Recurrent Solitary Fibrous Tumor in Intradural Extramedullary Space: Case Report and Review of the Literature

Solitary fibrous tumor/hemangiopericytoma (SFT/HPC) is a rare neoplasm arising from spindle cells and most commonly arising from pleura. Spinal SFT/HPC is a rare entity; hence, it is not on the top of the differential diagnosis list when a clinician faces a spinal lesion. In the review of the literature, there exist less than 50 case reports of intradural extramedullary SFT/HPC. Here, we present a 54-year-old female patient who underwent subtotal surgical excision of an intradural extramedullary spinal mass pathologically reported to be SFT/HPC and had symptomatic recurrence in the 3rd year of follow-up. Surgical intervention was unachievable and the patient was given 45 Gy to the surgical cavity followed by a 5.4 Gy boost to visible tumor with external radiotherapy. Patient reported significant relief of her symptoms. We aim to contribute to the formation of a treatment algorithm for this rare entity.


Introduction
Solitary fibrous tumors (SFT) (formerly known as hemangiopericytoma (HPC)) are rare mesenchymal neoplasms originating from spindle cells [1]. Although the majority arise from pleura, 60% of SFTs originate outside the pleura [2], and SFTs are deemed to arise anywhere in the body. CD34 is a sensitive marker though it is not specific, and lately specific markers are identified [3]. Although surgery with negative margins is the mainstay treatment, the role of chemotherapy and radiotherapy is under debate [4]. This report presents a case of intradural extramedullary (IDEM) SFT in the thoracic spine.

Case Report
Our patient is a 54-year-old female with a past medical history including asthma and peptic ulcer disease. Her family history is free of malignant diseases. In 2016, patient's chest X-ray examination found a suspicious lesion. Magnetic resonance imaging (MRI) was ordered for further investigation in November 2017 since the patient was complaining of back pain. MRI revealed a spinal lesion 12 × 10 mm in diameter at the level of second thoracic vertebra (T2) (Figures 1(a) and 1(b)). The patient was referred to surgery and one month later underwent total laminectomy at the level of second thoracic vertebra (T2) and subtotal excision of intradural extramedullary lesion with neuromonitoring. The lesion involved nerve roots; complete resection could not be achieved due to perioperative neuromonitor signals. Postoperative neurological examination showed motor strengths as 3/5 for the left thigh, 3/5 for the left knee, 0/5 for the left foot, and 4/5 for the left hand. Patient was started on corticosteroid. Left lower extremity strength improved to 4/5, and the patient was discharged on postoperative day 4. A follow-up physical examination and MRI examination were planned for the patient 45 days after the surgery. Pathology report confirmed a WHO 2016 Grade 1 solitary fibrous tumor positive for CD34 and negative for S100, epithelial membrane antigen (EMA), and p53 ( Figure 2). Thereafter, the patient underwent routine follow-up. MRI was performed 45 days after the surgery, and it was consistent with a remnant paracentral mass (Figures 1(c) and 1(d)). Control MRI a year after showed regression of the lesion (Figures 1(e) and 1(f)). In June 2020, no intraspinal mass lesion was revealed on MRI. In January 2021, the patient came for routine follow-up with a complaint of bilateral pain in her upper extremities as well as back pain. MRI examination of the 2 Case Reports in Oncological Medicine patient was consistent with an intradural extramedullary lesion 10 × 5 mm in diameter which was compressing the spinal canal in the left paramedian area at the level of T2. The patient was presented at the multidisciplinary tumor board for treatment options. Given the critical anatomic location and close involvement with nerve roots, the patient was found unsuitable for surgery. Eventually, the board decided on the referral to the radiation oncology clinic. A repeat contrasted thin-slice MRI of cervical and thoracic spine was performed (Figures 1(g) and 1(h)). The report confirmed the recent findings. Patient underwent 1 mm thin slice treatment-planning computer tomography (CT) with intravenous contrast infusion. CT-MRI image fusion was acquired with the preoperative and present MRI for better discrimination of the resection cavity and the boundaries of the present lesion to achieve improved target delineation for treatment planning [5]. Gross tumor volume (GTV) was defined as the postoperative surgical cavity and clinical target volume (CTV) was extended 2 cm above and below the GTV. Planned RT dose was 45 Gray (Gy) in 25 fractions (1.8 Gy/fraction) and a boost of 5.4 Gy in 3 fractions (1.8 Gy/fraction). Organs at risk (OAR) were determined as the spinal cord, lungs, and esophagus in accordance with the atlas of dose constraints in thoracic radiotherapy published by Kong et al. [6]. Maximum point dose constraint to the spinal cord was determined to be 45 Gray (Gy) in accordance with the Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) model to avoid any radiationrelated side effect, namely, myelopathy [7]. Treatment planning was done with conformal three-dimensional (3D), intensity-modified radiation therapy (IMRT), and volumetric modulated arc therapy (VMAT, hereinafter referred to as ARC) approach, respectively ( Figure 3). QUANTEC normal tissue tolerances were taken into consideration and the calculations were made accordingly ( Table 1) [8]. ARC was the chosen modality with optimal spinal cord protection compared to 3D and tolerable treatment duration for patient comfort and cooperation to stand immobile during the treatment compared to IMRT.

Discussion
Primary spinal cord tumors are rare neoplasms that constitute 4.5% of central nervous system (CNS) tumors in adults [9]. Their primary treatment is surgical resection if possible [10], and radiotherapy can ensue if total resection is not achieved [11]. They are classified according to their location as extradural, intradural extramedullary and intramedullary. Extradural tumors are most common followed by intradural extramedullary (IDEM) tumors. Meningioma, schwannoma, and neurofibroma comprise the majority of intradural extramedullary tumors [12]. Solitary fibrous tumors of intradural extramedullary space is a rare entity. They correspond to the   Case Reports in Oncological Medicine group IIA in the classification of Liu et al. [13] ( Table 2). The majority of type IIA spinal SFTs arise at the thoracic spine [12]. Table 3 summarizes the cases present in the PUBMED database. Due to their rareness, SFTs are not on the top of the differential diagnoses list when the clinician is faced with a lesion in the spinal cord [14]. MRI is the preferred modality to delineate the tumor and evaluate for invasion, but the imaging findings are variable and nonspecific [15]. Nevertheless, the diagnosis is less of a challenge with the advances in knowledge in pathological markers. CD34, although not specific, is deemed to be the histological hallmark of SFTs and has been found to be expressed in 79% of cases [16,17]. A retrospective study with 16 SFTs located in the spinal cord confirmed this finding with 100% SFTs being positive for CD34 and negative for EMA, GFAP, and MBP staining. S-100 positivity varied with 5 of the patients staining positive [18]. Lately, STAT6 and GRIA2 are proposed to be distinguishing markers for SFTs [19,20]. STAT6 expression is driven by NAB2-STAT6 gene fusion, and STAT6 has lately been sought to be a specific marker for SFT [19,21,22]. STAT6 positivity in SFTs and HPCs is considered as finding that supports the abolishment of the thin boundary between these 2 entities in late 1990s [3,23,24].
Achieving complete surgical resection is the main goal in extrathoracic SFTs, and it is associated with improved local control and survival. Routine long-term follow-ups must be ensured for the early detection of recurrence [25]. Although there are no standardized follow-up routines for these patients. The anatomical location of the tumor may not allow the total resection in some cases [4]. The role of radiotherapy is under debate in SFTs.
There are studies advocating that RT is not strictly indicated after complete resection due to close follow-up and low recurrence rate [4] and that adjuvant radiotherapy can be considered in the case of subtotal resection [26][27][28] or if we are facing a high grade SFT [29]. There are studies advocating adjuvant radiotherapy be standard of treatment [18,30]. Complete resection, low-grade tumor and young age were reported to be factors that led the clinicians to omit radiotherapy [31]. Krengli et al. revised 151 extrathoracic SFT patients while Wang et al. reviewed 16 spinal SFT patients in terms of the effect of GTR vs. GTR+RT on local recurrence (LC), disease-free survival (DFS), and overall survival (OS), and they both came up with the same conclusion: addition of adjuvant RT improved LS and DFS while GTR was the main predictor of OS [18,32]. While a retrospective review reveals that the median radiation dose for extracranial SFTs is 60 Gy [32], this dose seems to be unachievable in the spinal SFTs due to dose constraints of the spinal cord, which is 45-50 Gy. Retrospective single center study of Wang et al. reported a median dose of 40 Gy in patients with spinal SFT [18]. The treatment plan must be tailored according to location and extent of the tumor. Radiation doses may vary in between patients if the plan cannot limit spinal cord doses in the desired limits.
Herein, we present a case of IDEM SFT who had STR 27 months prior to presentation to our clinic with bilateral pain over the arms and legs and newly developed radiological evidence of recurrence. Radiotherapy was the treatment of choice for this patient due to the location of the tumor. To our knowledge, we present the 5 th case of a recurrent SFT treated with radiotherapy [18,[33][34][35].

Conclusion
Spinal type IIA SFT is a rare entity with less than 50 case reports in the literature. GTR is the required treatment option while the role of adjuvant radiotherapy and its indications is yet to be discussed. We contribute to the literature by presenting a rare case in which close follow-up ensued STR, and radiotherapy was performed when the lesion recurred. We believe that the increase in the number of cases in the literature will help and contribute to the embodiment of the therapeutic algorithm of the disease in question.

Conflicts of Interest
The authors declare that they have no conflicts of interest.