Although cardiac metastasis from soft tissue sarcomas has been mentioned in the literature [11–15], early diagnosis of cardiac metastasis from soft tissue sarcomas is difficult for musculoskeletal oncologists because of its rarity. As to MLS, approximately thirty cases with metastasis to the heart, including pericardium, have been reported so far [15]. Most of these patients had disseminated disease. The time intervals between the onset of primary disease and cardiac metastasis were reported to be relatively long, ranging from 4 to 25 years [16, 17]. The initial site of metastatic disease in our patient was the heart. He had a solitary cardiac tumor without obvious symptoms that presented two years after the primary surgery in the absence of metastases at other sites. Only 10% of patients with cardiac metastasis have been reported to show any symptoms [11]. This made it difficult to recognize the possibility of cardiac metastasis in our patient. On the other hand, he presented with slight dyspnea on exertion at the diagnosis of the second cardiac metastasis, probably because of inflow tract obstruction in the right atrium. Clinical symptoms depend on the location and extent of the lesion, which variously affect cardiac function. Careful physical examination alone is not sufficient for the early diagnosis of cardiac metastasis.
The lung is the most carefully monitored organ for the development of metastases in soft tissue sarcomas. However, pulmonary metastases often occur with a time lag behind extrapulmonary metastases in patients with MLS. Estourgie et al. reported that 55% of MLS patients with metastatic disease had extrapulmonary metastases [7]. Nishida et al. noted that 50% of metastases occurred in extrapulmonary sites, even in MLS patients with round cell components of less than 5% [10]. The reason for the tendency to metastatic spread in extrapulmonary sites was considered to be because of the affinity for adipose tissues such as the retroperitoneum, subcutaneous tissues, and bone marrow [5]. If physicians depend on imaging of the chest and primary site as the initial staging and follow-up studies, a significant number of metastases may be missed due to the high incidence of extrapulmonary metastases in patients with MLS [18]. For the staging of MLS at diagnosis, the 2017 National Comprehensive Cancer Network (NCCN) guidelines recommended chest imaging and abdominal/pelvic CT as well as total spine MRI [19]. Recent studies suggest that whole-body MRI is the most reliable modality for surveillance of all likely sites of extrapulmonary metastases [6, 20].
Positron emission tomography is used as an alternative imaging modality to screen for metastases in patients with malignancies. However, metastatic lesions of MLS are likely to have low uptake on Positron emission tomography [6]. False-negative results of Positron emission tomography in patients with MLS have been widely documented, with a reported sensitivity as low as 14% for the detection of spinal metastases [18].
When a cardiac metastasis is found incidentally, it is usually incurable. In our patient, the disease in the right atrium was not a recurrence of the first cardiac metastasis in the left ventricle but was considered a metachronous metastatic lesion. Surgical treatment may not be the best option for patients with cardiac metastasis. However, in selected patients with no evidence of distant metastases, surgical excision of the cardiac metastasis, if technically feasible, provides a chance to prolong survival in life-threatening situations such as mechanical obstruction or valvular disorder. If the surgical margin is positive after cardiac metastasectomy, radiotherapy can be useful as a supplemental procedure since MLS is regarded to be radiosensitive. Although surgery is the mainstay of treatment for localized MLS, systemic therapy is often given to patients with locally-advanced or metastatic MLS. Currently, doxorubicin, eribulin, and trabectedin are being used widely with some success in patients with advanced MLS.
To summarize, this report described a patient with MLS in the thigh, accompanied by ectopic and metachronous cardiac metastases. Periodic examinations with chest CT with careful attention may be able to document the presence of cardiac disease before the occurrence of severe complications. In patients with a solitary cardiac metastasis, especially when detected at an early stage, surgical excision, followed by some adjuvant therapies, may contribute to a reasonable outcome in patients with MLS.