Introduction
Solitary fibrous tumor (SFT) is a mesenchymal tumor of interstitial dendritic cells (1). Although it was initially considered to be a mesothelioma originating from the pleura, currently, it is reported in many locations due to the widespread presence of dendritic cells outside the thorax. SFTs originating from the prostate have been reported highly infrequently, and this is the 44th case to date.
SFTs are generally benign, although some may show malignant behavior (2). All reported cases have been surgically treated, but there is no standard treatment approach for these very rare tumors, particularly with regard of the benefit of radical surgery. Almost half of the reported cases were treated with palliative surgery, whereas others received radical surgery. Also, surveillance was attempted in one case (3). This report aims to present a rare case and the results of 36 months of surveillance firstly. Secondly, we reviewed all prostatic SFTs in the literature regarding histopathological features, treatment modality, and reported recurrence and progression data to identify the optimal treatment based on the available information.
Case Presentation
Case A 44-year-old man presented with a two-year history of pressure in the lower abdomen. He had no lower urinary tract symptoms (LUTS), or hematuria, and no constipation. He had no history of major medical illness. A rectal exam revealed a huge, firm prostate without nodule or induration. His renal function, prostate-specific antigen level, and urine analyses were also within normal limits. On dynamic, gadolinium-enhanced magnetic resonance imaging (MRI) of the abdomen, a 48×66-mm well-circumscribed mass with homogeneous enhancement was observed at the right anterolateral aspect of the prostate (Figure 1a, b). Twelve-core prostatic needle biopsy was performed.
On histological examination of biopsy specimens, the tumor was detected in all six transrectal prostate biopsies of the right prostate lobe and two biopsy specimens from the left lobe. The tumor had hypocellular and hypercellular areas, composed of bland spindle cells arranged in a haphazard pattern. The stroma consisted of a variable number of dense wire-like hyaline collagen deposits, with tumor cells arranged either singly or in small clusters next to the dense collagen (Figure 2a). The spindle-shaped cells had ill-defined borders and scanty eosinophilic cytoplasm. The nuclei were ovoid or elongated, with blunt or tapered ends and contained finely dispersed chromatin or had inconspicuous nucleoli (Figure 2b). Mitotic figures were infrequent. One or two mitotic figures were observed per 10 high-power fields (HPF). No atypical mitotic figure are encountered. These cells did not show prominent atypia or pleomorphism. No lymphovascular invasion or tumor necrosis was observed. Residual prostate parenchyma adjacent to the tumor was noted in some biopsy specimens.
Immunohistochemistry (IHC) analysis showed a strong expression of CD34, STAT-6, and vimentin by tumor cells (Figure 2c, d). Tumor cells were also immunoreactive for CD99, bcl-2, and progesterone (PR) (Figure 3a-c). No staining was observed for CD56, SMA, desmin, pancytokeratin, synaptophysin, CD31, S100, CD117, or DOG1. The proliferation rate, measured by Ki-67 nuclear staining, was evaluated as 5% in hot spots (Figure 3d).
These findings identified an SFT of the prostate. Radical surgery was discussed with the patient. However, he was hesitant about the possible side effects of the surgery, especially urinary incontinence and erectile dysfunction.
The findings were further evaluated according to the malignancy criteria proposed by England et al. (2) and the risk stratification model of Demicco et al. (4) and Pasquali et al. (5). This case had no malignancy criteria as suggested by England et al. (2). Furthermore, the tumor was classified as a low-risk and very low-risk using the models of Demicco et al. (4) and Pasquali et al. (5), respectively. As the biopsy revealed no malignancy and the patient was reluctant to surgery, conservative management was adopted. The tumor was stable on consecutive computed tomography (CT) and MRI scans, three and seven months after diagnosis. However, two years later, CT scan showed that the size of the mass had increased and at that time measured as 77x62x60 mm3, without any emerging symptoms. Approximately 1 cm additional growth was observed in the tumor (77x82 mm) in the 36th month CT images (Figure 1c, d). Three years after the initial biopsy, confirmatory prostate needle biopsy was performed. According to the histopathological evaluation of the follow-up biopsy, the tumor had the same features as when it was first diagnosed. Histologically, the spindle-shaped tumor cells were dispersed singly or in small groups within the collagenized fibrous stroma. There were no cytological atypia and pleomorphism in follow-up biopsies. The mitotic activity was the same as in the first biopsies. No atypical mitotic figures, tumor necrosis, or lymphovascular invasion were found. These findings indicated that tumor histopathology remained stable during the follow-up (Figure 2e-h).
Literature Review and Discussion
A literature search was conducted in MEDLINE using the following search parameters “(((solitary) AND (fibrous)) AND (tumor)) AND (prostate).” Forty-three cases were identified in 26 reports (3,6-31). Patient age, presenting symptoms, treatment modality, and microscopic findings in terms of malignancy criteria, follow-up time, recurrence, and metastasis information were noted. The cases were evaluated according to the malignancy criteria proposed by England et al. (2) (size >10 cm, mitotic activity >4/10 HPFs, nuclear pleomorphism, infiltrative boundaries, and the presence of necrosis) and divided into two groups (based on the presence of any criterion or none). Demographic characteristics and pathological results of these cases are summarized in Table 1. Most patients were older and suffered from LUTS, with an average tumor size of 8 cm.
Preference of Surgical Procedure
Recurrence and metastasis information was not reported for 15 patients. Local recurrence without metastasis was reported in five (11.6%) patients and metastasis in two (4.65%) patients. Twenty-two (51.2%) patients underwent radical surgery with a negative margin. None of these had local recurrence; metastasis was reported in only one case. Palliative surgery (enucleation or transurethral resection) was reported in 17 (39.5%) patients and radical surgery with a positive margin in one case (6). All the five local recurrences were reported in these cases. However, six of them had no local recurrence or metastasis during the reported follow-up.
Histopathology
Local tumor relapse was reported in five (11.6%) cases without distant metastasis (6,8,15,23,25). Three of them had at least one malignant criterion (mitosis in 7/10 HPFs in one case, nuclear atypia and necrosis in two cases) (6,8,15). The malignant criteria were not clearly specified in the other two cases (23,25). Tumor size was <10 cm in all.
Distant metastasis was reported in two cases (3,6). Both of them had at least one malignant criterion (mitosis in 13/10 HPFs in one case, mitosis present in more than 10/10 HPFs in the other; tumor necrosis in both; hypercellularity in one). Tumor sizes were 6 cm and 9 cm, respectively. Radical surgery with a negative margin was reported in one of them and unspecified in the other.
In one of four patients reported by Bakhshwin et al. (6) high-risk SFT by two prognostic systems Salas et al. (32) and Pasquali et al. (5). The patient underwent radical prostatectomy following transurethral resection of the tumor with pT0 disease. However, the patient had a recurrence at the bladder neck and subsequent biopsy-proven metastatic disease to the right obturator lymph node (6). Additionally, Tanaka et al. (3) reported distant metastasis in their case, although the initial prostatic needle biopsy reported stromal spindle cells with no mitosis. On follow-up MRI approximately ten months later, the mass had increased in size, another prostatic needle biopsy was performed and now showed that tumor cells with round and short spindle-shaped nuclei with some mitoses were present. A total resection of the mass was performed. In the permanent pathological examination, the tumor was found in the muscularis of the prostatic urethra or the bladder. The tumor consisted of spindle cells with fascicular and storiform patterns of growth, and mucinous degeneration and some necrosis were observed in the background. The tumor was hypercellular, and a significant number of mitoses (more than 10/10 HPFs) were present.
In contrast, six cases did not have any tumor recurrence; hence, radical surgery and negative margin were not performed (10,12,19,21,25,30). Median follow-up was 48 (12-168) months in these cases. Three of them had no malignant criterion (10,12,19). However, the others also had at least one malignant criterion (21,25,30). Nair et al. (21) reported a 10 cm tumor that was enucleated with an abdominoperineal approach. They had no evidence of loco-regional recurrence at follow-up after two years. They reported that there was a non-encapsulated tumor on microscopic examination with extended margins containing hyper- and hypo-cellular areas, spindle-shaped with bland nuclei having dispersed chromatin and inconspicuous nucleoli. The mitotic rate was 1/50 HPFs. Pins et al. (25) reported one of two cases, who was treated suprapubic prostatectomy. He had no recurrence after 21 months, though hypercellularity, nuclear atypia, and mitosis 20/50 HPFs were detected in his pathological examination. Xu et al. (30) reported three malignant prostatic SFTs in their study comparing mesenchymal tumors of the prostate. The tumor sizes were 7.6, 19 and 18 cm in largest diameter, respectively. The first was treated with radical prostatectomy with negative margin and excisional biopsies were performed for the others. The last had no information about the follow-up, but the first two cases were followed for six and 84 months without recurrence and metastasis. All tumors had necrosis, and the average mitosis was 5/10 HPFs.
Paraneoplastic Syndromes
On rare occasions SFT can present with paraneoplastic syndromes, the most commonly described being non-islet cell hypoglycemia (33). However, none of the authors reported hypoglycemia in cases of prostatic SFT.
Conclusion
Although there is little data, we suggest that probably the optimal treatment for prostatic SFT is radical surgery with a negative surgical margin. Surgeons should avoid partial resection of the tumor due to the risk of recurrence and metastasis. The malignancy criteria reported by England et al. (2) are a generally useful tool for predicting the prognosis of the disease. However, we did not observe that the tumor diameter affected the results in the literature. Contrarily, an excessive number of mitosis per HPF seems to be a poor prognostic factor. Surveillance should be performed in patients without malignancy criteria, particularly in cases with very low mitosis rates. Here we report the longest and un-complicated surveillance in the literature. However, one should be careful that insufficient sampling of the tumor with needle biopsies may not show where mitosis is high and nuclear atypia, hypercellularity, or necrosis is present. Close follow-up with repeated biopsy and imaging may be a treatment option in patients younger age and those without malignancy criteria.
Peer-review: Externally peer-reviewed.
Authorship Contributions
Surgical and Medical Practices: H.Y., A.T.E., Concept: H.Y., A.T.E., Design: H.Y., İ.E.A., Data Collection or Processing: H.Y., İ.E.A., E.Ö., Analysis or Interpretation: H.Y., İ.E.A., Literature Search: H.Y., C.Ö., A.T.E., Writing: H.Y., İ.E.A., C.Ö., E.Ö., A.T.E.
Conflict of Interest: No conflict of interest was declared by the authors.
Financial Disclosure: The authors declared that this study received no financial support.
