A healthy 39-year-old male presented with deep venous thrombosis (right external iliac and femoral veins) and was screened for occult neoplasms. A right kidney renal cell carcinoma was discovered and removed. A follow-up CT scan was performed one year later. In the meantime, the deep thrombosis had not completely resolved.
The patient went through radical nephrectomy for pT1a Renal Cell Carcinoma (RCC). The follow-up CT showed persistent deep thrombosis: homogenous hypodense endoluminal material, with heterogeneous enhancement, distending right external iliac and femoral veins.
A subsequently performed MRI revealed a vascular endoluminal mass, in relation to right femoral and external iliac veins, that measured 20cm in greatest diameter. Distal extraluminal growth was found.
At T1-weighting the mass was homogeneous and isointense, at T2-weighting the mass was hyperintense and slightly heterogeneous. Contrast uptake was heterogeneous.
Collateral veins were noted at the right groin and thigh.
The lesion was biopsied and then surgically removed. At histology, it was classified as grade 3 vascular leiomyosarcoma (femoral vein origin) with necrosis (<50%). Iliac and femoral arteries were not involved at surgery. No metastatic disease was found at diagnosis, including lymph nodes.
Leiomyosarcomas originate from smooth muscle.  Non-visceral leiomyosarcomas are subclassified into cutaneous, vascular and soft tissue.  The vascular form is the least frequent: 2% of the cases. 
Vascular leiomyosarcomas (vLMS) arise from the muscular wall of veins or arteries.  Veins are 5 times more frequently affected and vLMS are mostly found in central vessels.  vLMS are rare in the extremities, predominantly involving the femoral vascular bundle. [1,3]
vLMS usually show combined intraluminal and extraluminal growth, their size is variable and presenting symptoms are non-specific. [3,5]
Venous intraluminal growth normally follows the direction of flow and is associated with thrombosis.  Thus, vLMS may manifest with peripheral edema and be misdiagnosed as deep venous thrombosis (DVT). [3, 6] Extraluminal growth can compress/invade the artery and/or nerve resulting in pain.  Other patients present with palpable lump. 
vLMS might be first detected with ultrasound: tumour echogenicity is variable and sometimes difficult to differentiate from a blood clot. 
At CT both vLMS and bland thrombus may be hypoattenuating . Findings suggesting vLMs include vessel dilation and intraluminal heterogeneous enhancement. Bland thrombus does not enhance . Collateral vessels are common. 
On MR Imaging vLMS are often homogeneous and display low/intermediate signal on T1-weighting and intermediate/high signal on T2-weighting. Blood clots have high T1 signal. [5,8]
Imaging is essential for accessing tumour length, extraluminal extension, the relationship to surrounding structures and metastatic disease, thus helping determine treatment approach and prognosis. 
vLMS of the lower limb have a low incidence and most reports have been single cases, as so, there is little data on management and prognosis.  Treatment involves extensive resection, radiotherapy and/or chemotherapy. Vascular reconstruction may or may not be necessary. 
Metastases can present at diagnosis, precluding tumour resection, but most develop after diagnosis.  A review of 31 case reports of femoral vein vLMS concluded 45% developed metastases.  Lung and liver are frequent locations [1,3]. Overall, the prognosis for vLMS is poor, with a 5-year survival of 31–63%. 
The reported poor prognosis of vLMS is in part due to a late diagnosis. Atypical manifestation of DVT or little-to-no predisposing factors warrant consideration for alternative diagnosis. In our case a RCC was found at DVT work-up overshadowing and delaying the diagnosis, despite the non-resolving symptoms. We demonstrate the importance of having a great degree of suspicion in case of atypical DVT manifestations.
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