cerebral metastases of unknown origin are most frequently reported in association with primary lung cancer . however , we could not find any reports in the literature in which the primary site was not detected within the lung fields . the prognosis of cancer of unknown origin is poor , with median survival times of 612 months , and the advantage of chemotherapy over best supportive care remains unclear . we present a very unique case of cerebellar metastasis of unknown origin , in which primary lung adenocarcinoma was diagnosed by pathological examination of the cerebellar metastatic tumor . brain ct and mri revealed a cerebellar tumor with hemorrhage , as well as multiple small brain tumors ( fig . the cerebellar tumor was resected because of the hemorrhage , while the other tumors were treated using gamma - knife radiosurgery . 2a ) ; however , the primary tumor site was unidentifiable . serum carcinoembryonic antigen ( cea ) and sialyl lewis x antigen ( slx ) levels were 129.9 ng / ml and 150 u / ml , respectively . malignancy was not detected in the mammary glands , or in gynecological or gastroenterological areas . chest ct revealed enlarged submandibular ( only the right side ) , infraclavicular , and subcarinal lymph nodes ( fig . c ) . fluorodeoxyglucose ( fdg ) positron emission tomography revealed high fdg uptake in these lymph nodes ( maximal standardized uptake values of 3.8 , 8.2 , and 10.2 , respectively ; fig . the resected tumor was positive for napsin a and thyroid transcription factor-1 ( ttf-1 ) ( fig . 2b ) ; an epidermal growth factor receptor ( egfr ) exon 19 deletion mutation was also detected . these findings led us to strongly suspect lung adenocarcinoma ( ctxn3m1b , stage iv ) . thus , gefitinib ( 250 mg / day ) treatment was initiated . on day 19 of the therapy , serum cea and slx levels were 11.6 ng / ml and 29 u / ml , respectively . we successfully diagnosed a very rare case of lung adenocarcinoma with associated cerebellar metastasis of an unidentifiable primary site . napsin a and ttf-1 are useful immunohistochemical markers for primary lung cancer in cases where the primary site is unknown . ttf-1 and napsin a typically stain positive in cases of lung and thyroid cancers   and renal cell carcinoma , respectively . however , the use of napsin a and ttf-1 together is reportedly more beneficial in diagnosing primary lung adenocarcinoma ; hence , we arrived at a diagnosis of lung adenocarcinoma although the primary site was initially unknown . our case is unique because the cerebellar tumor was egfr mutation positive , prompting the initiation of gefitinib therapy . because gefitinib is effective in treating brain metastases of lung cancer with egfr mutation , brain metastases may have been controlled in this case . this case may offer some useful insights concerning treatment of cerebral metastases of unknown origin .
we present a very rare case of cerebellar metastasis of unknown origin , in which a primary lung adenocarcinoma was diagnosed by pathological examination of a cerebellar metastatic tumor , using immunohistochemical markers and epidermal growth factor receptor ( egfr ) mutation of primary lung cancer . 
 a 69-year - old woman was admitted to our hospital because of a hemorrhagic cerebellar tumor and multiple small brain tumors . 
 she underwent cerebellar tumor resection . on pathological examination , 
 the tumor was diagnosed as adenocarcinoma . however , the primary tumor site was unidentifiable even with several imaging inspections . on immunohistochemical analysis , 
 the resected tumor was positive for napsin a and thyroid transcription factor-1 . 
 in addition , an egfr mutation was detected in the tumor . 
 therefore , primary lung cancer was diagnosed and the patient was started on gefitinib ( 250 mg / day ) therapy .