Smooth Muscle Tumor Originating in the Pleura: A Case Report and Updated Literature Review

Smooth muscle tumors (SMTs) of the pleura are exceptionally rare. At present and to the best of these authors' knowledge, there are only 17 cases reported in the literature. We describe a case of a 51-year-old woman who complained of left sided pleuritic chest pain. Further, computed tomography (CT) revealed a left sided localized pleural-based mass involving the 9th rib. She underwent an interventional radiology guided percutaneous core biopsy of the lesion, which disclosed a “Smooth Muscle Tumor of Undetermined Malignant Potential (SMT-UMP).” A video-assisted thoracoscopic surgery (VATS) was performed for diagnosis and treatment purposes. Resections of the pleural-based mass and 9th rib were performed. SMT-UMP was the definitive diagnosis.


Introduction
Intrathoracic smooth muscle tumors are uncommon in the respiratory tract (upper and lower respiratory tract). They are seen occasionally in the gastrointestinal tract and commonly seen in the urogenital system [1][2][3]. However, the existence and diagnostic criteria of smooth muscle tumors originating in the pleura have been controversial and only rare and sporadic case reports have been mentioned in the literature [4]. Table 1 describes all cases that have been reported in the literature. We present a case of SMT-UMP with CT evidence of involvement of the 9th rib.

Case Report
A 51-year-old woman presented to the emergency department with persistent posterior left sided chest discomfort. Patient had nonspecific symptoms for over one year. Upon admission patient was noted to have a normal EKG findings; X-Ray of the chest was unremarkable. She underwent a CT which revealed a pleural-based mass 3.3 cm × 2.0 cm of the left lower lobe involving the 9th rib posteriorly, in Figure 1. A CT-guided transthoracic core biopsy of the tumor revealed a smooth muscle tumor of the pleura of undetermined malignant potential (SMT-UMP). The specimen showed a bland proliferation of spindle cells with abundant eosinophilic cytoplasm arranged in fascicles. No necrosis and rare mitotic activity were identified. Due to the pattern of spread that has been shown of these tumors (local growth without metastasis) and the lack of high risk features (no necrosis and rare mitotic activity) CT scan of the chest including the upper abdomen was performed for staging purposes and it did not show metastasis.
She was further treated with complete resection of the pleural-based mass and the 9th rib by video-assisted thoracoscopic surgery (VATS). The final pathology examination revealed a well-capsulated SMT-UMP of pleural origin measuring 3.5 × 3.0 × 2.4 cm with no evidence of rib involvement by the tumor, in Figure 2. Patient tolerated the procedure well without any surgical or medical complications. Unfortunately patient was lost to follow-up. Table 2 shows patient's immunohistochemical staining.

Discussion
SMT-UMP originating in the pleura are rare. They tend to have a female predominance (12 out of 18 patients) as per current literature review. Patient's age ranges from 21 to 73 years old (mean 42.7). Leiomyosarcomas (LMS) were found mainly in older patients and SMT of UMP on the other hand tend to happen in younger patients. Our case illustrates that SMT-UMP can present in a relatively older age. SMTs can be found incidentally on imaging studies done for unrelated issues or they can cause symptoms usually related to tumor size (the largest one has been reported as at least 21 cm) and location. In our case a middle aged women presented with nonspecific pleuritic chest pain where fatal conditions need to be ruled out. These patients should undergo a thorough history and physical examination. Radiological studies should be performed with chest X-ray (CXR), computed tomography, and magnetic resonance imaging that helps identify location, size, and radiological structure of the tumor [6].
Pleural tumors tend to grow locally toward the intrathoracic cavity [15]. There is not a single case reported in the literature showing that these tumors did metastasize. Proca et al. [6] reported a case that was followed up without any surgical intervention for four years and it showed that the tumor did grow locally inside and out of the thoracic cavity but no metastasis was reported. However, due to the rarity of these tumors and short follow-ups reported there is not enough data at the moment to determine if these tumors have the ability to metastasize.  Confirmation of diagnosis is always made with tissue sample and histological examination. A CT-guided biopsy can be performed, but it has the potential to seed the tissues directly in its path [6]. If surgical excision of the lesion can be done safely with minimal complications it should be done with diagnostic and treatment purposes instead of diagnostic needle biopsy in order to avoid potential seeding and spread of tumor with malignant potential.
Primary pleural tumors are rare since 75% of pleural tumors represent metastatic disease [13]. The differential diagnosis of spindle cell neoplasms from pleural origin includes smooth muscle tumor, solitary fibrous tumor, metastatic spindle cell carcinoma, synovial sarcoma, fibrosarcoma, malignant peripheral nerve sheath tumor, sarcomatous mesothelioma, and spindle cell thymoma [2]. Table 3 describes the main differences between these entities.
Immunohistochemical staining for smooth muscle actin and desmin provides a definitive diagnosis of smooth muscle origin.
In our patient, microscopic examination showed proliferation of bland spindle cells with elongated nuclei, eosinophilic cytoplasm, and rare mitotic figures. Focal areas of increased cellularity and atypia were present, but no necrosis was identified, in Figure 2. The tumor cells reacted with immunohistochemical stains for desmin, smooth muscle actin, SMMHC, CD34 (focal), and vimentin. Additional immunostains were performed, including S-100, BCL-2, CD99, and beta-catenin. All of these were negative. Less than 2% of the cells showed reactivity with proliferation index Ki-67; see Table 2. The pathological findings were diagnostic of a smooth muscle neoplasm. The absence of pleomorphism, increased mitotic figures, necrosis, and poor differentiation distinguished SMT of UMP from LMS [7].
Even when smooth muscle tumors of the pleura appear benign, well-encapsulated, smooth, and without evidence of necrosis and show rare mitotic activity they can possess malignant potential and present as or transform in LMS; see Table 1 [7].
Primary and preferred treatment is surgical resection if feasible which can be performed using minimally invasive surgery such as VATS, if after surgical resection there are positive margins to consider reresection (preferable option) versus observation (watch and wait approach) of the remaining disease with serial imaging studies during follow-up [6]. Smooth muscle tumors may increase in size with local invasion to the mediastinum and other structures, which can jeopardize complete resection with curative intent. If further surgery is contraindicated and disease was left behind perhaps the role of radiation could be explored [17][18][19][20]. Al-Daraji et al. [7] reported a case where there was a concern for possible incomplete resection at the apex and this patient received radiation with the intention to reduce the risk of local recurrence. This patient was reported to be alive at 14 months of follow-up and without recurrence. At present, there is no role for adjuvant chemotherapy.
The prognosis appears to be good if the tumor is excised completely with negative margins, but routine follow-up should not be neglected. Due to the rarity of this entity and  relative short follow-ups the behavior of this tumor cannot be properly evaluated.

Conclusion
Primary SMTs of the pleura are infrequent tumors and should be considered as a differential diagnosis when approaching a pleural mass. It seems to develop from the vascular smooth muscle cells. SMT-UMP tends to affect younger patients and LMS to tends to affect older patients. However, SMT-UMP can present in older patients as in our case. They appear to grow locally and invade nearby structures but there is not yet a single case reporting distant metastasis (the present case was not the exception). Tissue diagnosis and accurate histopathological evaluation are required. Although these tumors seem to possess low malignant potential they can be life threatening (they can grow very large causing serious symptomatology and/or degenerate into malignant tumors) and should be treated as such with appropriate surgical management and close follow-up.