the finding of a renal mass on imaging is suggestive of metastatic non - small cell lung cancer in the presence of a lung tumor but can also have another origin . we describe the case of a patient diagnosed with stage iv lung cancer based on a renal metastasis . a second opinion including review of histopathological data and additional imaging followed by lung surgery and cryoablation of the kidney lesion revealed two tumors of different origins , non - small cell lung cancer and a renal cell carcinoma . the presence of a renal mass diagnosed on a ct scan in a patient with lung cancer is not always synonymous with metastatic disease . confirmation of diagnosis by tissue sampling is mandatory , especially if a synchronous primary tumor is possible . the presence of metastatic non - small cell lung cancer ( nsclc ) can be suspected by imaging . however , tissue sampling is required to confirm the diagnosis , especially if a synchronous primary tumor is possible . in this case report , we present a case with stage iv lung cancer based on a renal metastasis . a 48-year - old female received palliative radiotherapy ( 30 gy ) directed to vertebrae th9-th12 and 4 cycles of chemotherapy [ gemcitabin 1,200 mg / m ( on days 1 and 8) , carboplatin 80 mg / m ] for an 18-fdg - pet avid , thyroid transcription factor-1 ( ttf-1 ) positive adenocarcinoma of the right lower lobe invading the thoracic vertebrae 10 , and an additional 18-fdg - pet negative lesion in the right kidney , cytology - confirmed adenocarcinoma , considered as a distant metastasis ( ct4nxm1 ) . therapy resulted in a slight reduction of both lesions . due to the determined stage ( iv ) the patient 's data were reviewed and additional 18-fdg - pet and ct scans were performed . except for the presence of the lesion in the kidney , no signs of metastases were found . therefore , the initial diagnosis was reconsidered as two primary tumors : an adenocarcinoma of the lung , stage iiib ( ct4n0m0 ) and an adenocarcinoma of the right kidney ( ct1a ) . two cycles of neo - adjuvant chemotherapy ( cisplatinum 80 mg / m , pemetrexed 500 mg / m on days 1 and 21 ) were administered prior to lung surgery . surprisingly , no tumor was found in the right lower lobe perioperatively . a hemicorporectomy of th10 and a resection of the processus transversus of th10 and th11 were performed , followed by spine stabilization . with regard to the lesion of the right kidney , an expectative approach was agreed because of stable disease ( no growth ) , its localization ( in the mid pole of the kidney ) and small size ( 2.3 cm ) . eleven months later , cryoablation was ultimately performed because the patient insisted on removal of the tumor , which revealed a papillary renal cell carcinoma . this case report emphasizes the importance of differentiating a primary from metastatic disease when a renal mass is diagnosed on a ct scan in a patient with lung cancer . although metastases to the kidneys are very uncommon , differentiation between these two entities is crucial for further management and prognosis ( palliative approach vs. curative intent ) [ 1 , 2 ] . on ct scan , a solitary metastasis to the kidney is more likely to be found in patients with higher tumor stage of the nonrenal malignancy or when other viscera are also affected . usually , the metastasis is an asymptomatic , small , endophytic , and solid mass . the finding of a corresponding hotspot on fdg - pet scan is suggestive of a metastasis , even if it appears to be benign on ct . however , if the primary tumor is not fgd - avid , this might not be the case . the possibility of a primary renal neoplasm should also be considered , but with caution , since the low sensitivity of detection of fdg - pet scans . histology obtained by percutaneous biopsy confirms or strengthens the diagnosis in case of small masses and doubts about performing a nephrectomy . ttf-1 is a typical marker for adenocarcinoma in the lung , while negative in primary renal neoplasms , as illustrated in fig . patients with localized lung cancers may obtain long - term survival with surgery . in case of a primary renal neoplasm ,