What Is Solitary Fibrous Tumour?

Overview

What are tumours? 

They are a group of uncontrolled dividing cells which accumulate together to form lumps in the body. The pathological route of growth of a tumour depends on whether it is cancerous (malignant) or non-cancerous (benign). The role of tumours in human health becomes alarming when the cells are cancerous. In this way, more cells are attracted to the site, allowing the tumour to grow by generating blood supply through vessel formation. A benign tumour, on the other hand, usually does not cause any harm to the body. 

What are solitary fibrous tumours?

A solitary fibrous tumour (SFT) can be defined as a mass of cells which develop from the soft tissue of the body. More specifically, it is a form of soft tissue sarcoma arising from mesenchymal cells. It remains so rare that it only forms less than 4% of soft tissue sarcomas.¹ This rare tumour presents with a benign course of growth; however, it has the potential to become cancerous in roughly a fifth of cases.² 

Locations of the SFTs

SFTs can be found in all regions of the body, such as in tissue layers like the peritoneum and organs, including the lungs. They can also grow in the spine, kidneys, and breast tissue. However, most SFTs that develop grow externally from the lungs within the pleural tissue, presenting as the most common form. The pleura tissue is a layer which covers the lungs to protect them. Furthermore, this form of tumour mostly affects the elderly.1 Before 1900, SFTs were only thought to be localised in the pleura.3 However, they have now been found in every region of the body, including the meninges, which are located in the brain. 

Prevalence of SFTs

The prevalence of SFT remains extremely low, with statistics showing that there is approximately one new case per million cases per year.4 Generally, SFTs appear in people over the age of 65, affecting people assigned male and female at birth, and are not very common among children. It is believed that some SFTs may not cause symptoms for some time until their size increases and they push against other tissues and organs. In the malignant form of SFT, symptoms are present in three-quarters of the cases.⁵ 

The recurrence rate of SFT is present much further down the line, up to ten years later.⁶ The pathological course of SFTs remains difficult to define due to each growth being unpredictable. Most of the collected data shows that characteristic traits specific to STFs cannot be fully defined. However, research has noted that indicative of a malignant tumour is having a diameter that extends beyond 10 cm alongside connections between neighbouring structures.⁵

SFT symptoms

When symptoms do present, this depends on the location of the growth. For example, if an SFT grows within the lung cavity; this is usually a pleural fibrous tumour, and symptoms present similar to a respiratory infection:

• Persistent coughing
• Pain in chest
• Shortness of breath

Chest pain is noted in approximately 60% of patients.⁷ However, other symptoms unrelated to pleural fibrous tumour specifically, include swelling of eyelids, toes, fingers, disorientation, unclear vision, and heightened pulsations surrounding the tumour.

Diagnosis

A range of exams can be performed to detect the presence of a solitary fibrous tumour in the body; this includes:

• Scanning: through assessment of an MRI, CT or PET scan, X-ray, or ultrasound, the size, location and regions of cancer cells can be detected.
• Tissue extraction: through a biopsy procedure, cells can be removed from the body surgically and examined in a lab to detect any cancer. 

However, in most cases, SFTs are often discovered incidentally by other means, such as a regular health checkup.⁵ This is often because of the lack of symptoms which accompany the tumour alongside its rarity. The official diagnosis of an SFT is made following its resection. It is important to know the size presented on an X-ray scan has the potential to differ largely on surgical review. As several cases have the potential to drastically evolve and become cancerous, it is important to identify the tumours at risk of this so that the best clinical management approach can be prepared. 

Identifying the SFTs

Furthermore, distinguishing SFTs from other forms, such as benign mesenchymal tumours, isn't easily distinguishable. Therefore, it is often done via a method of immunohistochemistry staining, whereby elevated levels of CD34 and CD99 are often found.9 Identification of the fusion genes between NAB2 and STAT6 is also an indicative marker of the distinction of SFTs by the process of a form of Immunohistochemistry staining. This is specifically for STAT6, which helps distinguish it from other subtypes of soft tissue tumours and makes it much more accessible.⁹ 

Causes and risk factors 

The main cause that leads to the progression of a solitary fibrous tumour remains undiscovered. However, new research has associated the development of SFTs with the fusion of the genes NAB2 and STAT6.8. It is seen as a distinct marker for the diagnosis of SFTs. This is a relatively new discovery following medical technology advancements in molecular diagnostic tools .⁴ 

A research study identified important risk factors for malignant SFT. This included being above the age of fifty-five, having a tumour size greater than fifteen centimetres with an increased cell division rate and tumour necrosis. However, there is a range of factors which can contribute to the development of SFTs indirectly or directly under the following groups:

• Genetics - gene fusion
• Age - age above 55 years
• Sex - more prevalent in people assigned male at birth 
• Cancer treatment - previous chemotherapy⁸

Research has not classified environmental influences as a risk factor for the development of SFTs. For example, radiation and smoking have not been associated with the development of solitary pleural fibrous tumours found in the lung lining.⁷ 

Treatment options 

SFTs are often treated according to an individual patient's case instead of a regimented approach. The following types of treatment approaches can be carried out:

• Surgery: via resection is often an option which is dependent on tumour location and location and is beneficial when the tumour is benign
• Chemotherapy: often used to destroy cancer cells, so it beneficial when the tumour is malignant. It is often administered in cases when the tumour regrows or is found in numerous cases.
• Radiotherapy: when a tumour cannot fully be resected, the application of radiation can help target specific regions to shrink the tumour
• Arterial embolus: interruption of blood flow to the tumour via initiating a blockage in the artery

Surgery provides a good outlook for regional SFTs, but in some cases, up to 15% of the recurrence of the tumour is possible in the following years. However, this is often found in incidences of resection surgery rather than complete removal of the tumour, which has a better success rate. The introduction of a risk model / highlights the drastic improvement in identifying the risk of recurrence and metastasis of an SFT.¹ 

Furthermore, patients with SFTs are treated by a multidisciplinary team, including surgical, radiation and medical oncologists, to determine the most effective strategy.

SFTs treatment with immunotherapy

Research has shown that specific gene mutations such as TP53 are associated with cancerous SFT, which is characteristically aggressive. In instances where the tumour has progressively worsened, chemotherapy outcomes remain ineffective. Therefore, the administration of Immunotherapy is an opening avenue for the treatment of SFTs. This is because, often, the surrounding regions display markers of immunosuppressed cells at the site of tumours. 

The use of immunotherapy would help to stimulate immune cells such as CD8+ and CD4+, which have a central role in the immune response and in destroying infected cells.¹ This action of immunotherapy is based on the microenvironment surrounding the tumour. Therefore, where tumours present with immune cells pose a great target for immunotherapy treatment. 

Summary 

Although the incidence of having solitary fibrous tumours remains extremely low, they most commonly present as benign mesenchymal soft tissue growths. Found in all regions of the body, they are most often found in the thoracic region, known as solitary fibrous tumours of the pleura (SFTP). In most cases these tumours remain non-cancerous, however, they can develop to be malignant. Physiological traits of SFTs include its slow growth rate alongside the lack of symptoms which can lead to accidental identification. The first line of treatment includes resection by surgery, followed by routine monitoring to assess for potential regrowth in the following years. Therefore, being aware of associated traits of SFTs and routine checkups is important for the identification and treatment of SFTs.