Diaphragmatic mesothelial cyst

A 62-year-old female was referred to our department for follow-up imaging after surgical treatment of lung adenocarcinoma. She had a right lower lobe lobectomy without adjuvant therapy in April 2020, and there have been no signs of metastatic disease or recurrence since. CT imaging studies demonstrated a cystic lesion under the diaphragm with initial diagnosis of a simple hepatic cyst. The final diagnosis was made after additional imaging studies and ruling out conditions with similar radiologic features.

On contrast-enhanced CT imaging, performed during follow-up in February 2022, an oval cystic abnormality measuring 52 x 44 x 41 mm was demonstrated in the epigastric region under the diaphragm, located between the left liver lobe and caudate lobe (Figure 1). The lesion appeared well-defined, homogeneous, non-enhancing and relatively dense with an average attenuation of 30 HU. The initial CT examination in April 2020 demonstrated the same lesion, which was diagnosed as a simple hepatic cyst. There was no progression of the lesion in comparison to previous imaging studies available within the last two years.

US demonstrated a thin-walled anechoic lesion with a single internal septation and posterior acoustic enhancement (Figure 2). The lesion was following the respiratory movement of the diaphragm and was not attached to the liver capsule, confirming its extrahepatic location. Colour Doppler signals were absent (Figure 3).

Previous PET-CT imaging performed during the staging of lung adenocarcinoma showed no abnormal metabolic activity of the lesion (Figure 4).

Mesothelial cysts are rare congenital abnormalities with benign features. They present as cystic lesions lined with a layer of mesothelial cells that originate from coelomic remnants [1]. They can be found on serous membranes in various locations throughout the body including the spleen, adrenal glands, ovary, falciform and round ligament, spermatic cord, mesentery and the diaphragm [1,2]. Mesothelial cysts are most commonly found in children and young adults.

Diaphragmatic mesothelial cysts (DMC) are extremely rare. They are usually located between the diaphragm and the posterolateral part of the right liver lobe, but they can be found anywhere along the diaphragm. DMC can be difficult to diagnose in some cases due to its close anatomical proximity to the adjacent liver [1].

On imaging studies, DMC is characterized by a well-defined, homogeneous, non-enhancing cystic structure with thin walls. Lesions are usually bi-lobulated or oval-shaped [1]. Coarse calcifications of the cystic walls and internal septations are uncommon in mesothelial cysts but have been described in few cases [3].

Ultrasound is usually the best imaging technique to confirm the diagnosis due to its non-ionizing radiation and accessibility. Typical US finding is a unilocular anechoic lesion located along the diaphragm. Ultrasound can show features such as septations and debris, making it superior to CT in demonstrating the internal structure of the cyst [4]. Lesions usually show no evidence of flow on colour Doppler [1,5]. 

CT and MR imaging findings demonstrate typical cystic features of the lesion, with a low signal on T1-weighted image and a high signal on T2-weighted image.

DMC are usually asymptomatic and are often discovered as an incidental finding on imaging studies. For asymptomatic lesions with typical imaging features, conservative management with ultrasound follow-up is recommended. Treatment options for large or symptomatic DMC include ultrasonography-guided aspiration with or without sclerotherapy and surgical excision [1].

Our initial diagnosis was a simple hepatic cyst due to the benign characteristics, a solitary form and the absence of metabolic activity on PET-CT that excluded malignant disease. The US examination revealed the diaphragmatic lesion that followed the respiratory movement, confirming the diagnosis of DMC.

The main differential diagnosis that can cause diagnostic confusion and alters treatment options is a hydatid cyst of the liver. It should be considered particularly in endemic areas. The presence of multiple cysts is the most important imaging finding that would suggest hydatid disease. Other features, such as calcifications and septations, are non-specific findings and can be seen in both mesothelial and hydatid cysts [1,5].

Teaching points: Diaphragmatic mesothelial cysts are extremely rare congenital abnormalities with benign features. They are usually asymptomatic. Imaging findings of DMC demonstrate typical cystic features. Ultrasound is the best method to confirm their benign nature and allow conservative management.

Written informed patient consent for publication has been obtained.