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Pulmonary mucinous cystic tumours of low or borderline malignant potential (PMTBMs) are extremely rare. These tumours have a very good prognosis and as such should be distinguished from usual type pulmonary adenocarcinoma. Here, we describe a case of PMTBM that arose in a 48 year old male non-smoker. He presented to respiratory physicians with right lower lobe pneumonia that failed to improve with antibiotic treatment. Sputum cytology revealed adenocarcinoma cells but at bronchoscopy no endobronchial tumour was seen. A right lower lobectomy was performed. Macroscopically, the lobectomy specimen contained an ill defined cystic lesion containing mucin. This measured 15 × 8 × 10 cm.
The tumour comprised tall columnar mucin secreting cells with minimal cytological atypia (fig 1) and no mitoses. The tumour cells lined spaces containing mucin and scattered single cells. No solid tumour areas were identified. Mucus dissection through the peribronchial spaces in a manner reminiscent of pseudomyxoma peritonei was seen (fig 2).
Tumour cells expressed cytokeratin 20 (CK20) strongly and CK7 weakly (fig 3). CK7 staining was also seen in pneumocytes and respiratory epithelium. There was focal expression of epithelial membrane antigen by tumour cells; however, there was no expression of carcinoembryonic antigen or chromogranin A. There was no evidence of metastatic carcinoma in the lymph nodes identified. The surrounding lung parenchyma showed bronchitis and organising pneumonia.
PMTBMs are very rare, with only 38 cases having been reported to our knowledge.1–7 The presentation varies from an incidental finding in an asymptomatic patient to persistent cough, chest pain, dyspnoea, pneumothorax, and pneumonia, which fails to resolve with antibiotics.1 At the time of surgery, the tumours are usually staged as T2 owing to their size. Adequate sampling of the specimen is as important as in ovarian tumours because cellularity is variable throughout these tumours. PMTBMs are histologically similar to appendiceal and ovarian mucinous tumours of borderline malignancy, with the microscopic features as described above. Lymph node involvement is not a feature.
Normal pulmonary parenchyma does not express CK20; however, pneumocytes and respiratory epithelium express CK7. Non-mucinous bronchioloalveolar carcinoma, mucinous bronchioloalveolar carcinoma, and conventional pulmonary adenocarcinomas with a bronchioloalveolar pattern at the periphery show constant but variable expression of CK7. These last two tumours also express CK20.8 In our case, the tumour cells stained strongly with CK20 and weakly with CK7.
The most difficult microscopic distinctions are between PMTBM and cystic bronchioloalveolar carcinoma and PMTBM and bronchioloalveolar carcinoma arising from a congenital cyst. Cystic bronchioloalveolar carcinoma tends to be more cellular than PMTBM, with cysts often formed secondary to necrosis,9 and previously normal x rays may exclude bronchioloalveolar carcinoma arising from congenital cysts. The exclusion of metastases from the ovary, appendix, and pancreas requires comprehensive clinical and radiological examination because immunohistochemistry is unlikely to be helpful. Other differential diagnoses include non-neoplastic mucinous cysts, mucinous cystadenoma of the bronchus, and mucoceles. These lack the cytological atypia and paucicellular mucus dissection of peribronchial spaces seen in PMTBM.
The term borderline implies a tumour of low malignant potential, rather than a tumour of no malignant potential, and this is reflected in the five year survival figures. Graeme-Cook et al stated that inherent in the diagnosis of borderline malignant tumour is an expected five year survival rate of between 75% and 95%.1 The optimal curative treatment for these tumours is surgery. In view of the excellent prognosis of PMTBM, these tumours should be distinguished from conventional pulmonary adenocarcinomas.