscosity as well as paramagnetic effects of intracellular deoxyhaemoglobin and methaemoglobin contents 5.
Beware of slow or turbulent flow artefacts.
Treatment and prognosis
Bland thrombus may resolve after thrombolytic and anticoagulant therapy, unlike tumour thrombus.
Differential diagnosis
tumour thrombus: usually shows a connection to an invading tumour, hypervascularity on colour Doppler and enhancement on CT and MRI with diffusion restriction. It also may expand the vessel lumen, less common with bland thrombus 4
venous thrombophlebitis: superficial venous system involvement only
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