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Myelolipomas are rare tumours composed of mature adipose tissue and normal haematopoietic cells. They commonly occur in the adrenal glands, and involvement of the lung is extremely rare.1,2,3,4,5,6 We report a case of pulmonary myelolipoma presenting as an asymptomatic nodule unchanging in size for 10 years after being detected.
A man in his 70s, a smoker, had a nodule in the left lower lobe (LLL) of the lung detected on a chest radiograph 10 years earlier. The nodule, 2 cm in maximal diameter, had not changed in size. The patient had never had haematological disorders including anaemia. He presented with intractable lumbago since 6 months, and MRI detected multiple bone tumours. CT of the chest indicated a large mass in the right lower lobe (RLL) and a small solitary nodule with rim‐like calcification in the LLL of the lung (fig 1A1A).). Sputum cytology showed carcinoma cells. He was clinically diagnosed as having lung cancer with multiple bone metastases. Chemotherapy and radiation therapy had no effect, and the patient died of respiratory failure. An autopsy was performed 7 h after death.
Macroscopically, on autopsy the lung specimens showed a demarcated nodule, measuring 2 cm and mixed yellow and red–brown in colour, adjacent to the bronchus in the LLL (fig 1B1B),), whereas a large tumour occupied the entire RLL. No hepatosplenomegaly or lymphadenopathy was observed at autopsy. Microscopically, the nodule in the LLL was surrounded by thin trabecular bone contiguous with bronchial cartilage (fig 1C1C),), and composed of approximately 60% mature adipose tissue and haematopoietic cells indistinguishable from usual bone marrow (fig 1D1D).). Haematopoietic cells comprised trilineage elements: myeloid, erythroid and megakaryocytic cells. Immunohistochemically, myeloperoxidase, glycophorin A, factor VIII‐related antigen and CD68 were expressed on myeloid cells (fig 2A2A),), erythroid cells (fig 2B2B),), megakaryocytes (fig 2C2C)) and macrophages (fig 2D2D),), respectively. Lymphocytes were hardly detected even by immunohistochemistry. No calcification or ossification was seen in the bronchial cartilage. The tumour of the RLL was pleomorphic carcinoma composed of poorly differentiated adenocarcinoma with giant cell components, and metastases were identified in the nodule of the LLL as well as in systemic organs including the thoracic vertebra and rib. Background bone marrow was normocellular without fibrosis. There was no evidence of extramedullary haematopoiesis (EMH) in the liver or spleen.
Myelolipomas are usually observed in the adrenal glands: about 3% occur in the thorax, mostly in the posterior mediastinum,7 and only seven cases of intrapulmonary lesions have been reported.1,2,3,4,5,6 Table 11 summarises the clinicopathological features of pulmonary myelolipoma in this case and the reported cases. Patients were aged 45–81 years (mean age 60 years), and there was a predominance of males. The background of these patients was non‐specific and no patient presented with symptoms or haematological diseases. There is no predilection of tumour location. The size of the tumour is mostly within 2.5 cm, except in one case. Adipose tissue is commonly a predominant element. Preoperative diagnosis by transbronchial biopsy is difficult; however, a case of peripherally located nodule has been diagnosed by fine‐needle aspiration biopsies.2
EMH usually occurs as a manifestation of myeloproliferative diseases or is a compensatory phenomenon in various chronic anaemias, which should be differentiated from myelolipoma. EMH generally develops in the liver and spleen, but rarely arises in the lung, presenting as mass or interstitial infiltrates with fibrosis.8,9 It frequently presents with multiple occurrences and indistinct margins in tumorous lesions, a predominance of haematopoietic cells and erythroid hyperplasia.10 In contrast, myelolipomas are almost solitary and usually well circumscribed. Adipose tissue is a major component and erythroid hyperplasia is infrequently observed.10 The most important distinctive finding in our case is that no haematological disorder or anaemic state was shown. Because the nodule had been detected 10 years earlier, it would be reasonable to think that the multiple bone metastases of the lung carcinoma were scarcely associated with its development.
Gowitt and Zaatari11 reported a patient with bronchial EMH in whom chronic myelogenous leukaemia later developed. The bronchial wall was thickened by calcification and ossification of the cartilage, and the EMH in the ossified area. The EMH extended into the surrounding tissues, and the margin of the lesion was ill defined. In the present case, the bronchial wall structure including bronchial cartilage was well preserved, which is a finding different from the EMH case discussed above.
Shuangshoti12 described a patient with osseous metaplasia of the bronchus who had tuberculosis for 6 years. Multiple bronchi contained small foci of submucosal osseous metaplasia continuous with the bronchial cartilage. A bone marrow component in the foci was not mentioned in that report, but haematopoietic cells were probably scattered with fat tissue in the lesion, judging from the associated picture. Chronic inflammation might be associated with the development of such lesions. In the present case, the lesion itself was histologically similar to this metaplastic lesion, but there were differences in the solitary lesion, the non‐ossified bronchial cartilage and the fact there was no history of chronic inflammatory pulmonary diseases.
A bone element in myelolipoma is controversial; Fowler et al10 described that no bone spicules were observed in myelolipomas, and cases in which bone spicule was observed were excluded from myelolipomas. In the lung, two previously reported cases and the present case show trabecular bone.6,7 Centrally located tumours in two cases and in the present case were located close to the bronchus.5,7 A bone component in pulmonary myelolipoma might be associated with the site of tumour development that is adjacent to the bronchial cartilage.
Myelolipomas can be detected incidentally at autopsy and do not often exceed 5 cm in diameter, and rarely present as a giant mass.13 Pulmonary myelolipomas are mostly small nodules, but a case of a myelolipoma, 7 cm in maximal diameter was reported.3 The lesion in this case had not changed size for 10 years, and had no apparent neoplastic feature. Chang et al14 reported an adrenal myelolipoma with balanced chromosomal translocation that suggested a true neoplastic nature. Myelolipoma is a heterogeneous concept in adrenal and extra‐adrenal sites. Some cases are genuine neoplasms, but others might be tumour‐like lesions. The histogenesis of myelolipoma is not evident, although there are several theories: embolic origin, heterotopic bone marrow and metaplasia from multipotential mesenchymal stem cells. The present case suggests that some pulmonary myelolipomas are not true neoplasms but can be metaplastic lesions arising in the peribronchial region.
We thank Hideaki Ninomiya, Department of Pathology, Kanazawa Medical University Hospital for assistance with immunohistochemistry.
Competing interests: None declared.