A case of lung cancer diagnosed by cytological examination of a fluid specimen obtained from a lesion on the buttock

Background : We report a case of lung cancer diagnosed by cytological examination of a fluid specimen collected from a lesion on the patient’s buttock, with no detectable primary lesion.Case : A man in his 60 s presented with a painful abscess around the anal region. Cytological examination of a fluid specimen obtained from the abscess revealed atypical cells with large and irregularly shaped nuclei, increased nuclear chromatin, and conspicuous nucleoli in a necrotic background, and multinucleated cells. Although cancer metastasis was suspected from these findings, the primary lesion had not yet been identified. CT led to the discovery of a mass lesion in the lungs, and bronchial scraping cytology revealed atypical cells similar to those observed in the fluid from a lesion on the buttock. Based on immunohistochemistry, the lung lesion was diagnosed as a primary non-small cell lung cancer.Conclusion : In our case, we diagnosed lung cancer based on cytological examination of a fluid specimen obtained from a lesion on the buttock. We identified the primary lesion by CT imaging, comparison of the findings of cytological examination of a fluid specimen obtained from a lesion on the buttock, and immunohistochemical examination of bronchial brushing specimens. Molecular pathological examination of the lung tumor revealed a high expression level of PD-L1. This patient showed complete response to pembrolizumab treatment, which led to shrinkage of the tumors in the lung, buttock, and lymph nodes. Since then, the patient has had a good clinical course.