Extrapleural Superficial Solitary Fibrous Tumor on the Posterior Shoulder: A Case Report and Review of the Literature

CASE REPORT

A 74-year-old man was evaluated by dermatology for a suspicious painless mass located over the posterior aspect of his right trapezius muscle. The mass had been present for at least 3 years but was noted to have had a rapid increase in size within the last 2 months prior to presentation. He denied any associated symptoms or recent changes in his health. Medical and surgical histories were significant for actinic keratoses, atrial fibrillation with cardiac ablation and placement of a permanent pacemaker, coronary artery disease, hypertension, hyperlipidemia, and benign prostatic hypertrophy. He denied any family history of malignancy, other than actinic keratoses. On examination, he was noted to have a palpable, nontender, mobile mass over the posterior aspect of his right upper trapezius muscle measuring approximately 3 × 3 cm. An incisional biopsy was performed by dermatology. Pathology revealed cellular spindle cell tumor without necrosis but with up to 6 mitoses/10 HPF. In addition, IHC staining was positive for CD34 and CD99. The histopathology was reviewed and the diagnosis of SFT was confirmed by the Department of Pathology by performing IHC staining for STAT6.

The patient was then seen in the surgical oncology office for further discussion and management. He was found to have a 3.5 × 1.2-cm mass with an overlying healing scar from his incisional biopsy. There was no evidence of satellitosis. Computed tomography of the chest, abdomen, and pelvis was performed to determine the extent of the tumor and revealed no evidence of metastatic disease. Prior to wide local excision, the patient was evaluated by the Multidisciplinary Cutaneous Oncology Clinic for any additional treatment recommendations. Neoadjuvant therapies were not recommended.

A full-thickness, wide local excision with 1-cm margins was performed. Additional trapezius muscle was taken for an oncological boundary of safety. All specimens were submitted to pathology. The primary resection defect measured 6.1 × 5.4 × 3.6 cm and was reconstructed with a local rotational-advancement flap.

Postoperatively, the flap reconstruction healed well without complication. Permanent pathology revealed positive deep margins with residual SFT. After extensive discussion with the patient, the decision was made to pursue adjuvant radiation therapy and forgo a secondary surgery. Radiation oncology plans for 30 treatments.

DISCUSSION

SFTs are of mesenchymal origin, most often located within the pleura. Extrapleural manifestations represent about 0.6% of all SFTs, with the most common location being the head and neck.²⁰ Previously described locations for extrapleural SFTs include the lumbar extradural space, intrameningeal space, cervical spine, deep soft tissue of the neck, orbital space, pelvic space, retroperitoneal space, vagina, thyroid gland, mammary gland, prostate, nasal mucosa, liver, renal pelvis, and the extremities.¹ Here, we present and discuss an extrapleural superficial SFT overlying the trapezius muscle.

Summarized in Table 1 is a compilation of superficial SFTs published to date. From this review, women are found to be the most common gender affected, while the most frequent anatomic location is the head and neck region. The mean age of the reported cases is 43.9 years (range, 8 months-88 years). The average size is 5.2 cm (range, 0.4-15.5 cm). After wide local excision, the majority of patients went on to be disease free. Imunohistochemically, CD34 expression was found to be more commonly reported than any other molecular marker. Underreporting of STAT6 in the literature may be secondary to more recent understanding and evidence of expression of NAB2-STAT6 translocation and gene fusion that is now utilized for confirmation of diagnosis.³⁰

Pathological diagnosis of SFTs requires recognition of histological features coupled with supportive IHC stains. Similar to their pleural counterparts, this is a spindle cell neoplasm with alternating areas of hyper- and hypocellularity and characteristic gaping and bifurcating vessels, also known as staghorn vessels (Fig 1). Hyalinization of the vessels and prominent perivascular hemangiopericytoma-like pattern of growth are subtle features to establishing the diagnosis. Cellular variants of SFTs in the past were classified as hemangiopericytoma but are now incorporated into the spectrum of SFTs. IHC staining that is CD34 positive (Fig 2) is indicative of perivascular cells, the putative cell of origin of this tumor. CD99 and bcl-2 can also show variable staining, but these stains are not lineage specific. Recently, a recurrent paracentric inversion involving chromosome 12q13 has been identified in SFTs of pleura and soft tissue that result in NAB2-STAT6 translocation and gene fusion.³⁰ This gene fusion results in oncogenic overexpression of activation factor STAT6 that drives tumor proliferation. STAT6 immunostain is reliable surrogate marker for this molecular change and was employed for confirmation of the diagnosis in this case (Fig 3). Strong criteria for malignancy have not been previously well-described in this tumor, but risk stratification models have been proposed. Concerning histological features for recurrence and metastasis include significant size greater than 5 cm, pleomorphism, atypia, high cellularity, mitotic figures more than 4/10 HPF, and tumor cell necrosis²⁸ (Fig 4). Age of patient, tumor size, and mitotic activity have shown discriminatory power in separating tumors into low, intermediate, and high risk for recurrence/metastasis in a cohort of approximately 100 patients.³¹

Figure 1. A representative routine hematoxylin-eosin section at 10× magnification showing hypo- and hypercellular areas of blood vessels and prominent perivascular growth of tumor.

Figure 2. A representative immunohistochemical stain at 10× magnification showing the positivity for CD34.

Figure 3. A representative immunohistochemical stain at 10× magnification showing the positivity for STAT6.

Figure 4. A representative routine hematoxylin-eosin section at 20× magnification showing mitosis and low-grade atypia.

Extrapleural location of SFTs does not necessarily portend a higher rate of metastasis/recurrence; however, its behavior is generally unpredictable. Reported recurrence and metastasis rates range from 10% to 37% in the literature.³² Positive margins after resection are reported to threaten a higher risk of metastasis and recurrence.^32,33 Recommended treatment is wide local excision of the tumor and its capsule. Failure to fully remove the capsule has a higher rate of local recurrence, but the risk in this case can be minimized with adjuvant therapy.³³ If a patient remains with positive margins postoperatively, radiation therapy and/or antiangiogenic inhibitors may be considered. In this case, the patient was found to have positive margins after initial resection but he elected to forego additional surgery and pursue adjuvant radiation therapy. Long-term follow-up is recommended and will be provided.

With extrapleural superficial SFTs seldom reported in the literature, it is our hope that reporting our experience will add to the database of published literature, raise awareness of extrapleural manifestations, and allow for earlier diagnosis and treatment.