Venous aneurysms are uncommon diseases that occur in the cervical vein, thoracic vein, and in the veins of extremities. Among the reported cases of venous aneurysms, iliac venous aneurysms are considered a rare form of the disease (1, 2).

Venous aneurysms of the common iliac vein can be asymptomatic or symptomatic through abdominal pain or swelling of the legs due to thromboembolism. Differentiation of a venous aneurysm from a true tumor is important for the proper management of the disease. We describe a case of a common iliac venous aneurysm mimicking a retroperitoneal mass, focusing on imaging findings.

A 63-year-old female visited our hospital for outpatient follow-up of chronic Hepatitis B virus. She had vague back pain and had no significant past medical history, except for cervical pain two years ago. Abdominopelvic CT was performed as a routine evaluation of the liver and other organs.

Abdominopelvic CT scan showed a well-defined mass measuring 4.5 × 3 × 4 cm near both the common iliac vessels at the level of the fifth lumbar spine. On the portal phase of the contrast-enhanced abdominopelvic CT scan, the lesion showed heterogeneous enhancement [70 Hounsfield units (HU)] in comparison with homogeneous enhancement of the common iliac vein (100 HU). On the delayed phase, the tumor showed homogeneous enhancement (100 HU) as the common iliac vein (Fig. 1A). The mass showed the same density relative to the common iliac vein. We were not able to distinguish between neurogenic tumor and vascular tumor.

MRI was performed for the discrimination of the tumor. T1-weighted scans showed a hypointense mass relative to the muscle and common iliac vein. T2-weighted scans showed a hyperintense mass with central hypointensity (Fig. 1B). In addition, the mass showed good enhancement similar to that of the common iliac vein (Fig. 1C). Fluorodeoxyglucose (FDG) positron emission tomography-CT was performed for further evaluation of hypermetabolic malignancy. Accumulation of FDG within the mass was not observed.

On ultrasonography (US) with color Doppler, faint blood flow into the mass was noted (Fig. 1D).

Conventional venography was performed for confirmation of aneurysm. Venography showed a saccular aneurysm on the anteromedial aspect of the left common iliac vein; complicated lesions such as thrombus or rupture were not observed (Fig. 1E). Therefore, surgical treatment was not required, and the patient was discharged with a recommended follow-up CT scan.

Venous aneurysm is an unusual disease, and the significance of this disease is unknown. Venous aneurysms are classified and divided into acquired and congenital lesions. These lesions are further classified into primary causes and secondary causes, such as trauma or factors that can have an influence on the modification of venous structures (3). Many studies have also suggested that a primary aneurysm of the iliac vein, such as the present case, is rare (2, 4).

Ysa et al. (2) reported a case of a thrombosed iliac venous aneurysm. In addition to their case, the study presented a retrospective review of 21 reported cases. They reported that the most common cause of iliac venous aneurysm is arteriovenous fistula secondary to trauma.

An iliac venous aneurysm may be presented with various symptoms such as swelling of the legs, abdominal pain, and dyspnea. Symptoms due to thrombus and venous stasis are common. In 32% of the cases, aggravated symptoms and persistent symptoms remain (2, 5). Various treatment strategies are needed for iliac venous aneurysms depending on the cause. Surgical procedures such as aneurysm removal or ligation of the fistula, medical anticoagulation, and elastic compression can be used (2). Arteriovenous fistula followed by trauma must be surgically removed (6). In addition, an asymptomatic primary venous aneurysm can be surgically resected (5).

Venous aneurysms can be detected with US, CT with contrast enhancement, and MRI. CT scan is particularly helpful in the detection of combined congenital anomalies and complicated aneurysms, such as thromboembolism (2).

In our case, a well-defined mass measuring 4.5 × 3 × 4 cm was observed near both common iliac vessels. Coronal and sagittal multiplanar reformation images of CT scan showed contiguity of the mass with inferior vena cava (IVC). Although the mass with delayed homogeneous enhancement same as IVC helped us to consider aneurysm originating from IVC, we could not exclude the possibility of a retroperitoneal mass such as leiomyosarcoma, angiosarcoma, or neurogenic tumor. Also, small tumors may present homogeneous attenuation, whereas large tumors have heterogeneous attenuation with extensive necrotic change. In such cases, MRI can be helpful. Neurogenic tumors such as shwannoma and neurofibroma located in the paravertebral area show characteristic features on MRI (7). Heterogeneous enhancement of the tumor on CT and MRI indicates mixed fast and slow flows. Krinsky et al. (8) suggested a pattern of enhancement called the layered gadolinium sign. This sign occurs due to very slow flow in the aneurysm, which results in poor mixing of in-flow within the aneurysm. This gadolinium sign can be helpful to the radiologist in diagnosing a venous aneurysm from solid neoplasms.

As seen in the present case, a venous aneurysm without symptoms can mimic a retroperitoneal mass. In such cases, imaging modalities are more useful. However, there are few cases focusing on radiologic findings relative to the reported cases of venous aneurysms. The authors used various imaging modalities, and these tools helped to distinguish a common iliac venous aneurysm from a mimicking pelvic mass. However, it is not necessary to perform all imaging modalities in every case.

In conclusion, it is important for the radiologist to recognize an iliac vein aneurysm mimicking a true mass, followed by an accurate radiologic diagnosis for the proper treatment of the disease.