Rare case of a synchronous pleural solitary fibrous tumour and a thymic cyst

Abstract

We present a unique case of a 41-year-old man with an anterior mediastinal mass and a pulmonary nodule, found incidentally on a thoracic CT. Further evaluation with an MRI revealed a thymic cyst which was multiloculated with several septations. Biopsy of the pulmonary nodule was performed and histology was consistent with a solitary fibrous tumour (SFT), on the benign spectrum. Both lesions were successfully resected simultaneously via a median sternotomy approach. Final full histological diagnoses confirmed a multiloculated thymic cyst and a completely excised SFT.

Background

Solitary fibrous tumours and multiloculated thymic cysts are two rare conditions with no known relation to each other. Pleural solitary fibrous tumours (SFT) are estimated to have a prevalence of 0.0028%¹ and thymic cysts are responsible for only 1% of all mediastinal cysts.² This report is the first described case of a patient who was diagnosed with an SFT, found incidentally and synchronously with a multiloculated thymic cyst.

Case presentation

A 41-year-old healthy man was referred to the respiratory team following a CT thorax in view of an incidental finding of a subpleural soft tissue density in the left anterior costophrenic angle (figure 1). The scan was carried out following a routine chest radiograph done preoperatively. In retrospect, the mass was also present on scans dating back to 2015, with no interval change in size. Moreover, the CT thorax also showed increased enhancement of the thymus (figure 2).

Figure 1.

Incidental finding of a left lung base round opacity on chest X-ray. Patient orientation vectors—R, right; L, left; H, head; F, foot.

Figure 2.

(A) Axial and (B) coronal lung window CT images demonstrating an oval-shaped homogenous well-defined solid nodule, contiguous with the oblique fissure (red arrows), in the left lower lobe. (C) Sagittal CT images of the chest in soft tissue window demonstrating a well-defined soft tissue density round lesion in the left costophrenic angle together with a multiloculated well-defined cystic lesion in the anterior mediastinum (red asterisk).

Clinically the patient was asymptomatic with no respiratory symptoms. He was an ex smoker and his only previous medical/surgical history was a laparoscopic appendicectomy for uncomplicated appendicitis. He did not demonstrate any clinical features suggestive of myasthenia gravis, any underlying autoimmune pathology or inflammatory processes. His performance status on the Eastern Cooperative Oncology Group scale was graded as 0.³

The imaging findings were discussed during a multidisciplinary team (MDT) meeting and the decision was to proceed with a CT-guided biopsy of the soft tissue subpleural mass. The MDT also recommended further imaging assessment of the anterior mediastinum by means of an MRI thorax as this is the imaging modality of choice for the anterior mediastinum.

Histology from the subpleural mass biopsy showed an SFT, with the overall features present on biopsy classifying the tumour on the low risk spectrum.

The MRI thorax confirmed the presence of a multiloculated well-defined cystic lesion in the anterior mediastinum measuring 2.5×2.3×3.2 cm (figure 3). The lesion showed several enhancing septations but no frankly solid enhancing portions. At that point the differential diagnosis of the mass based on imaging included a multiloculated thymic cyst, cystic thymoma and mature cystic teratoma. A biopsy was deemed to be futile following MDT discussion in view of the predominant cystic component of the mass and referral for complete resection was recommended.

Figure 3.

A multiplanar MR thorax with T1-weighted and T2-weighted sequences. (A) Liver acquisition with volume acceleration flex and T1-weighted MRI sequence showing a low signal intensity well-defined cystic lesion in the anterior mediastinum with several internal enhancing septations. (B) A T2-weighted MRI sequence showing high signal intensity within the multiloculated well-defined cystic lesion in the anterior mediastinum. Patient orientation vectors—A, anterior; P, posterior; R, right; L, left; H, head; F, foot.

The patient was admitted under the care of the cardiothoracic surgeons and he underwent a thymectomy via a median sternotomy approach and a left lower lobe wedge resection.

The final histological examination of the pleural lesion confirmed the presence of an SFT with hamartomatous features and no evidence of any malignant features (figure 4). The tumour measured 32×26×20 mm and was completely excised. Microscopically the spindle cells were highlighted by signal transducer and activator of transcription 6 (STAT6). This has been validated as the most specific marker for SFTs.⁴ Spindle cells were also highlighted by cluster of differentiation-34 (CD34) but were negative for anticytokeratin antibody (MNF116). The epithelial lining, on the other hand, was highlighted on a cytokeratin stain (MNF116) (figure 5). This immunoprofile had already been demonstrated in the biopsy specimen. There was no necrosis and no atypical mitosis. The Ki-67 proliferation index was <2%. A multicentre study by Diebold et al showed that a Ki-67 proliferation index of >10% was associated with a worse outcome.⁵

Figure 4.

Low power and high power views of the solitary fibrous tumour showing a biphasic neoplasm, comprised of slit-like spaces lined by a bland columnar-to-low cuboidal epithelium, set in a dense, hyalinised and fibrotic stroma.

Figure 5.

Immunoprofile stains of the solitary fibrous tumour showing the stromal cells highlighted on CD34 and STAT6 and the epithelial lining highlighted on a cytokeratin stain (MNF116).

The thymectomy showed the presence of a multiloculated cyst, correlating with the findings on MRI exam. There was no evidence of atypia or malignancy and the background thymic tissue showed reactive changes (figure 6).

Figure 6.

Low power and medium power views showing cysts within the thymic parenchyma, lacking an epithelial lining and containing acellular, eosinophilic debris with numerous cholesterol clefts. The background thymus shows reactive follicular hyperplasia.

Outcome and follow-up

The patient has remained well postoperatively with a good and uncomplicated recovery. He will be followed up on a regular basis by means of imaging for both the thymic cyst and the solitary fibrous tumour, as both these can recur.

Benign SFTs have a low rate of recurrence (<2% if pedunculated and <8% if sessile). Yearly radiological follow-up is sufficient for benign pedunculated tumours. However, if sessile, follow-up should be every 6 months for the first 2 years and yearly after that.⁶

With regards to multiloculated thymic cysts, the overall prognosis is a good one with very low overall risk of recurrence after surgical excision,⁷ however, follow-up on a regular basis is also necessary in view of possible asymptomatic recurrence.

Discussion

Although solitary fibrous tumours are typically described as tumours originating from the thoracic cavity, in actual fact, 50%–70% of them arise outside of the thorax and can occur at any site of the body, including the meninges⁸ and peritoneum.⁹ Overall they are rare tumours accounting for less than 4% of all soft tissue sarcomas and mesenchymal tumours.

Patients may present with symptoms of cough, shortness of breath and chest pain while in other cases, as in our case report, it is found as an incidental mass on imaging of the thorax carried out for another reason.⁹

While a core biopsy will lead to a diagnosis by means of supportive immunohistochemical markers highlighted above, it is usually not enough to fully establish the aggressive nature of the tumour and, therefore, the gold standard management is complete resection, which will provide a full histological examination.⁶ Features that suggest a more benign nature to the tumour include a hypocellular and collagen rich sample, low mitotic rate and no evidence of atypia. Although, this classification will help guide further management regarding need for adjuvant radiotherapy or chemotherapy as well as length of follow-up, literature shows that sometimes there is poor correlation between histological features of aggressiveness and risk of recurrence or relapse.⁸ There is no set guidance regarding length of follow-up but the majority of cases are followed up radiologically for up to 5 years.⁶ However, there have been cases of late recurrence even beyond a 10- year period. In one case series of 14 patients diagnosed with a solitary fibrous tumour, 9 patients were found to have had a recurrence beyond a 10 year follow-up period from the time of first treatment.⁸

Multilocular thymic cysts are a rare cause of a mediastinal mass accounting for less than 5% of all cases.^{10 11} They can be congenital or acquired, with the acquired form more likely to be multilocular as opposed to congenital which tends to be unilocular. Generally, multiloculated thymic cysts are thought to arise secondary to an inflammatory process and in fact may be found in relation to autoimmune conditions such as Sjogren’s or systemic lupus erythematosus. It has also been reported in patients with HIV.¹² In a number of cases, including our patient in this case report, no underlying predisposing condition is found and are, therefore, termed idiopathic.

Like solitary fibrous tumours they are often asymptomatic and diagnosed incidentally on imaging. Depending on their size, they may occasionally result in chest pain or dyspnoea.^{10 11} As evidenced by our case report, MR thorax is the ideal radiological investigation to assess mediastinal masses, however, this is not done routinely when investigating patients with thoracic pathology. In our case, this was done purely following the incidental mediastinal mass on CT as advised by the MDT. In most cases the MRI gives a wide range of differentials but cannot exclude the possibility of a malignant process. The presence of cysts identified on MRI as well as its position in the mediastinum could at least narrow down the preoperative differential diagnosis to a few possible pathologies. The main causes of anterior mediastinal cysts include mature cystic teratomas, cystic lymphangiomas, cystic thymomas and thymic cysts. The main limitation is that the cystic component of the mass will manifest the same characteristics on MRI, with the fluid component having low intensity on T1-weighted images (figure 3A) and high signal intensity on T2-weighted images (figure 3B). Mature cystic teratomas, however, typically appear as a heterogenous mass containing fat, fluid, soft tissue and calcification. Fat on MRI generally has high signal intensity on T1-weighted images and variable intensity on T2-weighted images. A limitation of this distinction is that some haemorrhagic cysts demonstrate a similar pattern.¹³ Therefore, such cases are best managed by complete surgical excision.¹¹ Once resected these cases should also be followed up radiologically as they can recur¹⁰ and because they may be associated with thymic neoplasms including thymic carcinoma. Histological specimens should, as a result, be thoroughly inspected for foci of coexisting neoplasms.¹¹

Based on our literature review, we have not found any known association between the two conditions and no other case of synchronous occurrence of the two conditions has been reported to date. Both solitary fibrous tumours and multiloculated thymic cysts are rare and both being diagnosed in the same patient makes our case a unique one.

Learning points.

• First described case of a pleural solitary fibrous tumour (SFT) with a synchronous multiloculated thymic cyst

• Discussion of such cases at multidisciplinary team offers the patient the best options with respect to investigations, diagnosis and management since both pathologies are rare. Most evidence is based on case reports or case series and no detailed guidelines are available

• Different imaging modalities are available and should be made use of in the assessment of an anterior mediastinal mass in view of the broad differential diagnosis not only limited to the thymus itself

• If an SFT is suspected complete surgical resection should be considered as the gold standard so as to properly classify the aggressiveness of the tumour

Ethics statements

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