Approach to pleural tumors or tumor-like lesions

1) Origin of lesion

The first step is to establish the origin of the lesion by assessing the epicenter, angle and definition of borders, and displacement of the pulmonary vasculature and extra-pleural fat.

Fig. 3: Determining the origin of the lesion. 1) Is it pulmonary or extra-pulmonary? (a) Pulmonary lesions are centered in the lung, form acute angles with non-tapered margins, and engulf pulmonary vasculature. (b) Extra-pulmonary lesions are centered on the chest wall, form obtuse angles with tapered margins, and displace pulmonary vasculature. Incomplete border sign, where the inner border is well-defined and the outer, more lateral border is ill-defined is a classic sign of an extra-pulmonary lesion. 2) If extra-pulmonary, is it pleural or extra-pleural? (c) If the extra-pleural fat is displaced outward, it is pleural. (d) If the extra-pleural fat is displaced inward, it is extra-pleural.

2) Morphology of lesion

Morphology of the lesion, including size, shape, and borders, can help guide diagnosis. Pleural lesions may be solitary or multiple, focal or diffuse, unilateral or bilateral, and calcified or noncalcified.

3) Benign or malignant features

While there is overlap between the appearances of benign and malignant pleural thickening, features such as circumferential, irregular nodular thickening (>1 cm), infiltration of the chest wall and diaphragm, and involvement of the mediastinal pleura were more associated with malignant thickening [2,4,7].

Malignant lesions show avid uptake on FDG-PET and tend to be hyperintense on T2-weighted images with post-contrast enhancement, whereas benign thickening is hypo- to isointense on T2 and post-contrast images [2].

Tumoral lesions of the pleura

Primary lesions – malignant

Fig. 4: Primary malignant pleural tumors.

Mesothelioma

Mesothelioma is the most common primary malignancy of the pleura and is locally aggressive, with a strong association with asbestos exposure.

Unilateral pleural effusion is the most common finding on chest radiography. Lobulated pleural thickening with heterogenous post-contrast enhancement, unilateral volume loss , and calcified pleural plaques associated with asbestos exposure may be seen. CT defines the extent of the tumor and is necessary for staging, demonstrating local invasion of the chest wall and diaphragm, mediastinal lymph node involvement, and metastasis to the retroperitoneal space [1].

While not routinely used, magnetic resonance imaging (MRI) has a superior soft-tissue resolution and can help detect early local invasion  [1,2]. Mesothelioma shows avid uptake on FDG-PET scans and can help guide biopsy to the most FDG-avid regions for better yield and show treatment efficacy on follow-up imaging [1,2].

Fig. 5: A 53-year-old male presented to the emergency department with intermittent dry cough and occasional bloody sputum. No recent travel or significant weight loss. (a) Chest radiograph showed total opacification of the left hemithorax, the heart silhouette and mediastinal structures are obscured and shifted to the contralateral side (yellow arrow) suggestive of a massive pleural effusion with left lung collapse. Coronal (b) and axial (c) chest contrast-enhanced CT (CECT) display focal irregular thick pleural soft tissue attenuation along the base of the left hemithorax (blue arrow) associated with massive pleural effusion. (d) Coronal fused PET-CT depicts the pleural soft tissue density with high FDG uptake (orange arrow). No extra-pleural or intraparenchymal extension is seen. Histopathology showed malignant mesothelioma (epithelioid type).
References: Department of Radiology, Hamad General Hospital - DOHA/QA (2020)

Lymphoma

Primary pleural lymphoma results from aggregates in the subpleural connective tissue of the visceral pleura and is associated with immunodeficiency and chronic pyothorax. The most prevalent subtype is B-cell non-Hodgkin’s lymphoma [3].

CT appearance is variable, ranging from solitary nodule, to focal or diffuse nodular thickening with homogeneous enhancement and high uptake on FDG-PET. Pleural effusion is commonly found. Calcifications and necrotic changes can be seen post-chemotherapy [2]. Extra-pleural involvement is frequent and is a differentiating factor from mesothelioma.

Primary lesions - Benign

Fig. 6: Primary benign pleural tumors.

Pleural lipoma

Usually found incidentally, lipoma is the most common benign pleural tumor. Lipomas are well-defined, non-enhancing, and of fat-density on CT. If enhancing septa or intervening areas of soft-tissue are seen within the lesion, the possibility of liposarcoma should be investigated [1,2].

Fig. 7: A 62-year-old male presented with a history of abnormal chest radiograph on a pre-employment exam. He immigrated from India, and reports no constitutional or chest symptoms. (a) Axial images of chest CT scan in lung window shows two well-circumscribed lobulated peripheral mass lesions, seen at the posterior aspect of the right lower lobe (blue arrows). The form obtuse angles with the chest wall. No sclerosis or destruction of the underlying ribs is seen. (b) Soft tissue window shows the lesions to be of fat density. These findings are consistent with pleural lipomas.
References: Department of Radiology, Hamad General Hospital - DOHA/QA (2020)

Solitary fibrous tumor

Solitary fibrous tumor of pleura (SFTP) arises from the visceral pleura and is usually benign (malignant in 20% cases). On CT, SFTP is a pleural-based hypodense soft tissue mass with homogeneous post-contrast enhancement [3]. Larger tumors may exhibit heterogeneous enhancement with areas of necrosis, hemorrhage, cystic changes, or calcifications. [1,3].

Differentiating benign from malignant fibrous tumors on imaging is challenging. Larger (>10 cm), more numerous lesions occupying more than half of the affected hemithorax with high FDG uptake, calcifications, and effusion may suggest malignancy [3]. Conversely, a linear soft tissue extending into the pleura representing a stalk is suggestive of a benign nature.

Patients may present with classic but rare paraneoplastic syndromes of hypoglycemia and hypertrophic osteoarthropathy [3].

Fig. 8: A 30-year–old asymptomatic female patient, and had a routine chest radiograph (a), in which a soft tissue density at the right upper quadrant/posterior costophrenic angle (yellow arrow) is identified, with no associated rib destruction or pleural effusion. In the sagittal (b) and axial (c) views of the patient‘s chest contrast-enahnced CT (CECT), a pleural-based well-circumscribed enhancing soft tissue mass with homogenous enhancement (blue arrow) is noted at the posterior costophrenic angle with a broad base and obtuse angle with lung parenchyma. No rib destruction, diaphragmatic or hepatic invasion is detected. Histopathological examination showed a benign solitary fibrous tumor.
References: Department of Radiology, Hamad General Hospital - DOHA/QA (2020)

Metastasis

Pleural metastases are most commonly from bronchogenic carcinoma, breast carcinoma, and lymphoma [1,2]. Focal masses or nodular pleural thickening with post-contrast enhancement, pleural effusion, and avid FDG uptake are seen.

Differentiation from mesothelioma may be difficult. Bilateral involvement favors metastasis, whereas unilateral involvement with associated lung volume loss favors mesothelioma [5].

Fig. 9: 57-year-old male presented to the emergency department with chest pain, fever, hemoptysis and chest pain provoked by dry cough. These symptoms were progressive over 3 weeks duration. (a) Chest radiograph demonstrates a left pleural based mass with a sharply defined medial border and ill-defined lateral border that is merged with overlying chest wall (yellow arrow). This incomplete border sign confirms extra-pulmonary location of the mass. (b) Axial and (c) coronal CECT display lobular mixed cystic solid mass (blue arrow) along the left posterolateral chest wall, that forms an obtuse angle with the adjacent locally thickened pleural surface (red arrow). Another smaller soft tissue lesion of similar CT characteristics is noted beneath left pectoralis minor muscle (orange arrow). Histopathological exam demonstrated metastatic pulmonary adenocarcinoma.
References: Department of Radiology, Hamad General Hospital - DOHA/QA (2020)

Tumor-like lesions

Pleural thickening

Benign pleural thickening can be focal or diffuse, with varying patterns of involvement, morphology, and clinical findings (Fig. 10).

Fig. 10: Focal and diffuse benign pleural thickening.

Focal

Apical cap

Apical caps are benign idiopathic age-related pleural thickening confined to the apices. Imaging shows well-defined thickening with a smooth or undulating border and can be bilateral [1]. 

Pleural plaque

The most common manifestation of asbestos inhalation, pleural plaques arise from parietal pleura. They are usually found in the diaphragmatic dome and undersurface of the lower posterolateral ribs, with sparing of lung apices and costophrenic angles. Patients are typically asymptomatic. 

Pleural plaques appear as geographic lesions in radiographs. CT is more sensitive demonstrating discrete, raised parietal pleural thickening in characteristic locations, with calcifications seen in 15%–20% of cases [2].

Fig. 11: A 43- year old male was referred to the clinic with abnormal chest radiograph findings. He is asymptomatic, with no constitutional or chest symptoms. (a) Axial images of CT chest show multiple discrete raised soft tissue pleural thickenings on the undersurface of the posterior ribs bilaterally (yellow arrows), more on the left side with calcifications. (b) Coronal images show similar lesions with calcifications on the diaphragmatic domes bilaterally (blue arrows). Costophrenic angles are clear, with no pleural effusion.
References: Department of Radiology, Hamad General Hospital - DOHA/QA (2020)

Diffuse

Diffuse pleural thickening

Diffuse pleural thickening manifests due to fibrosis and adhesions of both the visceral and parietal pleura due to severe pleural inflammation with an exudative effusion. It is associated with empyema, hemothorax, connective tissue disorder, and asbestosis [1,6]. There is associated volume loss, restrictive lung disease, and accumulation of extra-pleural fat.

On imaging, it shows continuous irregular pleural thickening >5 cm wide, 8 cm in height, 3 mm thick, spanning multiple ribs with blunting of costophrenic angles [2].

Fig. 12: 55-year-old male patient admitted with a 10 day history of fever, cough, and left sided chest pain. Chest radiograph (a) shows a large, left sided opacity abutting the left chest wall forming obtuse angles (yellow arrow), likely representing a loculated pleural effusion. Diffuse infiltrates are seen in the left lung, more in the lower zone, with blunting of the left costophrenic angle (blue arrow). Coronal (b) and axial (c) cuts of CECT show a moderate left sided loculated pleural effusion (*) with diffuse pleural thickening and enhancement (orange arrows). Pathological diagnosis was confirmed as infected multilocular pleural fluid collection (empyema).
References: Department of Radiology, Hamad General Hospital - DOHA/QA (2020)

Other tumor-like lesions

Tuberculosis

Unilateral pleural effusion is a common manifestation of extra-pulmonary primary mycobacterium tuberculosis infection. Another manifestation is tuberculous empyema, which is loculated and associated with pleural thickening showing post-contrast enhancement on CT. Residual pleural thickening with calcification is expected post-treatment, potentially leading to fibrothorax [7].

Concomitant parenchymal involvement may also be seen as micronodules in subpleural and peribronchovascular interstitium [7].

Fig. 13: A 34-year-old male patient presented with chest pain and cough. (a) Axial and (b) coronal contrast-enhanced chest CT chest (mediastinal window) shows a unilateral right diffuse irregular nodular pleural thickening and enhancement of parietal and visceral pleura (yellow arrows), extending over the mediastinum (orange arrows), associated mild pleural effusion, without evidence of parenchymal or lymphatic disease. It initially raised the possibility of mesothelioma. (c) High-power view of the pleural biopsy of the same patient showing Langerhans giant cells (white arrow) in a background of caseous necrosis indicative of pleural tuberculous granuloma.
References: Department of Radiology and Department of Pathology, Hamad General Hospital - DOHA/QA (2020)

Pleural endometriosis

Thoracic endometriosis results from the implantation of endometrial tissue in the pleural space and the right hemidiaphragm via the paracolic gutter. These implants may undergo cyclical necrosis, with subsequent fenestrations in the right hemidiaphragm allowing air to reach right pleural space from the abdominal cavity, leading to recurrent catamenial pneumothorax. 

Patients may present with right scapular or thoracic pain occurring around the time of menstruation. Coexistent abdominal and pleural endometriotic implants may be seen [6].

Fig. 14: 37 year old female with known history of pelvic endometriosis previous history of right-sided pneumothorax presented with dyspnea. (a) Coronal and (b) axial images of chest CT show encysted right pneumothorax (*) with collapsed and compressed lung parenchyma tethered by adhesions (yellow arrows). Mediastinal shift to the left is seen. The right diaphragmatic surface shows subtle irregular appearance (blue arrow), seen more clearly in the coronal mediastinal window (c). Upper abdominal cuts show ascites (orange arrow). Intra-operatively, multiple small dark red fleshy hemorrhagic deposits on the right diaphragmatic pleural surface with multiple tiny fenestrations in the right hemidiaphragm. The samples were too small for histopathological evaluation; however, the clinical history of endometriosis with recurrent right-sided pneumothorax and the intra-operative findings is consistent with catamenial pneumothorax.
References: Department of Radiology, Hamad General Hospital - DOHA/QA (2020)

Pleural pseudotumor

Pleural pseudotumor is a fluid collection within a lung fissure, usually the minor fissure, due to fluid overload states. On chest radiograph, a lenticular or biconvex opacity in the fissure can be seen [6].

Fig. 15: 77-year-old female, with a known history of congestive heart failure presented with shortness of breath. (a, b, d) Axial and (c) coronal cuts of the chest CT scan show global bilateral interstitial edema and mosaic appearance impressive of pulmonary edema. Pleural effusions are seen in the major (yellow arrows) and minor (blue arrows) fissures of the right lung. Global cardiomegaly is also noted.
References: Department of Radiology, Hamad General Hospital - DOHA/QA (2020)

Non-pleural mimic of tumor

Extra-pleural hematoma

In an injury of the internal mammary or intercostal vessels without disruption of the parietal pleura, blood accumulates in the extra-pleural space, leading to hematoma. Recognition of extra-pleural hematoma is essential as rapid expansion can lead to respiratory or circulatory collapse. On CT, it is a high-attenuation collection with inward displacement of extra-pleural fat [6].

Fig. 16: A 27-year-old male presented with blunt trauma to his chest. Axial cuts of his CT chest in (a) lung, (b) soft tissue, and (c) bone window, and (d) 3D-reconstruction are shown. There are fractures along the posterolateral left sixth, seventh, and eighth ribs (yellow arrows) associated with a small focal extra-pleural hematoma (blue arrows).
References: Department of Radiology, Hamad General Hospital - DOHA/QA (2020)