although the prevalence of metastases to the thyroid gland is variable in previous reports , metastasis to the thyroid gland is known to be an uncommon condition . fourteen cases of metastatic tumor to the primary thyroid carcinoma have been reported previously in the literature . the majority of reported primary thyroid carcinomas were papillary thyroid carcinoma ( ptc ) , including follicular variant papillary thyroid carcinoma ( fvptc ) ( 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ) . to our knowledge , there has been no reported case of a tumor metastasizing to medullary thyroid carcinoma ( mtc ) . we report a case of tumor - to - tumor metastasis involving metastatic colonic adenocarcinoma and medullary thyroid carcinoma . a 53-yr old man underwent an anterior resection of his cancerous sigmoid colon and adjuvant chemotherapy on november 14 , 2005 . about one year after surgery , a fluorine-18-fluorodeoxyglucose - positron emission tomography integrated with computed tomography ( f - fdg pet / ct ) scan showed focal hypermetabolism in the right lobe of the thyroid gland ( standardized uptake value , [ suv ] 4 ) and pulmonary nodules in the right lung , suggesting hematogenous metastatic lesions . two years later , a pet scan still revealed a nodule , showing focal activity in the thyroid gland ( suv 2.5 ) ( fig . a thyroid gland ultrasonography showed a marked hypoechoic solid nodule with a lobulated margin and inner microcalcification in the right mid pole , suggesting malignancy ( fig . the patient underwent ultrasound - guided fine needle aspiration biopsy ( fnab ) of the thyroid nodule . serum calcitonin and carcinoembryonic antigen ( cea ) levels were mildly elevated ( 17.3 pg / ml ( reference range : 0 - 10 pg / ml ) for calcitonin ; 29.31 ng / ml ( reference range : 0 - 4.7 ng / ml ) for cea . thyroid stimulating hormone was 2.47 iu / ml ( 0.25 - 4.0 iu / ml ) , thyroglobulin antigens were 9.96 ng / ml ( 0 - 35 ng / ml ) , antithyroglobulin antibodies were 0.19 iu / ml 0 - 0.3 iu / ml ) . the serum level of intact parathyroid hormone was 40.83 pg / ml ( 15 - 65 the 24-hr urine cortisol / metanephrine / cathecholamin levels were within the normal range . rearranged during transfection ( ret ) proto - oncogene mutations were not detected . subsequently , the patient underwent a total thyroidectomy and bilateral central neck dissection . on gross examination , the capsule of the right lobe of thyroid was intact , smooth , and the surface was irregularly bosselated . the cut sections revealed a well - circumscribed , round gray - tan nodular mass , measuring 1.51.2 cm . there is an ill defined white solid mass with central irregular yellow necrosis , measuring 0.80.7 cm in the gray - tan nodular mass ( fig . an immunohistochemical stain of cea and caudal type homeobox protein cdx-2 showed a strong , diffuse positivity in colonic adenocarcinoma . in contrast , the medullary thyroid cancer cells were positive for chromogranin - a and calcitonin and negative for the colonic adenocarcinoma marker . after thyroidectomy , the patient continued palliative chemotherapy for the colon cancer and supportive care . one year later , he died from dyspnea due to the aggravation of pulmonary metastases . while the coincident occurrence of multiple primary malignant tumors in the same host is not unusual , tumor - to - tumor metastasis is a rare phenomenon . ( 13 ) , proposed criteria for the diagnosis of tumor - to - tumor metastasis ; 1 ) the presence of more than one primary malignant tumor must be proved , 2 ) the recipient tumor must be a true neoplasm , and 3 ) the donor malignant tumor must be a true metastasis , with established growth and invasion in the tumor . direct contiguous growth of one tumor into another adjacent tumor ( collision tumor ) , embolism of tumor cells , and metastasis to leukemic nodes are not defined as metastases . previous studies reported the prevalence of metastases to the thyroid gland varied greatly ( 14 , 15 , 16 , 17 ) . however , the previous studies included preexisting or coexisting thyroid conditions , such as benign thyroid diseases ( goiter and adenoma ) , and primary thyroid neoplasms . cases involving metastasis to primary thyroid carcinoma only , have a prevalence rate estimated to be less than 1% , and only 14 cases have been reported in the literature ( table 2 ) . rosai ( 3 ) reported the first documented case of metastatic breast carcinoma to a papillary thyroid carcinoma ( ptc ) in 1992 . the most commonly reported non - thyroid malignancies to metastasize to the thyroid gland are renal cell carcinoma and lung cancer ( 18 ) . colorectal carcinoma was an uncommon donor , accounting for only two of the cases ( 2 , 4 ) . willis ( 19 ) proposed a hypothesis for why the thyroid gland receives few metastatic deposits despite its rich blood supply . according to the willis hypothesis , fast arterial flow through the thyroid and the high oxygen saturation and iodine content of the thyroid gland prevent of metastatic tumor survival in the thyroid ( 19 ) . due to the rarity and complexity of tumor metastasis , some views suggested that the thyroid tumor makes an environment in which metastatic tumor cells can easily grow by altering the normal thyroid structure as stated above ( 20 ) . mtc is a relatively rare type of primary thyroid carcinoma , accounting for only about 5% of all thyroid carcinomas . the majority of previously reported recipient primary thyroid carcinomas were ptc , including fvptc ( 12 among 14 cases ) ( 1 , 2 , 3 , 5 , 6 , 7 , 9 , 11 ) . the other two cases were oncocytic / hurthle cell carcinomas ( 4 , 8) . as far as we know , fnab diagnosis of both primary thyroid malignancy and non - thyroid malignancies metastasizing to the thyroid gland at the same time is difficult and often incorrect ( 18 ) . in most cases , fnab can diagnose primary thyroid carcinoma but not the metastases to the thyroid . in the present case , fnab allowed us to diagnose mtc , but we did not find the colorectal cancer with the technique . after surgical resection of the thyroid gland and several specific stains , we were able to diagnose the tumor - to - tumor metastasis ( mtc and colorectal cancer ) . although tumor - to - tumor metastasis to the primary thyroid carcinoma is very rare , metastasis to the thyroid gland should be considered , when a patient with history of other malignancies presents with a new thyroid finding . in this case , although the patient already had pulmonary metastasis of colonic adenocarcinoma , he underwent surgical treatment . the prognosis of the patient was determined by the coexisting advanced colon cancer . in conclusion , the present patient is the first example of colonic adenocarcinoma metastasizing to medullary carcinoma of the thyroid .
metastasis to the primary thyroid carcinoma is extremely rare . 
 we report here a case of colonic adenocarcinoma metastasis to medullary thyroid carcinoma in a 53-yr old man with a history of colon cancer . 
 he showed a nodular lesion , suggesting malignancy in the thyroid gland , in a follow - up examination after colon cancer surgery . 
 fine needle aspiration biopsy ( fnab ) of the thyroid gland showed tumor cell clusters , which was suspected to be medullary thyroid carcinoma ( mtc ) . 
 the patient underwent a total thyroidectomy . using several specific immunohistochemical stains , 
 the patient was diagnosed with colonic adenocarcinoma metastasis to mtc . to the best of our knowledge , 
 the present patient is the first case of colonic adenocarcinoma metastasizing to mtc . 
 although tumor - tumor metastasis to primary thyroid carcinoma is very rare , we still should consider metastasis to the thyroid gland , when a patient with a history of other malignancy presents with a new thyroid finding.graphical abstract