Synovial sarcoma mimicking a traumatic pseudoaneurysm in the upper extremity

Abstract

Synovial sarcomas are a rare but aggressive malignancy that primarily affects young patients. Diagnosis is often difficult and delayed due to its insidious onset, heterogenous presentation and mimicry of other pathologies. We present the case of a patient with a history of a slow-growing left arm mass that arose after a traumatic fracture of the humerus. Multimodal imaging was undertaken and reported the mass as being consistent with a vascular malformation of the brachial artery. The patient underwent surgical repair of the artery and intraoperative biopsies confirmed a diagnosis of synovial sarcoma. This case highlights the importance of maintaining suspicion for soft-tissue sarcomas in young patients presenting with a mass, and demonstrates the way in which these tumours may mimic other pathologies both clinically and radiologically. Early referral to a specialist sarcoma centre is key for further investigative workup.

Background

Synovial sarcomas are rare and account for less than 10% of all soft-tissue sarcomas.^1–3 Its name is a misnomer, as synovial sarcomas do not originate from synovial tissue; rather, they typically arise from the deep mesenchymal tissues of the extremities as well as the head and neck.^{2 4} Primarily, adolescents and young adults are affected.^{5 6} Due to their esoteric nature and heterogenous presentation, the diagnosis of synovial sarcomas is difficult and often delayed. We present a case of a patient who presented with a slowly enlarging mass in the cubital fossa. This was initially presumed to represent a pseudoaneurysm following trauma to that site, but was subsequently diagnosed as a synovial sarcoma following excisional biopsy.

Case presentation

Our patient is an otherwise healthy 27-year-old man who was referred to the vascular surgery unit at a tertiary hospital by his general practitioner with a provisional diagnosis of a traumatic arteriovenous fistula arising from his distal left brachial artery. This is in the context of a supracondylar fracture in the same arm over 15 years ago.

He was found to have a large mass at the medial aspect of his left arm just proximal to the cubital fossa. He reported first noticing a lump following his fracture, and denied any abnormalities in the area prior to this. The mass slowly enlarged over a 15-year interval. It recently began to cause pain and limitation of moment when the patient was given anticoagulation following a diagnosis of left arm deep vein thrombosis (DVT) while on holiday overseas. Documentation of this DVT was not available at the time of review. On examination, the mass was tender but there was no palpable thrill. There were no overlying skin changes or distal neurovascular compromise.

Investigations

An initial ultrasound scan showed a 6.5×4.0 cm ovoid mass with an anomalous communication to the brachial artery (figure 1) reported as a post-traumatic arteriovenous fistula or pseudoaneurysm. A subsequent CT angiogram of the arm confirmed the presence of a heterogeneously enhancing cystic structure adjacent and deep to the distal brachial artery. The appearance of this was reported to represent a partially thrombosed pseudoaneurysm of the brachial artery (figures 2 and 3).

Figure 1.

(Left) Ultrasound of the left elbow demonstrates an ovoid 6.5 cm solid and cystic mass deep to the brachial artery. (Right) Colour Doppler images demonstrate the relationship of the mass to the brachial artery (arrow) with perfusion of the mass through a narrow ‘neck’ (arrowhead) simulating the appearance of a partially thrombosed pseudoaneurysm.

Figure 2.

CT angiogram of the left upper limb. Sagittal maximum intensity projection demonstrates the relationship of the ovoid mass (arrow) deep to the brachial artery.

Figure 3.

CT angiogram of the left upper limb. Axial image showing the left brachial artery (arrow) along the superior aspect of the mass which is heterogeneous in attenuation, containing a focus of calcification along its margin (arrowhead).

Treatment

Consequently, the patient proceeded to surgical repair of the presumed traumatic pseudoaneurysm. Intraoperatively, a large defect in the brachial artery with active bleeding into a pseudoaneurysm sac was found. However, an unusual solid tan-coloured mass within the sac was also identified. There was no macroscopic appearance of invasion into adjacent structures and multiple biopsies were sent for histopathological analysis. Repair of the brachial artery proceeded with an interposition great saphenous vein graft, which was harvested using a separate sterile set-up including gown, gloves and instruments. The patient recovered well from surgery and was discharged several days later.

Outcome and follow-up

Histopathology revealed that the biopsied mass entirely comprised tumour consisting of a spindle cell population. A portion of brachial artery adjacent to the mass showed focal mural expansion with tumour infiltration. On further immunohistochemistry, a diagnosis of malignant spindle cell tumour consistent with monophasic synovial sarcoma was suggested.

The patient was urgently referred to a designated regional sarcoma centre and further molecular testing confirmed monophasic synovial sarcoma with SS18 gene rearrangement. Staging was completed with MRI showing residual tumour at the site as well as a CT chest and positron emission tomography scan which demonstrated no evidence of metastatic disease. Subsequent radiotherapy was performed with good response, with interval reduction in the size of the treated tumour seen on repeat MRI (figure 4). The patient is now planned for further limb-salvaging surgical resection with flap reconstruction in the forthcoming months.

Figure 4.

Axial T1-weighted fat-saturated postcontrast image of the left elbow following surgical excision and neoadjuvant radiotherapy shows an ill-defined rim of residual enhancing tumour deep to the left brachial artery (arrow).

Discussion

Synovial sarcomas are an uncommon but aggressive malignancy which predominantly affect patients under 30 years of age.^{5 6} It is hallmarked by a pathognomonic chromosomal translocation t(X;18)(p11;q11) leading to the formation of a SS18-SSX fusion gene, which is detected in up to 95% of patients.^{1 3 5} There are two main histological subtypes: monophasic (entirely comprised spindle cells) and biphasic (epithelial glandular structures and spindle cells).⁷

Diagnosis of synovial sarcomas can be difficult due to the large variety of possible tumour sites, including extremities, head and neck, thorax, axilla and even abdomen.2 4 8 9 The clinical history is often that of a slow-growing mass which may be present for years, associated with pain or tenderness, but can also present with motor and sensory deficits or painless swelling.^{1 2} Synovial sarcomas are difficult to diagnose by imaging alone as there are no pathognomonic imaging features. Ultrasound and CT typically demonstrate a well-circumscribed soft-tissue mass with heterogeneous internal density. CT can also detect focal or disseminated calcifications internally.¹ MRI is the imaging modality of choice for local staging due to superior soft-tissue contrast resolution and its ability to visualise neurovascular, muscular and bony invasion.¹⁰ Gadolinium-enhanced sequences are useful in increasing suspicion for an aggressive lesion and also to more accurately delineate the tumour plane.^{11 12} Tissue diagnosis is confirmed via either incisional or core biopsies. However, care must be taken to minimise risk of dissemination of malignant cells along the biopsy tract or surgical field.¹³

The primary treatment for synovial sarcomas is complete surgical resection combined with radiation therapy.4 7 14 15 A recent study combining two prospective trials has reported that for small, localised disease, a surgery-only approach can be a safe and viable treatment option.¹⁶ Additionally, neoadjuvant or adjuvant cytotoxic chemotherapy is the mainstay of treatment in cases of advanced disease^{5 15 17} with several studies showing improved overall patient survival.^18–20 However, due to the relative rarity of this disease and its involvement of paediatric populations, there is limited prospective and randomised data analysing survival outcomes for different treatment options.

Synovial sarcomas are generally thought to have a poor prognosis. Reported 5-year survival rates range from 61% to 78%^{2 3} and distant metastases occur most commonly in the lungs, bones and regional lymph nodes. A multicentre study analysing data collected across three decades demonstrated that factors associated with a survival advantage include head and neck cases, female patients, tumour size (<5 cm) and histology (biphasic).^{3 21} The current literature also indicates that paediatric patients have better overall prognosis compared with adult populations with synovial sarcoma.^{1–3 22}

Our case represents a particularly unique diagnostic challenge of an upper limb synovial sarcoma arising in the same location as a previous fracture. Clinical assessment and imaging were both suggestive of a post-traumatic vascular lesion; therefore, further preoperative investigation with MRI was not undertaken. However, it was discovered that the previous fracture history was ultimately a red herring. As a result, unplanned surgical resection was undertaken. Further wide excision to obtain clear margins is the next step in these cases,²³ in conjunction with adjuvant radiotherapy or chemotherapy in patients undergoing limb-sparing surgery.^23–25 There is conflicting evidence regarding the effect of unplanned resections in soft-tissue sarcomas. Some studies show poorer outcomes while others suggest that prognosis is not significantly impaired.^{2 23–26}

Although no definitive causal link between trauma and the subsequent development of soft-tissue sarcomas have been proven, there are at least two cases in the literature describing the diagnosis of synovial sarcoma after localised trauma.^{27 28} It is more likely that the trauma itself unmasks a pre-existing soft-tissue sarcoma in an otherwise healthy and asymptomatic patient.

The majority of soft-tissue sarcomas, including synovial sarcomas, are often misdiagnosed due to their insidious onset, well-circumscribed appearance (when small) and heterogenous presentation. Clinical characteristics that appear to be associated with late diagnosis and unplanned resection include long duration of symptoms, unusual tumour location, small size and mimicry of other pathologies.^29–31 This case demonstrates the importance of maintaining suspicion for soft-tissue sarcomas in young patient populations presenting with a mass, despite history and imaging findings suggesting otherwise. Early referral to a specialist sarcoma centre is key for further investigative workup. Where possible, these lesions should not be biopsied without prior discussion with a treating sarcoma surgeon. Additional education of both primary healthcare providers and other medical specialists with regards to the complex diagnostic nature of soft-tissue sarcomas is crucial to avoid delayed diagnosis and treatment.

Learning points.

• Synovial sarcomas are a rare, although aggressive malignancy which affect younger patient populations.

• Soft-tissue sarcomas, including synovial sarcomas, can often pose a diagnostic challenge for clinicians that rarely encounter this pathology.

• This case report highlights the way in which these tumours may mimic other non-aggressive pathologies on clinical and imaging grounds, the need for an index of suspicion in any young patient presenting with a soft-tissue mass (especially if >5 cm) and the importance of early referral to a specialist sarcoma centre for full assessment.

• Further education of clinicians can aid in preventing misdiagnosis.