Mesenchymal cystic hamartoma of the lung mimicking simple pulmonary aspergilloma: a case report

Mesenchymal cystic hamartoma (MCH) of the lung is a rare pulmonary neoplasm that has favorable outcomes following surgical resection, although it occasionally presents with severe symptoms such as pneumothorax and hemothorax. MCH is radiologically characterized by cyst formation when the tumor reaches a size of 1 cm or more [1], and presents solitary or multiple cystic lesions in the lung on computed tomography (CT) scanning. Preoperative diagnosis of MCH is reportedly difficult due to similar radiographic features as other cystic neoplasms, such as pleuropulmonary blastoma, pulmonary metastasis of endometrial stromal sarcoma, lymphangiomyomatosis (LAM), and cystic adenomatoid malformation (CAM) [2]. Herein, we present a case of MCH mimicking simple pulmonary aspergilloma (SPA) on diagnostic imaging.

We experienced a case of MCH with a rare presentation mimicking aspergilloma. In the present case, SPA was initially suspected in addition to lung cancer, because of the CT finding of a single cystic lesion with wall thickening and an intracavitary nodule without FDG-uptake, along with elevation of serum Aspergillus antigen levels. A PubMed search for “mesenchymal cystic hamartoma, lung, from 1986 to 2022” revealed no previous reports of MCH requiring differentiation from Aspergillus infection, and, to the best our knowledge, the present case is the first such report.

Previous reports have described MCHs presenting as nodules or cysts, some with septae within the cyst, but with no specific CT findings [3]. SPA, on the other hand, is characterized by a cavitary lesion with crescent-shaped air around the fungus ball (crescent sign) on CT images. The intracavity fungus ball is sometimes mobile, and the lesion reportedly shows no increase in size over several months and typically presents no evidence of surrounding inflammation [4]. CT in the present case showed a single cystic lesion with mild wall thickening with poor inflammatory findings and an intracavitary nodule, with imaging findings similar to SPA, although mobility of the nodule was not evident. Although none of the previously reported MCH cases mentioned aspergillus infection in the differential diagnosis, a case of MCH presenting with cystic lesions and internal nodules was also reported by Leopizzi et al. [5]. MCH and SPA are similar in that they are both slow-growing tumors, which might make it difficult to distinguish between them on imaging. Although the result for Aspergillus antigen was a false-positive in this case, the cause was not determined. False-positive results have previously been reported to be caused by the use of β-lactam antibiotics and the ingestion of foods (grains, etc.) contaminated with galactomannan in the environment in patients with impaired intestinal mucosal barrier function, such as immunocompromised hosts [6]. In the present case, however, there was neither a history of antibiotic usage nor immunodeficiency.

MCH of the lung, which was first described in 1986 [1], is a rare tumor originating from primitive mesenchymal cells, and differs from ordinary hamartomas in that it forms cysts. MCH is diagnosed based on the clinical findings of slow growth and lack of symptoms relative to the size of the lesion, and by pathologic examination showing that the cyst wall is composed of primitive mesenchymal cells covered with normal respiratory epithelium [1]. Although the mechanism by which MCH, which initially develops as a nodule, forms a cavity when it reaches a size of about 1 cm has not been identified, previous reports have proposed the theory that cavitation occurs due to the destruction of bronchiolar and alveolar structures by proliferating cells, or the liquefaction of surrounding structures due to mucus secretion [1, 3]. In the present case, there was no obvious necrosis within the lesion, but there was mucus production within the microcyst that extended into the lumen, suggesting that mucus accumulated, expanded, and then drained out through the local bronchi and other pathways to form a cyst cavity. We consider that the cyst wall expanded, protruding into the lumen during its growth process, and increased while forming microcysts, which was detected on X-ray and CT over the course of one year. Since diagnostic biopsy of the cystic wall is unrealistic, there are no reports of a definitive preoperative diagnosis of MCH. In this case, diagnosis by bronchoscopic biopsy was considered to be difficult because of the small size of the lesion, which had a maximum diameter of 1.6 cm. Even if bronchoscopic bronchial biopsy had been performed, the treatment policy of surgery would have been unchanged regardless of the biopsy results, due to the observed growth in size of the lesion. While most cases are asymptomatic, there have been reports of pneumothorax, hemothorax, and, rarely, death due to hemoptysis in MCH cases [5, 7]. In addition, there have been reports of malignant transformation of these tumors, although only in infant cases [1, 8, 9]. Since the outcome is not always favorable, it might be reasonable to perform surgery with diagnostic and therapeutic intent. Additionally, we consider that careful postoperative follow-up is necessary in cases of MCH.

We experienced a case of MCH mimicking SPA. Although rare, MCH should be included in the differential diagnosis of cases of suspected SPA or lung cancer with cystic wall thickening on imaging.