Solitary Fibrous Tumor of the Pleura: Surgical Treatment and Recurrence

Abstract

Background Solitary fibrous tumors of the pleura (SFTPs) are primary pleural tumors originating from the mesenchymal tissue. Surgical treatment was the first choice for management of SFTPs. There were no defined guidelines for the follow-up of these tumors and the postoperative therapy due to the rarity of these tumors.

Methods We conducted a retrospective, multicenter study from two high-volume centers in Italy. Data of patients diagnosed with pleural solitary fibrous tumors between January 2003 and October 2022 were prospectively recorded and retrospectively analyzed. The aim of this study was to identify predictive prognostic factors and the correlation between tumor characteristics and recurrence.

Results In all, 107 patients undergoing R0 surgical resection of pleural solitary fibrous tumor were included in the study. Patients were divided in two groups: benign and malignant. All the patients were treated with surgery with the aim to obtain R0 resection. Lung resection was necessary when the tumor adhered strongly to the lung parenchyma or infiltrated it. Twenty of the 107 patients had tumor recurrence. At a multivariate analysis, histological characteristics (high mitotic index) and maximum standardized uptake values (maxSUV) were related to recurrence. The mean disease-free survival (DFS) was 143.3 ± 6.1 months.

Conclusion In our experience, histological features of malignancy and maxSUV are significantly related to recurrence, which can occur even years after the first diagnosis. Surgical excision with negative surgical margins results in good long-term outcomes. After surgery, a long-term and strict follow-up should be done, in order to detect recurrence early. R0 of the recurrence is associated with long-term survival.

Keywords

thoracoscopy/VATS - surgery - complications - pleural disease (including drainage) - chest

Introduction

Solitary fibrous tumors of the pleura (SFTPs) are primary pleural tumors originating from the mesenchymal tissue. They were firstly described by Wagner et al as primary neoplasms of the pleura in 1870[1]; then, in 1931 Klemperer and Coleman distinguished localized from diffuse mesothelioma.[2] According to Stout and Murray in 1943, localized mesothelioma had a mesothelial origin.[3] Later, the development of immunohistochemistry analysis showed that SFTPs arise from the mesenchymal cell layer and were different from pleural mesothelioma.[4]

Surgical treatment was the first choice for management of SFTPs. However, some tumors recurred after several years from an R0 resection, but there were no defined guidelines for the follow-up of these tumors and the postoperative therapy due to the rarity of these tumors.

In this study, we aimed to analyze the behavior of SFTPs and the predictive prognostic factors in order to define the follow-up and potential postoperative treatments.

Materials and Methods

This was a retrospective, multicenter study from two high-volume centers in Italy. Data of patients diagnosed with pleural solitary fibrous tumors (SFTs) between January 2003 and October 2022 were prospectively recorded and retrospectively analyzed. All patients undergoing R0 surgical resection of pleural SFT were eligible. The aim of this study was to identify the predictive prognostic factors and the correlation between tumor characteristics and recurrence.

The exclusion criteria were the following: (1) no R0 resection of initial tumor; (2) metastatic disease; (3); mesothelioma, lung cancer, or other known cancers; (4) medical inoperability because of comorbidities; and (5) incomplete follow-up.

Patients were divided in two groups: benign and malignant. According to England et al's criteria,[5] they were classified as malignant if one or more of these criteria were present: >4 mitosis/10 high power fields (HPF), necrosis, pleomorphism, high cellularity, and atypical nuclei.

The difference between two study groups were assessed in order to define whether the different histology could affect outcomes. Additionally, a univariate and a multivariate analysis were performed in order to identify survival prognostic factors. The study was approved by the Local Ethics Committees of the coordinating center of the study. All the patients gave a written informed consent for the surgical treatment and for the anonymous use of their data for scientific purpose only.

Study Population

The patients' clinical records were analyzed for age, sex, tumor size, signs and symptoms, side, positron emission tomography (PET) maximum standardized uptake values (maxSUV), surgical procedures, postoperative course, pathological features, recurrence, follow-up period, and survival ([Table 1]). All the patients underwent thoracic computed tomography (CT) scan followed by flourine-18 fluorodeoxyglucose PET/CT (¹⁸F-FDG-PET/CT), and the histological diagnosis was both preoperative, with fine needle aspiration biopsy, and postoperative. SFTPs were positive for vimentin, CD34, CD99, and Bcl-2, and negative for keratin.[6]

Table 1 Study population with SFTPs

Variables	Total population (n = 107), % or IQR	Benign (n = 48)	Malignant (n = 59)	p-value
Age at diagnosis (y)	60 (36–87)	58	66	0.51
Men	58 (54%)	28	30	0.43
Women	49 (46%)	20	29	0.43
Smoker	26 (24%)	10	16	0.45
Asymptomatic	52 (48%)	22	30	0.60
Symptoms[a]	55 (52%)	26	29	0.60
Chest discomfort	34 (32%)	12	22	0.02
Cough	14 (14%)	9	5	0.30
Dyspnea	7 (6%)	5	2	0.17
Size	13 (1–32)	13	16
>10	36	20	16	0.11
≤10	71	28	43	0.11
Preoperative diagnosis
maxSUV (range)	2.9 (1.4–4.6)	1.7	3.1	0.003
FNAB	64 (60%)	25	39	0.14
Surgery
Thoracotomy	82 (77%)	39	43	0.30
VATS	25 (23%)	9	16	0.30
Tumor excision	77 (72%)	31	46	0.12
Wedge	26 (24%)	15	11	0.13
Lobectomy	4 (4%)	3	1	0.21
Pleural location
Parietal	68 (64%)	29	39	0.54
Visceral	39 (36%)	19	20	0.54
Recurrence	20 (19%)	4	16	0.013

Abbreviation: FNAB, fine-needle aspiration biopsy; IQR, interquartile range; maxSUV, maximum standardized uptake values; SFTPs, solitary fibrous tumors of the pleura; VATS, video-assisted thoracic surgery.

^a Some patients presented more than one symptom.

Survival and Recurrence

In-hospital mortality and operative mortality were defined as the number of patients who died in the first 30 days after surgery. Follow-up consisted of a total body CT scan every 6 months for the first year and every year for the following years. Overall survival (OS) was calculated from the time of the resection to the last follow-up or death. Disease-free survival (DFS) was defined as the time from first tumor excision to date of recurrence. The site, time, number, and management of recurrences were also recorded for each patient.

Statistical Analysis

Categorical variables and non-normally distributed variables are presented as number and percentage and as median and interquartile range [IQR] respectively. χ ² test and Wilcoxon or Kruskal–Wallis tests were used for statistical analysis. The Kaplan–Meier method was used to obtain the survival curve. A p-value of less than 0.05 was considered significant. MedCalc statistical software was used for analysis.

Results

There were 137 patients diagnosed with SFTPs from October 2022 to January 2003. Patients excluded from the study were those with the following: no R0 resection of initial tumor (n = 7); metastatic disease (n = 2); mesothelioma, lung cancer, or other known cancers (n = 5); medical inoperability because of comorbidities (n = 7); and incomplete follow-up (n = 9). Finally, 107 patients, comprising 58 men and 49 women, were included in the study, with a mean age of 60 years (range: 36–87 years). Only 26 of them were smokers. Fifty-two patients were asymptomatic (48.5%) and 55 had clinical manifestations: the most common was chest discomfort in 34 patients (31%), 7 patients complained dyspnea (6.5%), and 14 patients had cough (13%). We noted a correlation between size and clinical presentation, since most of the asymptomatic patients had tumor size ≤10 cm (71 cases) and most of the symptomatic ones had tumor size greater than 10 cm (36 cases). The tumor was detected with thoracic CT scan and then all patients underwent a ¹⁸F-FDG-PET/CT. They appeared as rounded well-defined masses. Tumor size was between 1 and 32 cm, in particular, 71 cases between 1 and 10 cm (67%), 26 cases between 11 and 20 cm (24%), 9 cases between 21 and 30 cm (8%), and 1 case greater than 30 cm (1%). The median size was 13 cm and there was no statistically significant difference between benign and malignant tumors in terms of size. Its localization was right hemithorax in 66 cases and left hemithorax in 41 cases. The mean maxSUV was 2.9 for the entire study population. It was 1.7 (range: 1.4–3.5) for the benign group and 3.1 (range: 2.9–4.6) for the malignant group. Regarding recurrent SFTs, the mean maxSUV was 2.4, with a range of 1.2 to 3.9. In particular, the mean maxSUV for benign recurrences was 1.5 and that for malignant recurrences was 2.8 ([Table 2]).

Table 2 Patients with recurrence

Variable	No. or mean +- SD (% or IQR)	Benign (n = 4)	Malignant (n = 16)	p-value
Patients	20	4	16	0.013
Disease-free survival (mo)	143.3 ± 6.1 (25–175)	159 ± 2.9	132 ± 8.2	0.003
maxSUV (range)	2.4 ± 1.5 (1.2–3.9)	1.5 ± 1.3	2.8 ± 1.4	<0.001
Pattern of recurrence
Local	7 (35%)	3	4	0.06
Regional	13 (65%)	1	12	0.06
Distant (extrathoracic)	0
Pleural location
Parietal	12 (60%)	2	10	0.53
Visceral	8 (40%)	2	6	0.54
Treatment of recurrence
Excision + wedge	15 (75%)	2	13	0.19
Excision + lobectomy	3 (15%)	1	2	0.53
No surgery	2 (10%)	1	1	0.26
VATS	7	2	5	0.48
Thoracotomy	13	2	11	0.48

Abbreviation: IQR, interquartile range; SD, standard deviation; VATS, video-assisted thoracic surgery.

There was a statistically significant difference between maxSUV of benign and malignant SFTPs (p = 0.003).

Surgical Findings

All the patients were treated with surgery with the aim to obtain R0 resection. We considered adequate margins of at least 2 cm. An isolated mass excision was performed in 77 cases (72%), and lung resection was performed in the remaining 30 cases as well. In 26 cases (24%), mass excision was associated with pulmonary wedge resection, while in 4 cases (5%), it was associated with pulmonary lobectomy. Lung resection was necessary when the tumor adhered strongly to the lung parenchyma or infiltrated it. The surgical approach was thoracotomy in 82 cases and video-assisted thoracic surgery (VATS) in 25 cases. VATS was preferred for small tumors (tumors excised in VATS were <10 cm) not infiltrating the surrounding structures. Chest tube was removed after 1 to 22 days (mean: 6 days).

Ten patients presented the following postoperative complications: prolonged air leak (n = 7); anemia for bleeding (n = 2) that did not require a surgical revision; and lobe atelectasis due to retention of bronchial secretions (n =1).

Pathological Findings

Patients with tumor size between 1 and 5 cm had surgical resection and the histological diagnosis was confirmed by postoperative examination. In 64 cases (size > 5 cm), the histological diagnosis was made by CT-guided biopsy. In most cases, they were characterized by a proliferation of spindle cells with connective tissue. The immunohistochemistry confirmed the positivity for vimentin, CD43, CD99, and Bcl-2. Forty-eight cases had malignant features, in particular mitosis >4/10 HPF. In 39 cases, tumors originated from the visceral pleura and in 68 cases it originated from the parietal pleura.

Recurrence

Twenty of the 107 patients had tumor recurrence. Recurrence was more common among patients with malignant tumor than among those with benign tumors. Characteristics of patients with recurrence are summarized in [Table 2]. At a multivariate analysis, histological characteristics (high mitotic index) and maxSUV were related to recurrence (p = 0.001 and 0.004). There was no statistically significative difference between tumor size and recurrence (p = 0.72; [Table 3]).

Table 3 Multivariate analysis for recurrence

Variable	HR	95% CI	p-value
Age (>70 y)	1.1	0.7–2.8	0.25
Comorbidity (yes vs. not)	0.8	0.4–1.2	0.52
>4 mitosis/10 high power fields (HPF)	5.8	2.7–4.3	0.001
maxSUV	3.7	1.8–5.8	0.004
Parietal vs. visceral pleura	0.53	0.8–2.4	0.85
Tumor size (>10 cm)	1.08	0.7–1.6	0.72

Abbreviation: CI, confidence interval; HR, hazard ratio; maxSUV, maximum standardized uptake values.

The mean DFS was 143.3 ± 6.1 months. DFS of benign and malignant tumors was 159 ± 2.9 and 132 ± 8.2 months, respectively (p = 0.003; [Fig.1]).

Fig. 1 Disease-free survival according to histological features (benign vs. malignant). Recurrence was more common among patients with malignant tumors than among those with benign tumors.

We considered the presentation of an SFT after more than 5 years from surgery as a recurrence, because in all cases the tumor presented the same histologic features of the first tumor.

In the literature, the definition of recurrence and new onset of a second primary SFT is not defined in terms of DFS. Furthermore, other authors described cases of SFTs that recurred more than 5 years after the first tumor.[7]

All recurrence had the same histologic features of the primary tumor including the benign SFTs, which maintained a mitotic rate inferior to 4 mitosis/10 HPF; 7 were local and 13 were regional. Two of the patients who presented recurrence did not undergo surgery because of their poor preoperative status, while 18 of them underwent a second surgery to excise the tumor. In all cases, tumor excision was associated with pulmonary resection (15 wedges and 3 lobectomies; [Table 2]). A parenchymal resection was needed because of the tenacious adhesions of the tumor to the lung and because of the infiltration of the parenchyma, in order to obtain free margins. In all cases, we obtained clear margins. It could be possible that in some cases we had an R1 resection that was not recognized at the histological examination, explaining the recurrence of benign SFT tumors. No death or major complications occurred.

Survival

The median follow-up time was 85 months. OS was 150.9 ± 5.3 months: OS of patients with benign SFTPs was 143 ± 6.9 months and that of patients with malignant SFTPs was of 150 ± 6.5 months (p = 0.78; [Fig. 2]). The overall 5- and 10-year survival rates were 95 and 70%, respectively, with a median survival of 88 months. Only one of them with malign SFTP died of recurrence; the others died for reasons not related to the tumor.

Fig. 2 Overall survival according to histological features (benign vs. malignant). There is no significant difference in terms of overall survival.

Three patients died within the first 30 days after surgery for hemodynamic failure.

At a multivariate analysis, the presence of comorbidities (such as cardiovascular disease, respiratory disease, and other neoplastic disease) was the only significant prognostic factor (p < 0.001). According to histological features, there were no significant difference in terms of OS ([Table 4]). Patients with benign SFT can have a shorter survival than patients with malignant SFTs, because OS is related to the comorbidities (p < 0.001).

Table 4 Multivariate analysis for overall survival

Variable	HR	95% CI	p-value
Age (>70 y)	1.5	0.1–3.1	0.30
Comorbidity (yes vs. not)	3.8	2.50–3.75	<0.001
Tumor (benign vs. malignant)	0.2	0.1–2.1	0.43
maxSUV	1.1	0.3–3.4	0.71
Parietal vs. visceral pleura	0.8	0.2–2.1	0.35
Reintervention for recurrence	0.34	0.7–1.5	0.53
Recurrence	0.54	0.4–2.5	0.74

Abbreviation: CI, confidence interval; HR, hazard ratio; maxSUV, maximum standardized uptake values.

Discussion

Surgical resection is the gold standard management of SFTPs. They require a microscopically radical (R0) resection. In fact, the literature reports that OS of patients who received surgery is significantly better. In our study, all patients were treated with an R0 resection and the OS is long both in patients who presented and did not present recurrence.[8] However, a system to predict the risk of recurrence has not been assessed yet. In the literature, there are only case series or anecdotal reports describing the approaches of different centers to this neoplasm. We divided the patients into two groups, benign and malignant, in order to analyze differences in terms of prognosis. The aim of our study was to analyze the behavior of SFTPs and to predict their recurrence in the postoperative years based on radiological and histological features, in order to detect and treat recurrence early.

First, the clinical data of our cases were in line with the previous literature. The mean age was 60 years, similar to the data in the literature. Most of the patients were asymptomatic, while symptomatic patients presented nonspecific symptoms such as chest discomfort, cough, and dyspnea. Symptoms were mainly related to the dimension of the mass. Thirty-one of the symptomatic patients presented tumors with of a diameter greater than 10 cm. SFTPs are described to be related with paraneoplastic syndromes such as Doege–Potter syndrome and a nonislet cell hypoglycemia due to tumor production of insulinlike growth factor-2 (IGF-2).[9] Also, Pierre–Marie–Bamberger syndrome can be present, due to the secretion of hepatocyte growth factor (HGF) or hyaluronic acid by tumor.[10] [11] [12] None of our patients presented paraneoplastic syndromes.

All patients discovered the presence of an intrathoracic mass through radiological examination (CT scan) done for other reasons or to investigate the symptoms they complained about. On CT scan, tumors were homogeneous and defined masses adjacent to the chest wall, forming with it an obtuse angle ([Fig.3]). In four cases, the tumor was localized in the anterior mediastinum, going in differential diagnosis with the most frequent mediastinal tumors such as lymphoma or thymoma. In seven cases, the tumor was in the fissure space mimicking a pulmonary tumor. Larger masses can mimic other tumors such as sarcomas or big peripheral lung cancer. In our population, the size of the tumor was between 1 and 32 cm, and there was no relationship between dimensions and malignancy. CT scan alone is not enough for the diagnosis of SFTP. All patients who were referred to our attention underwent ¹⁸F-FDG-PET/CT. In the literature, there is no definition of standard uptake values useful to differentiate benign from malign tumors.[13] [14] In our study, malignant tumors had higher maxSUVs than benign tumors. They were between 1.4 and 4.6, and in particular maxSUVs of benign tumors were between 1.4 and 3.5 and maxSUVs of malign tumors were between 2.9 and 4.6.

Fig. 3 Radiological presentation of solitary fibrous tumors of the pleura.

Histopathology and immunohistochemistry are necessary for the diagnosis of SFTPs: they are positive for vimentin, CD34, CD99, and Bcl-2, and negative for keratin[15] and nuclear localization of STAT-6.[16] Tumors arose from the visceral pleura in 39 patients and from the parietal pleural in 68 patients.

We used the England et al criteria[5] for distinction between benign and malignant tumors and distinguished 48 malignant tumors from 59 benign tumors, in accordance with some studies in which benign tumors were more common than malignant tumors.[17] [18] [19]

The treatment of SFTPs is surgical, with the aim to obtain a complete resection of the tumor with negative oncologic margins. Surgical approach varies from case to case. When the characteristics of the tumor made it possible, we preferred a minimally invasive surgical approach. We used VATS for tumors of size less than 10 cm, not infiltrating surrounding structures. In the remaining cases, we preferred a lateral thoracotomy. In the cases in which the tumor adheres tenaciously to the lung parenchyma, a lung resection is necessary. The aim is to save as much lung as possible, so wedge resection is preferred. Anyway, if the tumor invades most of the lobe or if it is central, an anatomical lobectomy is necessary. In 30 cases, excision of tumor was performed together with a pulmonary resection, wedge in 26 cases and lobectomy in 4 cases. Surgical outcome was good since no patient died during surgery and postoperative complications presented in few cases.

Second, the behavior of SFTPs is hard to predict, since recurrence is not exclusive of malignant tumors. Different scoring systems have been proposed to predict the risk of recurrence and adverse outcome. De Perrot staging of SFTPs identifies the following[20]: (1) stage 0, pedunculated tumor without signs of malignancy; (2) stage I, sessile or inverted tumor without signs of malignancy; (3) stage II, pedunculated tumor with histologic signs on malignancy; (4) stage III, sessile or inverted tumor with histologic signs of malignancy; and (5) stage IV, multiple synchronous metastatic tumors. Tapias et al[21] proposed a scoring system based on clinical and histological features in order to predict recurrence after SFTP resection. They affirmed that the risk of recurrence is higher in lesions originating from the parietal pleura. In the meta-analysis conducted by Liu et al,[22] the recurrence rate for benign SFTP was 3% and that for malignant SFTPs was 22%, and there was no statistically significant difference between tumors originating from parietal or visceral pleura. Diebold et al[23] affirmed that a high proliferation rate by MIB-1 (>10%) is associated with adverse outcome. They also proposed a score based on mitosis, necrosis, tumor size, and MIB-1. Validation of these scores is necessary in future studies with a larger number of cases and a longer follow-up time.

An SFT is characterized by a pathognomonic gene fusion between NAB2 (NGFI-A binding protein 2) and STAT6 (signal transducer and activator of transcription 6), which is suggested to be the central molecular alteration driving tumor development and progression. The presence of fusion has been confirmed in nearly all SFTs and the introduction of STAT6 as a robust immunohistochemical marker has significantly improved diagnostics.[24] Georgiesh at al[25] aimed to investigate the clinicopathological and prognostic impact of different fusion variants, and they found a correlation between a variant of NAB2-STAT6 fusion and an increased risk of recurrence. Anyway, the prognostic value of fusion variants should be further explored, in large patient cohorts. This gene fusion has not been investigated in our patients. It would be good to investigate this gene fusion and its prognostic value in future studies.

In our study, 20 patients had recurrence, most of them after more than 5 years from surgery. None of them had metastasis. Histological features of malignancy and higher preoperative maxSUV were significantly related to recurrence.

There was no difference in recurrence rate between SFTPs originating from visceral or parietal pleura. Disease-free interval suggested that patients diagnosed and surgically treated for SFTP, in particular patients with malignant form, need a long annual follow-up period, in order to detect tumor recurrence early and to perform surgery rapidly. The treatment of recurrence was surgical in all cases except two, because of their poor general conditions. The surgical approach was mainly open through lateral thoracotomy, since they were all reintervention. The excision was associated to a lung resection, wedge in 15 cases and lobectomy in 3 cases.

Third, the prognosis for patients with SFTP is favorable with a 5- and 10-year OS rate of 95 and 70%, respectively. Recurrence is frequent for this kind of tumor, with a mean DFS of 143.3 ± 6.1 months. To date, the role for adjuvant therapy remains controversial. Chemotherapy and radiotherapy are not largely used and data available in literature are limited. Bylicki et al[26] highlighted the importance of the De Perrot classification as prognostic factor and used it as a guideline for adjuvant therapy: they gave postoperative chemotherapy based on doxorubicin to patients with stage III/IV cancer. Despite it, the majority of these patients presented tumor recurrence. In the same study, chemotherapy for recurrence had a modest response. Suter et al[27] described long-term survival using adjuvant radiotherapy in patients in whom an R0 resection was not obtained. In our experience, all recurrences were treated only with a surgical approach. Long-term survival after a second intervention for recurrence was excellent, with 95 and 70% of patients alive after 5 and 10 years, respectively. One of them died from causes related to the SFTP.

Fourth, according to our experience, the gold standard treatment of SFTPs is a radical surgical excision. The surgical approach, VATS or thoracotomy, varies from case to case on the basis of the localization of the tumor, its dimensions, and its relations with the surrounding structures. Recurrence is frequent in these tumors. It can present even more than 5 years after the surgical resection. In our experience, recurrence is related to the histological features of malignancy and to maxSUV. Even benign tumors, when presented with higher maxSUV, had a higher risk of recurrence. Since the gold standard treatment of recurrence remains surgical excision, an early detection is necessary to obtain a radical excision of the recurrence. We propose a long-term follow-up with a CT scan every 6 months for the first year and every year for the next years, with particular attention for patients with higher maxSUV at PET-CT scan and with histological features of malignancy.

Conclusion

In our experience, histological features of malignancy and maxSUV are significantly related to recurrence, which can occur even years after the first diagnosis. In patients with preoperative diagnosis of malignancy and higher maxSUV, it should be indicated to perform an enlarged resection with margins of more than 2 cm, in order to prevent recurrence. Surgical excision with negative surgical margins results in good long-term outcomes. After surgery, a long-term and strict follow-up should be done, in order to detect recurrence early. R0 resection of a recurrent tumor, when feasible, should be performed because it is associated with long-term survival.

Study Limitations

Our study presented several limitations that should be considered before drawing definitive conclusions. The retrospective nature and the limited number of patients were the main limitations. Also, we did not focus on NAB2-STAT6 fusion. In future studies, it could be interesting to also focus on the molecular implications of NAB2-STAT6 fusion to find the prognostic value of fusion variants. Another limitation is that non-R0 resected SFTs were excluded from analysis. It could also be interesting to investigate the correlation between R1–R2 resection and recurrence.

Conflict of Interest

None declared.

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Publication History

Received: 11 June 2023

Accepted: 25 October 2023

Article published online:
27 November 2023