Medical imaging for the diagnosis, recurrence and metastasis evaluation of clear cell sarcoma

Abstract

Clear cell sarcoma (CCS) of soft tissue is extremely rare, accounting for approximately 1% of all soft tissue tumours. It is very difficult to diagnose CCS based on clinical manifestations. Magnetic resonance imaging (MRI) provides high-resolution images of soft tissues and pathological features such as mucus, necrosis, bleeding, and fat through high and low signals on T1 weighted image (T1WI) and T2 weighted image (T2WI). On the other hand, the paramagnetism of melanin in CCS shortens the relaxation time of T1 and T2, and high signal intensity on T1WI and low signal intensity on T2WI can be found. This is different from most other soft tissue sarcomas. At present, the treatment method for CCS is surgical resection. MRI can effectively display the tumour edge, extent of surrounding oedema, and extent of fat involvement, which is highly important for guiding surgical resection and predicting postoperative recurrence. As an invasive sarcoma, CCS has a high risk of metastasis. Regardless of the pathological condition of the resected tumour, MRI or computed tomography (CT) should be performed every 1-2 years to assess recurrence at the primary site and to screen for metastasis in the lungs, liver, and bones. If necessary, PET-CT can be performed to evaluate the overall condition of the patient.

Key Words: Clear cell sarcoma; Recurrence; Metastasis; Computed tomography; Magnetic resonance imaging; Editorial

Core Tip: In this editorial, we comment on a case report by Liu et al published in the recent issue. According to the authors of this article, the objective of the study was to investigate the metastasis, surgical treatment and postoperative follow-up of clear cell sarcoma (CCS). In this editorial article, we will focus specifically on the roles of computed tomography and magnetic resonance imaging in the diagnosis, surgical treatment, evaluation of recurrence, and detection of metastasis of CCS.

INTRODUCTION

In this editorial, we comment on a case report by Liu et al[1] published in the recent issue. According to the authors of this article, the objective of the study was to investigate the metastasis, surgical treatment and postoperative follow-up of clear cell sarcoma (CCS). In this editorial article, we will focus specifically on the roles of computed tomography (CT) and magnetic resonance imaging (MRI) in the diagnosis, surgical treatment, evaluation of recurrence, and detection of metastasis of CCS.

CCS

CCS of soft tissue is extremely rare, accounting for approximately 1% of all soft tissue tumours, with an annual incidence of approximately one million[2]. Due to its extremely low incidence, CCS was listed as an ultrarare sarcoma at the consensus meeting held by the Connective Tissue Oncology Society in 2019[3]. The list of ultrarare sarcomas included 56 types of soft tissue sarcomas, all of which are challenging to diagnose and treat.

CCS typically occurs in young people aged 30 to 40 years. Primary CCS typically occurs in the limbs, especially in the feet or ankle joints, followed by the knee joints. The retroperitoneal space, kidneys, gastrointestinal tract, chest cavity, lungs, mediastinum, bones, and head and neck are rarely affected. Tumours that develop in the limbs are often located deep in the soft tissue or under the fascia and are clinically characterized by slow growth without pain[2,4-6]. These tumours can easily be misdiagnosed as benign tumours. Tumours that occur in important areas can cause compression, pain, and functional abnormalities as the tumour grows. If it occurs in the gastrointestinal tract, obstructive symptoms may occur. Some patients have lymph node metastasis and distant metastasis at the initial diagnosis. It is very difficult to diagnose CCS and metastasis based on clinical manifestations.

CT and MRI are important imaging methods. The manifestations of CCS on CT include soft tissue lesions with homogeneous density, and in cases of necrosis, heterogeneous density may be present. Due to insufficient information provided, the diagnostic value of these methods is limited. MRI provides high-resolution images of soft tissue and pathological features such as mucus, necrosis, bleeding, and fat through high and low signals on T1 weighted image (T1WI) and T2 weighted image (T2WI)[7]. According to the latest ESMO guidelines, MRI is the best imaging method for soft tissue sarcomas, except for those in the retroperitoneal and visceral areas[8]. CCS originates from neural crest cells in soft tissues[9,10]. The deposition of melanin within the lesion makes it difficult to distinguish from malignant melanoma through immunohistochemistry[11]. At the same time, the two have similar morphological and clinical features[12], which makes differential diagnosis difficult. The specific signals on MRI that indicate CCS show important clinical value. In cases of CCS, the paramagnetism of melanin shortens the relaxation time of T1 and T2, and there is high signal intensity on T1WI and low signal intensity on T2WI[7]. This is different from most other soft tissue sarcomas, which show low signal intensity on T1WI and high signal intensity on T2WI. However, the MRI signal depends on the amount of melanin in the tumour. A low melanin content may not lead to an increase in signal intensity on T1-weighted images[12]. On the other hand, connective tissue, necrosis, cystic degeneration, and other factors within the lesion can also reduce the signal intensity on T1-weighted images and increase the signal intensity on T2-weighted images. Other components within the lesion, such as fat, haemoglobin, and protein fluid, can also cause changes in MRI signals. However, in most cases, the combination of multiple MRI techniques, including fat suppression, sensitivity-weighted imaging, and enhanced MRI, can accurately diagnose CCS.

At present, the treatment method for CCS is surgical resection[13]. MRI can effectively display the edge of the lesion, the extent of surrounding oedema, and the extent of fat involvement, which is highly important for guiding surgical resection and predicting postoperative recurrence. Tumour cells may be present in the oedematous area surrounding the tumour, which can predict the risk of local recurrence[14,15]. One study[16] revealed that if tumour cells are found in the reactive oedematous area around the tumour, the local recurrence risk may be as high as 40%. The fat gap around the tumour can help clinicians evaluate the involvement of neurovascular bundles and surrounding organs. If the fat gap is eliminated, local invasion may occur.

As an invasive sarcoma, CCS has a high risk of metastasis. Up to 40% of patients experience lymph node metastasis. Remote metastasis mostly occurs in the lungs and bones, and there are also a few reports of metastasis in the gastrointestinal tract, liver, pancreas, and sellar region[17-19]. Insufficient surgical resection, a tumour size > 5 cm, and vascular invasion are important factors for metastasis[20]. Early detection of metastasis during postoperative follow-up is crucial for avoiding a poor prognosis. Regardless of the pathological condition of the resected tumour, MRI or CT should be performed every 1-2 years to assess recurrence at the primary site and to screen for metastasis in the lungs, liver, and bones. If necessary, PET-CT can be performed to evaluate the overall condition of the patient[21].

CONCLUSION

In summary, CCS is an extremely rare sarcoma characterized by invasiveness and high rates of postoperative recurrence and metastasis. The clinical symptoms are often atypical, and early diagnosis is difficult. MRI have high soft tissue resolution, and multiple signals can reflect the internal components of the tumours. It can help with early diagnosis, surgical guidance, and recurrence and metastasis detection during postoperative follow-up, which has important clinical significance.

ACKNOWLEDGEMENTS

The authors thank the participants and referring technicians for their selfless and valuable assistance in this study.