Skeletal muscle metastasis is relatively rare compared with bone metastases. A few cases of radiologically apparent or clinically symptomatic skeletal muscle metastases have been reported in different tumor types [2
]. 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 [5
]. 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% [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 [8
]. Ogiya et al. [8
] 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. Niikura et al. [9
] 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 [10
]. However, skeletal muscle metastasis may be an incidental finding in imaging studies without symptoms [10
]. 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 [5
]. 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 [7
]. 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 [11
]. 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 [7
]. 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 [8
]. Therapeutic options include radiotherapy, chemotherapy, and surgical excision. In previous reports, surgical excision was recommended in selected patients such as those with a painful isolated mass [5
]. 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.