Posterior Tibial Artery Pseudoaneurysm: A Case Report

Abstract

Introduction:

Pseudoaneurysms are abnormal arterial aneurysmal sacs contained by only the tunica adventitia and/or surrounding soft tissues and may mimic soft tissue tumours both clinically and radiologically. Several overlapping features may exist: anatomic location close to a neurovascular bundle, haemorrhagic components, heterogeneous signal intensity on magnetic resonance imaging (MRI) and a history of a painless, slow growing mass. A key differentiating factor is the pulsation artefact, or ‘ghosting’ artefact, on MRI owing to sufficiently high velocity flow in a pseudoaneurysm, as well as the ‘ying-yang’ duplex ultrasound appearance of blood flow ‘to-and-fro’ within the sac.

Case details:

This case report is of a male in his 60s who was referred to our institution for a soft tissue sarcoma and potential biopsy. He was eventually diagnosed with a large posterior tibial artery pseudoaneurysm and biopsy was not performed. We present the multi-modality imaging findings used to make this diagnosis and discuss useful radiological features to distinguish vascular from soft tissue tumours.

Conclusion:

The case highlights the importance of considering vascular lesions in sarcoma care to avoid iatrogenic injury, which may be catastrophic. This is the second reported case of a pseudoaneurysm mimicking a soft tissue neoplasm in this location.

Introduction

Pseudoaneurysms are abnormal arterial aneurysmal sacs contained only by the tunica adventitia or surrounding soft tissues, occurring secondarily to a breach of the inner vessel wall. They demonstrate variability in their degree of chronicity and regression.^[1] The two most common causative factors are trauma and iatrogenic injury; rarer atraumatic pseudoaneurysms are reported in vasculitis, connective tissue disorders and may also be induced by a nearby osteochondroma.^[2] Pseudoaneurysms, and other vascular lesions such as high flow arteriovenous malformations, may mimic soft tissue sarcomas due to anatomic location, radiological appearance and a paucity of characteristic clinical findings such as an audible bruit or pulsatility. However, they remain an important diagnosis to consider due to the potential morbidity and/or mortality from an inadvertent biopsy or aspiration.^[3] In this article, we describe a case report and demonstrate high-yield imaging findings of a 60-year-old male with a suspected lower limb sarcoma who had a final diagnosis of posterior tibial artery pseudoaneurysm. We discuss key multi-modality imaging findings to help in this distinction and hope to raise awareness of pseudoaneurysms in sarcoma care.

Case Report

A male in his 60s had a Technetium-99m-MDP (methyl diphosphonate) whole body bone scan [Figure 1] as part of his workup for prostate cancer. It demonstrated an incidental mild area of osteoblastic activity within the left proximal tibia and fibula. There was a photopenic area interposed between the two bones, suggesting the presence of an extrinsic mass.

Figure 1.

Tc-99m-methylene diphosphonate (MDP) whole body bone scans demonstrate mild uptake within the proximal left tibia and fibula (red arrows), indicating mild osteoblastic activity. There is also a relatively photopenic area (blue arrow) interposed between the two bones, suggesting an expansile mass with low osteoblastic activity is present. No further concerning areas of uptake are present

The patient subsequently returned to the clinic for examination, where he confirmed over a year’s history of an enlarging mass in his left proximal lower leg with no pain, systemic symptoms or history of trauma. The patient had no past medical or family history of note. On examination, the patient had a deep mass palpable at the posteromedial aspect of the mid lower leg. The mass was hard, non-tender, and not significantly fluctuant or pulsatile. Distal lower leg pulses were palpable and symmetrical. A soft tissue sarcoma was suspected, and further radiological studies were performed.

The follow-up radiograph [Figure 2] demonstrated smooth eccentric remodelling of the proximal tibia and fibula. The narrow zone of transition confirmed the mass to be chronic and slow-growing. A subsequent non-contrast magnetic resonance imaging (MRI) demonstrated a T1 and T2-weighted predominantly hypointense lesion with areas of intrinsic hyperintensity [Figure 3]. There was remodelling but gross preservation of the adjacent bony cortices and mass effect on surrounding muscles. A suspicion for soft tissue sarcoma was raised, and the patient was referred to the musculoskeletal sarcoma MDT for consideration of biopsy.

Figure 2.

AP radiograph of the lower leg demonstrating smooth extrinsic scalloping of the tibial and fibular cortices (red arrow) with secondary lateral bowing of the fibula. There is a narrow zone of transition indicating the chronicity and a slow-growing nature of the lesion

Figure 3.

(A) Axial T1 and (B) T2 fat-saturated (T2FS) MRI images demonstrate a predominantly hypointense mass with areas of hyperintensity located within the soft tissues between the tibia and fibula in the region of the interosseous membrane and posterior tibial neurovascular bundle. The tibialis posterior (TP) and anterior tibial neurovascular bundle (white arrow) are anteriorly displaced. The soleus (So) and lateral head of gastrocnemius (LGn) muscles are posteriorly displaced. There is smooth remodelling of the tibia from extrinsic pressure with preservation of the cortex (blue arrow), indicating a slow-growing and chronic process. (C) On window widening of the T2FS MRI sequence, a horizontal band of artefact in the phase-encoding direction from pulsation artefact of the pseudoaneurysm is present

MRI scans were re-reviewed at the sarcoma multidisciplinary meeting. On windowing of the T2 fat saturation MRI sequence [Figure 3C], a linear horizontal band of artefact, in the direction of the phase-encoding gradient, was present. This represented a pulsation (‘ghosting’) artefact suggesting intrinsic high-grade vascularity. An MDT decision to not proceed with biopsy was made. Instead, a follow-up CT angiogram of the lower limbs was performed, which revealed diffuse homogenous enhancement of the mass and contiguity with the posterior tibial artery [Figure 4], leading to the final diagnosis of a posterior tibial artery pseudoaneurysm. The patient was reassured of the diagnosis and referred to the vascular surgical team for further management.

Figure 4.

(A) Axial, (B) coronal bilateral maximal intensity projection (MIP), and (C) coronal unilateral CT angiogram images of the lower limb demonstrate diffuse homogenous post-contrast enhancement of the mass lesion, which is contiguous with the posterior tibial artery, confirming the diagnosis of a pseudoaneurysm. There is remodelling but preservation of the adjacent bone cortex (white arrow)

Discussion

A clinical diagnosis of pseudoaneurysm may be difficult to reach when the presenting complaint is vague, or the characteristic signs such as audible bruits and palpable thrills are absent, which was the case in this patient, given that the lesion was deeply located between multiple muscle compartments.^[3] In such cases, the diagnosis may only be reached on imaging. The cause of the pseudoaneurysm was not identified in our patient. Pseudoaneurysms are commonly traumatic or iatrogenic in aetiology. Atraumatic pseudoaneurysms are reported in vasculitides and connective tissue disorders and are rarely induced by a nearby osteochondroma.^[4] We identified one case report of a large posterior tibial artery pseudoaneurysm in this location by Singh et al. (2011), which was secondary to a prior penetrating injury to the lower leg.^[5] The pseudoaneurysm similarly led to chronic fibula remodelling, resulting in a gracile bone. Preliminary radiological investigations also raised suspicion of a soft tissue tumour and recommended sampling. However, the patient subsequently presented with a pathological fracture of the fibula. Post-traumatic aneurysms of the posterior tibial artery distally are reported following calcaneal trauma, although these are likely to be suspected and diagnosed clinically, given the superficial location.^[6]

The MRI appearance of pseudoaneurysms can radiologically mimic soft tissue sarcomas, which are comprised of a heterogeneous group of tumours.^[7–9] For example, synovial sarcoma may appear simply as a complex cystic and haemorrhagic lesion in its early stages.^[10–12] Haemorrhagic components may be difficult to distinguish from a vascular lesion such as a pseudoaneurysm. Both may demonstrate T1 and T2 heterogeneous signal intensities with elliptical layering of haemorrhage and thrombus. The presence of soft tissue elements, absence of trauma and correlation with demographics may allow this distinction. Soft tissue sarcomas are also likely to demonstrate a more aggressive bone destruction pattern owing to a high mitotic count. This is characterised by cortical destruction, permeative change and aggressive periosteal reactions, as opposed to chronic bone remodelling from the pulsations of a pseudoaneurysm.

Pulsation artefact adhering to the phase-encoding direction on MR is a clue to relatively high velocity vascular flow within the lesion and may be the only differentiating feature of a pseudoaneurysm, such as in this case.^[4,13] The vascular pulsations affect data collection in the phase-encoding direction on certain MR slices, resulting in motion artefact across the field of view. This may be overlooked without appropriate windowing and potentially lead to fatal consequences when obtaining a biopsy sample. It is important to note that these artefacts may not be present in dedicated vascular protocol MRIs where images are negated to optimise image quality.

Peripheral nerve sheath tumour (PNST), including schwannoma and neurofibroma, was also considered as a differential diagnosis due to MRI features of a heterogeneous intensity and a slow-growing lesion near the tibial neurovascular bundle.^[14–16] These can occur in isolation or in association with other systemic disorders such as neurofibromatosis.^[14] Patients may present with symptoms of nerve entrapment/radiculopathy, especially in larger lesions; a large proportion still remain asymptomatic.^[15] MR typically demonstrates an encapsulated heterogeneous lesion with minimal associated oedema; the ‘target’, ‘fascicular’ and ‘fat-splitting’ signs may also be observed; haemorrhage and increased oedema may occur in rare malignant varieties. Pulsation artefact is not generally present and is again a differentiating feature.^[4]

The optimal imaging modality for pseudoaneurysm assessment depends on the location. CT angiography is used for imaging the aorta, for more peripheral vessels, for example, the common femoral artery; duplex ultrasound is of high utility.^[1] Sonographic findings are of a cystic structure with hyperechogenic eccentric layers within it representing haematoma or thrombus. Duplex ultrasound will demonstrate an alternating ‘to-and-fro’ waveform and ‘swirling colour flow’ which demonstrates the characteristic ‘ying-yang sign’.^[17,18] Interrogation with duplex ultrasound at the time of biopsy of a potential soft tissue lesion is imperative to avoid a catastrophe. A major drawback, as with any ultrasound imaging, is the operator’s experience. In addition, triple-phase Tc99m phased bone scans can raise concerns for a pseudoaneurysm with increased radiotracer activity in blood flow and pooled images that are subsequently washed out in delayed imaging; this modality may not be diagnostic in isolation.^[19] Conventional angiography is another useful imaging modality which can aid in diagnosis, especially but usually reserved for cases where prior imaging has been inconclusive.

Peripheral pseudoaneurysms can be treated with ultrasound-guided thrombin injections where the aneurysmal neck and sac are relatively small. Larger lesions require endovascular intervention with covered stenting and sometimes require open surgical repair.

Conclusion

Vascular lesions (including pseudoaneurysm and high-flow arteriovenous malformation) should always be excluded in the diagnostic workup of soft tissue sarcoma. This case highlights caution in MRI interpretation of solid soft tissue tumours, such as STS and PNST, which may demonstrate overlapping features with vascular lesions. Subtle signs such as the pulsation (or ‘ghosting’) artefact may be overlooked without appropriate windowing. CT angiography or US duplex studies should be performed where a pseudoaneurysm is considered in the differential diagnosis. Doppler ultrasound is an important tool to exclude high-flow vascularity at the time of any US-guided biopsy to prevent iatrogenic and potentially fatal complications.