breast cancer is the second most prevalent cancer in women in south korea . over the past few decades , significant advances have been made in adjuvant therapy of breast cancer , but 20% to 30% of patients treated with adjuvant therapy after mastectomy , still experience recurrence of cancer for the rest of their life . recurrence or metastasis can occur in any organ , but metastases to skeletal muscle are rare as the first manifestation of metastasis of breast cancer . a 51-year - old woman was referred to division of medical oncology on march 23 , 2011 , for elevated serum levels of the tumor marker ca 15 - 3 . at that time , she had been taking tamoxifen ( 20 mg daily ) for 6 months following mastectomy in 2009 . she had no symptoms and there was no clinical evidence of tumor recurrence on the breast computed tomography ( ct ) scan , bone scan , and f-18 fluorodeoxyglucose ( fdg ) positron emission tomography / computed tomography ( pet / ct ) performed in february 2011 . the woman 's medical history showed that she had been diagnosed with right breast cancer ( ct2n2m0 ) in january 2009 , and underwent modified radical mastectomy after three cycles of neoadjuvant chemotherapy with docetaxel and epirubicin . an histopathological examination showed invasive ductal carcinoma with nuclear grade 3 , and 4 metastatic lymph nodes out of the 5 dissected axillary lymph nodes . immunohistochemistry studies showed positive staining for the estrogen receptor ( er ) protein , and negative staining for progesterone receptor ( pr ) protein and the human epidermal growth factor receptor 2 ( her2 ) ( score 0 ) . she received an additional three cycles of adjuvant chemotherapy with docetaxel and epirubicin until june 2009 , and then adjuvant radiotherapy on the right chest wall and regional lymphatics . in november 2009 , a 2.3-cm diameter mass was found on surveillance ct of the left breast , and invasive ductal carcinoma was confirmed by core needle biopsy ( ct2n2m0 ) . the expression of er , pr , and her2 was concordant with the prior right breast cancer : er positive , pr negative , and her2 negative ( score 0 ) . she received adjuvant radiotherapy and six cycles of adjuvant chemotherapy with cyclophosphamide , methotrexate , and 5-fluorouracil ( cmf ) until october 2010 . her serum level of ca 15 - 3 increased from 24.10 u / ml in october 2010 to 121.94 u / ml ( normal range , 0 - 28 u / ml ) in march 2011 , when she was presented to our department . because there was no clinical evidence of tumor recurrence , we planned to carefully follow up tumor markers and imaging tests . her serum level of ca 15 - 3 continued to increase to 829.77 u / ml in august 2011 , but there was no evidence of tumor recurrence on the chest ct and bone scan , performed in april and july 2011 . in october 2011 , she complained of intermittent stabbing pain in the epigastric area . on physical examination , there was no palpable mass in her abdomen , but she had mild tenderness with deep palpation in the epigastric region . ct scans showed multiple round or oval masses with the density of soft tissue in the subcutaneous and rectus abdominis muscle of the epigastric region ( figure 1a ) , and enlargement of lymph nodes in the peripancreatic and para - aortic regions . pet / ct also showed multiple nodules of various sizes with increased fdg uptake ( suvmax , 7.1 ) in the upper anterior abdominal muscles ( figure 1b ) , and multiple enlarged lymph nodes with increased fdg uptake ( suvmax , 4.3 ) in the upper para - aortic region . a core needle biopsy of the abdominal wall mass was performed , and a pathological examination revealed metastatic carcinoma ( figure 2a ) , that was positive for er , negative for pr , and positive for her2 ( score 3 ) . thus , the her2 status was discordant between the primary breast cancer and the metastatic carcinoma . she received six cycles of trastuzumab and docetaxel chemotherapy followed by three cycles of trastuzumab monotherapy . after the third and sixth cycles of combination chemotherapy , follow - up ct scans showed partial regression of multiple metastatic masses in the subcutaneous and rectus abdominis muscles . however , after an additional three cycles of trastuzumab monotherapy , follow - up ct scans showed an increase in size of the metastatic carcinoma in the subcutaneous and rectus abdominis muscles , and her disease progressed . a 52-year - old woman visited our hospital with a palpable left neck mass in april 2010 . her medical history showed that she had been diagnosed with left breast cancer ( ct2n0m0 ) in december 2004 , and she underwent modified radical mastectomy of the left breast in our hospital . an histopathological examination showed invasive ductal carcinoma with nuclear grade 2 , and the absence of axillary lymph node metastases . immunohistochemistry studies showed positive staining for er and pr and negative staining for her2 ( score 0 ) . she received five cycles of adjuvant cmf chemotherapy followed by tamoxifen ( 20 mg daily ) for 2 years . we performed ct scans of the neck and breast , bone scan , and pet / ct . ct scans showed multiple enlarged lymph nodes in the left axillary region and the left lower cervical chain , and pet / ct showed multiple conglomerated enlarged lymph nodes with high fdg uptake ( suvmax , 7.02 ) in the left axillary , retropectoral , subclavian , internal mammary , and cervical lymph nodes . there were no metastatic sites except the lymph nodes . a core needle biopsy of the left axillary lymph node was performed , and pathological examination revealed metastatic adenocarcinoma , which was positive for er , negative for pr , and equivocal for her2 ( score 2 ) . she was postmenopausal based on the serum follicle - stimulating hormone level , and she started receiving the nonsteroidal aromatase inhibitor letrozole in may 2010 . she also received radiotherapy to her left chest wall and regional lymphatics including the cervical chain ( total dose , 5,040 cgy ) between august 18 and september 30 , 2010 . follow - up ct scans showed a complete regression of the previously invaded lymph nodes in november 2010 . about 10 months later , she complained of a mild throbbing pain in the right buttock area that was aggravated by walking . on physical examination , there was no palpable mass and tenderness in the buttock area . ct scans of the abdomen - pelvis and pet / ct were performed in august 2011 . abdomen and pelvis ct scans showed no notable findings , but pet / ct showed multiple new enlarged lymph nodes with increased fdg uptake ( suvmax , 15.4 ) in the right iliac chain and a nodular lesion of about 2.5 cm in diameter ( suvmax , 20.1 ) in the right gluteus maximus muscle . we decided to switch from letrozole to the steroidal aromatase inhibitor exemestane on disease progression . three months later , a follow - up abdominal ct scan showed no discernible change , so the patient continued to take exemestane . after 6 months of taking exemestane , pet / ct showed an increase in size of the right gluteal muscle mass and right iliac lymph nodes ( figure 3a ) . there was no evidence of local recurrence or other distant metastases . based on pet / ct images , a poorly defined , mildly enhanced mass of 3 cm in diameter was identified in the right gluteus maximus muscle upon abdominal ct scan ( figure 3b ) . therefore , we performed ultrasound - guided core needle biopsy of this gluteal muscle mass in february 2012 . pathological examination revealed invasive ductal carcinoma ( figure 2b ) , which was negative for er and pr , and equivocal for her2 ( score 2 ) . gene amplification was not observed by fluorescence in situ hybridization ( her2/cep17 ratio , 1.26 ) . thus , expression of er showed discordance between the primary breast cancer and the metastatic carcinoma . after two cycles of chemotherapy , a follow - up abdominal ct scan showed partial regression of the gluteal muscle mass and iliac lymph nodes , with a decrease in diameter of the gluteal muscle mass from 3.0 to 2.1 cm . a 51-year - old woman was referred to division of medical oncology on march 23 , 2011 , for elevated serum levels of the tumor marker ca 15 - 3 . at that time , she had been taking tamoxifen ( 20 mg daily ) for 6 months following mastectomy in 2009 . she had no symptoms and there was no clinical evidence of tumor recurrence on the breast computed tomography ( ct ) scan , bone scan , and f-18 fluorodeoxyglucose ( fdg ) positron emission tomography / computed tomography ( pet / ct ) performed in february 2011 . the woman 's medical history showed that she had been diagnosed with right breast cancer ( ct2n2m0 ) in january 2009 , and underwent modified radical mastectomy after three cycles of neoadjuvant chemotherapy with docetaxel and epirubicin . an histopathological examination showed invasive ductal carcinoma with nuclear grade 3 , and 4 metastatic lymph nodes out of the 5 dissected axillary lymph nodes . immunohistochemistry studies showed positive staining for the estrogen receptor ( er ) protein , and negative staining for progesterone receptor ( pr ) protein and the human epidermal growth factor receptor 2 ( her2 ) ( score 0 ) . she received an additional three cycles of adjuvant chemotherapy with docetaxel and epirubicin until june 2009 , and then adjuvant radiotherapy on the right chest wall and regional lymphatics . in november 2009 , a 2.3-cm diameter mass was found on surveillance ct of the left breast , and invasive ductal carcinoma was confirmed by core needle biopsy ( ct2n2m0 ) . the expression of er , pr , and her2 was concordant with the prior right breast cancer : er positive , pr negative , and her2 negative ( score 0 ) . she received adjuvant radiotherapy and six cycles of adjuvant chemotherapy with cyclophosphamide , methotrexate , and 5-fluorouracil ( cmf ) until october 2010 . her serum level of ca 15 - 3 increased from 24.10 u / ml in october 2010 to 121.94 u / ml ( normal range , 0 - 28 u / ml ) in march 2011 , when she was presented to our department . because there was no clinical evidence of tumor recurrence , we planned to carefully follow up tumor markers and imaging tests . her serum level of ca 15 - 3 continued to increase to 829.77 u / ml in august 2011 , but there was no evidence of tumor recurrence on the chest ct and bone scan , performed in april and july 2011 . in october 2011 , she complained of intermittent stabbing pain in the epigastric area . on physical examination , there was no palpable mass in her abdomen , but she had mild tenderness with deep palpation in the epigastric region . ct scans showed multiple round or oval masses with the density of soft tissue in the subcutaneous and rectus abdominis muscle of the epigastric region ( figure 1a ) , and enlargement of lymph nodes in the peripancreatic and para - aortic regions . pet / ct also showed multiple nodules of various sizes with increased fdg uptake ( suvmax , 7.1 ) in the upper anterior abdominal muscles ( figure 1b ) , and multiple enlarged lymph nodes with increased fdg uptake ( suvmax , 4.3 ) in the upper para - aortic region . a core needle biopsy of the abdominal wall mass was performed , and a pathological examination revealed metastatic carcinoma ( figure 2a ) , that was positive for er , negative for pr , and positive for her2 ( score 3 ) . thus , the her2 status was discordant between the primary breast cancer and the metastatic carcinoma . she received six cycles of trastuzumab and docetaxel chemotherapy followed by three cycles of trastuzumab monotherapy . after the third and sixth cycles of combination chemotherapy , follow - up ct scans showed partial regression of multiple metastatic masses in the subcutaneous and rectus abdominis muscles . however , after an additional three cycles of trastuzumab monotherapy , follow - up ct scans showed an increase in size of the metastatic carcinoma in the subcutaneous and rectus abdominis muscles , and her disease progressed . a 52-year - old woman visited our hospital with a palpable left neck mass in april 2010 . her medical history showed that she had been diagnosed with left breast cancer ( ct2n0m0 ) in december 2004 , and she underwent modified radical mastectomy of the left breast in our hospital . an histopathological examination showed invasive ductal carcinoma with nuclear grade 2 , and the absence of axillary lymph node metastases . immunohistochemistry studies showed positive staining for er and pr and negative staining for her2 ( score 0 ) . she received five cycles of adjuvant cmf chemotherapy followed by tamoxifen ( 20 mg daily ) for 2 years . we performed ct scans of the neck and breast , bone scan , and pet / ct . ct scans showed multiple enlarged lymph nodes in the left axillary region and the left lower cervical chain , and pet / ct showed multiple conglomerated enlarged lymph nodes with high fdg uptake ( suvmax , 7.02 ) in the left axillary , retropectoral , subclavian , internal mammary , and cervical lymph nodes . a core needle biopsy of the left axillary lymph node was performed , and pathological examination revealed metastatic adenocarcinoma , which was positive for er , negative for pr , and equivocal for her2 ( score 2 ) . she was postmenopausal based on the serum follicle - stimulating hormone level , and she started receiving the nonsteroidal aromatase inhibitor letrozole in may 2010 . she also received radiotherapy to her left chest wall and regional lymphatics including the cervical chain ( total dose , 5,040 cgy ) between august 18 and september 30 , 2010 . follow - up ct scans showed a complete regression of the previously invaded lymph nodes in november 2010 . about 10 months later , she complained of a mild throbbing pain in the right buttock area that was aggravated by walking . on physical examination ct scans of the abdomen - pelvis and pet / ct were performed in august 2011 . abdomen and pelvis ct scans showed no notable findings , but pet / ct showed multiple new enlarged lymph nodes with increased fdg uptake ( suvmax , 15.4 ) in the right iliac chain and a nodular lesion of about 2.5 cm in diameter ( suvmax , 20.1 ) in the right gluteus maximus muscle . we decided to switch from letrozole to the steroidal aromatase inhibitor exemestane on disease progression . three months later , a follow - up abdominal ct scan showed no discernible change , so the patient continued to take exemestane . after 6 months of taking exemestane , pet / ct showed an increase in size of the right gluteal muscle mass and right iliac lymph nodes ( figure 3a ) . there was no evidence of local recurrence or other distant metastases . based on pet / ct images , a poorly defined , mildly enhanced mass of 3 cm in diameter was identified in the right gluteus maximus muscle upon abdominal ct scan ( figure 3b ) . therefore , we performed ultrasound - guided core needle biopsy of this gluteal muscle mass in february 2012 . pathological examination revealed invasive ductal carcinoma ( figure 2b ) , which was negative for er and pr , and equivocal for her2 ( score 2 ) . gene amplification was not observed by fluorescence in situ hybridization ( her2/cep17 ratio , 1.26 ) . thus , expression of er showed discordance between the primary breast cancer and the metastatic carcinoma . after two cycles of chemotherapy , a follow - up abdominal ct scan showed partial regression of the gluteal muscle mass and iliac lymph nodes , with a decrease in diameter of the gluteal muscle mass from 3.0 to 2.1 cm . . a few cases of radiologically apparent or clinically symptomatic skeletal muscle metastases have been reported in different tumor types [ 2 - 4 ] . it has been suggested that skeletal muscle is relatively resistant to metastatic disease because of its hostile microenvironment . factors that make skeletal muscle hostile include muscle motion resulting in mechanical tumor destruction , inhospitable muscle ph , the muscle 's ability to remove tumor - produced lactic acid associated with angiogenesis , and the activation of lymphocytes and natural killer cells in skeletal muscle . however , according to data from a large autopsy series , subclinical metastases to skeletal muscle may be more common than generally thought , and the incidence has been reported to range from 0.2% to 17.5% . the underdiagnosis of skeletal muscle metastases in clinical practice may be related to the observation that they are often manifested as part of the disseminated disease and furthermore , in some cases , it is difficult to detect skeletal muscle metastasis with the generally used ct scans . skeletal muscle metastasis from breast cancer is also uncommon , and is often manifested as disseminated disease with multiple organ metastasis . reported a case of breast cancer with an isolated metastasis into the abdominal wall muscle , with a review of 13 previously reported cases of which four presented as an isolated skeletal muscle metastasis without other distant metastases . the metastatic sites were the paraspinal muscle , scalene muscle , iliopsoas muscle , and extraocular muscle . in our patients , one relapsed with an abdominal wall muscle metastasis without other distant organ metastasis , and the other showed gluteal muscle metastasis with involvement of iliac lymph nodes . we performed a muscle biopsy for proper diagnosis , and a pathological examination revealed diffuse infiltration by cancer cells with disruption of the muscle fascicles . recently , several studies have reported a discordant her2 status between primary and metastatic sites in breast cancer . reported that the incidence of discordance for er , pr , and her2 between primary and metastatic tumors was 18.4% , 40.3% , and 13.6% , respectively . however , to the best of our knowledge , no previous reports examined the hormone receptor or her2 status of the primary tumor and metastatic skeletal muscle lesions . in our patients , we confirmed discordant er , pr , and her2 status between the primary breast cancer and the metastatic skeletal muscle lesions . therefore , our cases support the need for the biopsy of metastatic skeletal muscle lesions to determine accurate diagnosis and proper management . skeletal muscle metastasis generally manifests itself as a painful mass in the involved area ; our patients also complained of mild muscular pain . however , skeletal muscle metastasis may be an incidental finding in imaging studies without symptoms . therefore , more careful monitoring of imaging results for musculoskeletal structures is required when evaluating the response . ct is generally used for staging and response evaluation , and provides information about the extent of the mass in skeletal muscle and its relationship with adjacent structures . however , most of the body 's musculature is outside the scanned region of the chest and abdominal ct , and some lesions may be isodense compared with the surrounding muscle , making it difficult to differentiate the metastatic lesion from the surrounding muscle . magnetic resonance imaging ( mri ) is the gold standard for imaging muscle disease ; it shows features of muscle metastases that are relatively typical , consisting of round or oval intramuscular masses with well - defined margins and marked enhancement . however , mri is not commonly used in daily practice because it has high cost , long scanning times , and a limited field of view . the number of intramuscular metastases detected has increased since the introduction of pet / ct . a recent study , in which unsuspected intramuscular metastases were found by pet / ct in 20 of 39 cases , showed that pet / ct has higher sensitivity than mri for detecting skeletal muscle metastases . in our second patient , we found a gluteal muscle metastasis by pet / ct that was initially missed on ct . therefore , pet / ct may be a sensitive tool for detecting skeletal muscle metastases . the prognosis and appropriate treatment of skeletal muscle metastasis are currently uncertain . , surgical excision was recommended in selected patients such as those with a painful isolated mass . we treated our patients with chemotherapy and confirmed a partial response and relief of symptoms . further studies are needed to determine the prognosis and proper diagnostic and therapeutic strategies for skeletal muscle metastasis in breast cancer .
the skeletal muscle is an unusual site for metastasis from breast cancer . 
 we present two cases of breast cancer that relapsed as skeletal muscle metastasis without other distant organ metastasis . 
 we performed the core needle biopsy of metastatic sites and confirmed discordance in estrogen receptor , progesterone receptors , and human epidermal growth factor receptor 2 expression between primary breast cancer and skeletal muscle metastases . in the second case 
 , we found the skeletal muscle metastasis through f-18 fluorodeoxyglucose positron emission tomography / computed tomography scans ( pet / ct ) . 
 intramuscular hot spots on pet / ct scans should be considered as a sign of metastasis even in the absence of abnormalities on computed tomography scans . 
 our patients received systemic chemotherapy , and showed a partial response . 
 further studies are needed to determine the prognosis and proper management of isolated skeletal muscle metastasis in breast cancer .