Epidemiology
Pleural tumors are a small but significant subset of thoracic malignancies. The majority of pleural tumors tend to be malignant and mesothelioma is the most common diagnosis. Most patients have had environmental exposure to asbestos.
Pathophysiology
Pleural tumors are generally characterized as benign or malignant. Benign tumors are usually slow growing and do not cause symptoms until they exert mass effect, or in the case of MPM, when they cause pain by tumor invasion or dyspnea secondary to pleural effusion.
Diagnosis/clinical features
Many pleural-based tumors are diagnosed incidentally on imaging performed for unrelated reasons. The primary radiographic evaluation consists of CXR and CT scan. Once the pleural-based lesions have been identified, tissue diagnosis is obtained via image-guided core-needle biopsy or through thoracoscopic biopsy. When symptoms are present, they are most commonly chest wall pain from tumor invasion or dyspnea from restriction or cough.
Treatment
Benign tumors of the pleura are followed with serial imaging or resected. Treatment options for MPM are varied and range from palliation of symptoms to radical surgery within a multimodality regimen.
Prognosis
The prognosis for benign lesions of the pleura is excellent and recurrences are rare after complete resection. MPM, however, is often advanced at presentation and prognosis is poor. However, early-stage patients with epithelioid histology and negative resection margins have promising long-term survival.
The diagnosis and treatment of pleural diseases was a formidable task for practitioners of ancient and medieval medicine as most maladies of the thoracic cavity proved to be rapidly and universally fatal. The development of general anesthesia, thoracic imaging, and closed suction drains has greatly facilitated the diagnosis and treatment of pleural disease, including primary tumors of the pleura.
The pleura refers to a continuous mesothelial tissue layer of mesenchymal origin that lines the chest cavity and blankets thoracic structures.1,2 Standard nomenclature differentiates the parietal pleura, which covers the chest wall, mediastinum, and diaphragm, from the visceral pleura, which invests the lung parenchyma. The hilum of the lung and the inferior pulmonary ligament serve as the anatomic interfaces between these two layers. Blood supply and lymphatic drainage follow that of structures—parietal pleura supplied by intercostal, internal mammary, superior phrenic, and anterior mediastinal arcades. Lymphatics drain to intercostal, mediastinal, and phrenic lymph nodes. Communications across the diaphragm and chest apex link the thorax to the peritoneal cavity and ipsilateral axillary lymph nodes respectively. The visceral pleura contain both a pulmonary and a bronchial blood supply but drains solely into the pulmonary venous circulation. Visceral pleural lymphatics drain to mediastinal lymph nodes. Autonomic and somatic fibers innervate the pleura. The visceral pleura carry autonomic fibers only. Pain, carried through somatic routes, is generated from parietal pleural inflammation, such as by direct tumor invasion or local irritation.
The evaluation of a patient with pleural pathology begins with a thorough history and physical examination (Fig. 21-1). Identification of environmental exposures, particularly asbestos, can provide important insight to the etiology. Other risk factors for pleural disease such as a known prior malignancy or a history of tuberculosis should be identified. A history of cough or fever suggests infectious etiology, whereas a history of weight loss and chest pain portends a malignant diagnosis. Surprisingly, many pleural tumors are diagnosed incidentally on radiographic examinations performed for screening of unrelated diseases.
Dyspnea and chest pain are the most common presenting symptoms in patients with pleural tumors. For symptomatic patients, the plain chest radiograph (CXR) is typically the first diagnostic test performed, although this is neither sensitive nor specific. Common CXR findings are pleural plaques and effusions. Most CXR findings lead to computed tomography (CT), or possibly, magnetic resonance imaging (MRI). Chest CT scans are relatively inexpensive and provide excellent high-resolution anatomic information. Due to this, chest CT scan is the most commonly used noninvasive modality to differentiate pleural from parenchymal disease. MRI is more expensive, relatively difficult to obtain at most centers, and confounded by motion artifact, but it can add information regarding mediastinal, vascular, chest wall, or diaphragmatic invasion.
Once the presence of a suspicious pleural lesion is confirmed, tissue diagnosis is typically obtained for definitive diagnosis. This can be accomplished by cytologic examination of fluid obtained via thoracentesis of a pleural effusion or image-guided core-needle aspiration of a pleural-based mass. Should these tests prove nondiagnostic, a single-port video-assisted thoracic surgery (VATS) pleural biopsy is indicated. In addition to clarifying the diagnosis, VATS can both confirm the lesion as pleural-based and facilitate pleurodesis should this be appropriate. Care should be taken to plan the biopsy incision within the line of a possible future thoracotomy so that the site can be excised at the time of definitive surgery and limit potential port site implants (especially for mesothelioma).
Benign lesions of the pleura are relatively common and usually detected incidentally. These include lipomas, plaques, and pleural thickening. Rounded atelectasis is often misdiagnosed as a pleural lesion. The most common presenting symptom, if any are present at all, is dyspnea secondary to parenchymal compression. Chest pain is unusual for benign tumors.
Pleural lipomas are benign, slow-growing tumors that arise from fat cells present in the pleural lining. They are well circumscribed, small tumors but can grow to large sizes leading to compressive symptoms. They have a characteristic low density, homogenous appearance on CT imaging.3 Malignant transformation of lipomas is extremely rare. Pleural lipomas can typically be differentiated from their malignant counterpart, liposarcoma, which tends to have a heterogeneous appearance on imaging.
When the adipose tissue of a suspected lipoma consists of fetal brown fat, it is referred to as a hibernoma.4 These lesions are distinguished on pathology by increased vascularity and immature fat cells with multiloculated cytoplasm. Some reports have also suggested that the skin over these lesions is warm due to the heat generated via an uncoupling of the mitochondrial electron transport. Small lipomas can be followed with serial imaging, whereas larger lipomas are typically excised for cure.
Although not technically tumors, pleural plaques and pleural thickening represent a reaction to environmental irritants and can mimic malignancy.5 Plaques are reactive fibrosis, which are typically located on the chest wall or diaphragm and incited by asbestos exposure. Calcification of these plaques is pathognomonic for asbestosis (Fig. 21-2). Diffuse pleural thickening is a more extensive process that can involve circumferential encasement of lung parenchyma with accompanying pleural symphysis.6 Etiology includes diffuse inflammatory conditions, radiation exposure, infection (empyema), and idiosyncratic medication response (bromocriptine and methysergide).
Rounded atelectasis can be seen in the setting of asbestos exposure as well as in association with other inflammatory conditions. It is thought to be initiated in the setting of an exudative pleural effusion that ultimately leads to thickening, contraction, and fusion to an adjacent fold of pleura.7 This process creates atelectasis of the dependent lung parenchyma to produce a rounded peripheral nodule. It is a silent condition whose diagnosis is usually made incidentally during the workup of a solitary pulmonary nodule or a pleural-based mass. Clues on radiographic imaging can help differentiate rounded atelectasis from other lesions. CXR characteristics include volume loss of the ipsilateral lung, peripheral location, presence of pleural thickening adjacent to the lesion, and the pathognomonic “comet tail” finding, a convergence of the pulmonary vasculature as it enters the hilar side of atelectatic lung. The “comet tail” is best seen on CT scan. As rounded atelectasis is usually metabolically inactive, positron emission tomography (PET) scan is sometimes helpful in ruling out a malignant diagnosis.7
Although most solitary tumors of the pleura (SFTP) are composed of benign-appearing cells, they can nonetheless undergo malignant transformation and cannot simply be classified as “benign” lesions.8 SFTP are most commonly discovered as incidental radiographic findings. They can grow very large and cause chest pain, cough, and dyspnea from mass effect. Although unusual, malignant transformation is evidenced by rapid growth or even distant metastasis. Occasionally, SFTP are secretory and the liberation of factors such as insulin-like growth factor 2 (IGF2) can create hypoglycemia.9 SFTP are also associated with pulmonary osteoarthropathy.
Chest CT scan is typically adequate for diagnosis of SFTP although occasionally MRI or PET scanning can provide useful diagnostic and anatomic information. SFTP has pathologic correlates in the kidney, cervix, meninges, thigh, pelvic fossa, retroperitoneal, and serosal surfaces. In earlier literature, this entity was referred to as “localized mesothelioma” or “fibrous mesothelioma.” However, it has now become clear that these tumors arise from pleural mesenchymal cells most closely resembling pericytes (a dendritic-like smooth muscle cell) rather than mesothelium. The majority of SFTP arise from the visceral pleural layer, presenting as pedunculated lesions (Fig. 21-3A).
The histologic features include bipolar spindle cells with varying areas of cellularity, often with small vessels and perivascular hyalinization (Fig. 21-3B). These tumors stain for CD34 (a vascular marker) and are distinguished from sarcomatoid mesotheliomas by absence of staining for cytokeratin and calretinin.10,11 There are no definitive histologic features that discriminate benign from malignant SFTP. However, tumors with a mitotic rate greater than 4 mitoses per 10 high-power fields and greater than 10 cm in size are more likely to recur locally. The treatment of SFTP is surgical resection (Fig. 21-3A). Adjuvant therapy is not typically warranted, particularly if the lesion is completely resected.
Malignant pleural mesothelioma (MPM) is a rare thoracic malignancy, directly linked to asbestos exposure. The incidence of MPM remains low at about 2000 to 3000 new cases12 per year compared with lung cancer that has over 160,000 new cases per year, but has gradually increased over the past 5 decades. Although the use of asbestos dates back to thousands of years, it became widespread with increased industrialization as it was used as a form of insulation in many industries. These include construction, shipbuilding, pipefitting, and car brake assembly, due to its properties of light weight, low cost, and high heat resistance. Asbestos is a natural silicate mineral that can be categorized as serpentine and amphibole. Serpentine asbestos is composed of spiral fibers compared with amphibole asbestos whose fibers are linear and needle-like. This shape of amphibole is believed to cause chronic irritation that may lead to malignant transformation of the pleura.
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