the incidence of primary heart tumors is very low ; they are detected in only 0.1% of all autopsies . metastatic heart tumors arise from a number of different primary tumors such as lung , breast , and hematological malignancies 2 . more than 90% of changes in the heart are asymptomatic ; therefore , they remain undetected because the heart is rarely investigated if no symptoms are present 3 . a few cases of sudden death caused by lifethreatening arrhythmia derived from myocardial metastasis have been reported in the literature 4 . , there have been no reports concerning the stabilization of circulation dynamics by mounting an implantable cardioverter defibrillator ( icd ) for lifethreatening arrhythmia derived from myocardial metastasis . a left segmental mandibulotomy and reconstructive surgery with plates was performed in 2014 . at the time of the surgery , there were no abnormal findings on computed tomography ( ct ) and positron emission tomography / ct ( pet / ct ) . s1 monotherapy was therefore administered as postoperative chemotherapy but was discontinued 6 months later because of extreme nausea . over the next 6 months , the patient complained of cough and neck pain and developed swelling in the left cheek . chest xray and ct revealed a tumor in the right lower lung , which was suggestive of lung cancer . a transbronchial lung biopsy was performed via bronchoscopic examination , which revealed squamous cell carcinoma . although the previously resected gingival tumor was proven to be squamous cell carcinoma , it was difficult to differentiate the origin of the primary cancer ( fig . the swelling in the left cheek was thought to represent a recurrence of the gingival tumor , and cutaneous nodules were found in the left submandibular region . the patient was then referred to our hospital for ongoing treatment . upon physical examination , the patient 's body temperature was 36.9c , pulse rate was 46 beats per min , and systolic blood pressure was 154 mmhg . respiratory sounds were reduced in the right lower chest , and she had a clear sensorium . swelling of the left cheek was clearly evident , and a cutaneous tumor was found in the left submandibular region . laboratory examinations of her blood revealed no abnormal elevation of tumor markers related to lung cancer ( table 1 ) . an electrocardiogram ( ecg ) performed upon admission showed complete atrioventricular block , which had not been previously apparent ( fig . 2 ) . echocardiography revealed a nodule in the atrial septum , which was indicative of myocardial metastasis ( fig . pet / ct further revealed an accumulation of 18ffluorodeoxyglucose in the atrial septum and left ventricle , a tumor in the left cheek , and a tumor in the right lower lobe and cervical vertebra ( fig . clinically , these results suggested that the tumor in the left cheek and the cutaneous tumor in the left submandibular region represented a recurrence of gingival cancer because these tumors were located close to the principal gingival cancer , which had been previously resected . the principal differential diagnosis of the tumor in the right lower lobe was either metastasis from the original gingival cancer or primary lung cancer . swelling of the mediastinal lymph nodes , cavity formation in the tumor , and the absence of multiple lung metastases suggested a diagnosis of primary lung cancer , although we could not completely rule out gingival cancer at this stage . as a hypothetical diagnosis , the patient was diagnosed with squamous cell lung cancer [ clinical t2an2m1b ( oss ) stage iv ] . at this point , we did not perform a biopsy of the tumor in the left cheek or the cutaneous nodule in the left submandibular region . the right lower lung , myocardial , and cutaneous tumors in the left submandibular region all showed welldifferentiated squamous carcinoma . laboratory data upon hospital admission electrocardiogram ( ecg ) upon admission showed a complete atrioventricular block that had not been apparent earlier ( a ) . echocardiograph showing a nodule in the atrial septum , which was suspicious of myocardial metastasis ( a).the heart tumor increased in size following the first cycle of chemotherapy ( b ) . scout image of positron emission tomography / computed tomography ( a ) showing the accumulation of 18ffluorodeoxyglucose in a tumor in the left cheek ( b ) , a tumor in the right lower lobe ( c ) , and myocardial tumor ( d ) . she was rescued by resuscitation , and an icd was mounted in her chest to prevent lifethreatening arrhythmia 3 days after ventricular fibrillation . the anticancer drug nabpaclitaxel in combination with carboplatin was administered to prevent the recurrence of the gingival and lung tumors 7 days after icd mounting . however , although the status of the gingival and lung tumor remained stable , the heart tumor progressed after the first cycle of chemotherapy ( fig . the patient 's general condition deteriorated , and she died 4 weeks after the anticancer drug was administered . upon autopsy , specimens of the cutaneous nodule in her left submandibular region and the tumor in the right lower lobe revealed welldifferentiated squamous carcinoma . immunostaining of these two structures was closely matched , and the two tumor types were thus likely to be derived from a common primary cancer ( table 2 ) . dysplastic cells were not detected in the bronchoepithelial cells harvested from the tumor in the right lower lobe . myocardial metastases in the atrial septum and left ventricle were confirmed to be welldifferentiated squamous carcinoma ( fig . 1 ) and had infiltrated the atrioventricular node ( fig . other areas of metastases were confirmed in the lungs , liver , and kidneys . ultimately , the cause of her death was cancer , mainly due to the progression of myocardial metastases . immunostaining of biopsies taken from the cutaneous tumor in the left submandibular region and right lower lung . in this case , the patient underwent a left segmental mandibulotomy for gingival cancer . at the time of the surgery after a period of approximately 12 months , the patient developed swelling in her left cheek , a cutaneous nodule in the left submandibular region , and a tumor in the right lower lobe . it was difficult to differentiate whether the primary cancer responsible for these lesions was a recurrence of the gingival or lung cancer because the tissue diagnosis and immunostaining from all tissue types showed welldifferentiated squamous carcinoma . we finally determined that all of the lesions were due to the recurrence of gingival cancer rather than primary lung cancer because of the history of gingival cancer , and no dysplastic cells were detected in bronchoepithelial cells harvested from the tumor in the right lower lobe . the frequency of tumor metastasis to the heart is reported to be 7.1% in cancer patients and 2.3% of all autopsies 5 , whereas primary cardiac tumors are reported in 0.1% of all autopsies . the frequency of tumor metastasis to the heart is approximately 20 times that of primary cardiac tumors . reports have cited several sources for the primary lesion in cases involving tumor metastasis to the heart , including lung cancer , breast cancer , malignant melanoma , malignant lymphoma , and leukemia 6 . the frequency of lung metastasis in cases that had metastasized from head and neck cancer was reported to be 83.4% and 86.7% , respectively , and all of those patients died within a year . although multidisciplinary treatment was performed in these cases , curative therapies have not yet been established for head and neck cancer . autopsies have shown that the frequency of tumor metastasis to the heart from primary oral cancer is approximately 7% 8 . in other reports , it was shown that oral cancer metastasized to the node in the myocardium , and metastasis was found in the heart cavity in a form resembling vegetation 8 , 9 . on the other hand , the frequency of tumors metastasizing to the heart in lung cancer is much higher , approximately 1836% 10 . in 67 cases of myocardial metastasis or metastasis to the pericardium in lung cancer , 59 cases exhibited metastasis in only the pericardium , whereas one case exhibited metastasis in the myocardium alone 11 . little is known regarding the mechanisms underlying metastasis to the heart , although one report showed that 18 of 23 cases of myocardial metastasis or metastasis to the pericardium in lung cancer were caused by retrograde lymphatic metastasis 12 . in our patient , myocardial metastasis in both the atrial septum and left ventricle was confirmed to represent differentiated squamous carcinoma . lymph ducts surrounding the myocardial metastases were not invaded by cancer , which suggested that myocardial metastases had developed by hematogenous metastasis . because infiltration to the pericardium could not be confirmed on autopsy , it was clear that carcinomatous pericarditis did not exist . therefore , our case was not consistent with hematogenous metastasis from gingival cancer to the myocardium . we therefore diagnosed this case as a recurrence of gingival cancer with metastases to the cheek , skin , lung , and myocardium . with respect to the diagnosis of myocardial metastasis , more than 90% of myocardial metastasis myocardial metastasis was suspected because of various factors , including an stt wave change in the ecg and the appearance of a myocardial lead obstacle such as atrial fibrillation , atrial flutter , premature ventricular contraction , complete atrioventricular block , and acute cardiac infarction 13 , 14 . it has been suggested that in such cases , confirming wall movement decline by echocardiography , evaluating changes in the brightness of the myocardium , and assessing increases in the myocardial thickness of the wall are required 15 . it has also been reported that heart magnetic resonance imaging ( mri ) can detect a tumor in the heart in a more sensitive manner than echocardiography 16 . the use of qualitative assessments of the myocardium in lung cancer by pet / ct has also been reported 17 . when our patient was admitted to the hospital , her ecg showed complete atrioventricular block , an observation that had not been made previously furthermore , a nodule in the heart cavity , which had been confirmed by echocardiography , was strongly indicative of myocardial metastases . in addition , the accumulation of 18ffluorodeoxyglucose was confirmed in the heart by pet / ct . implantable cardioverter defibrillator insertion is a treatment that clearly involves numerous risks , including tumor embolus , and imaging becomes impossible as tumors develop . however , we elected to use an icd implant because we expected better circulation dynamics and an improvement in general condition . at least temporarily , lifethreatening arrhythmia was prevented ; thus , the patient avoided a sudden death . this was considered to be the causative factor underlying the main heart problem . generally , it is difficult for cancer to infiltrate the myocardium because of the dense arrangement of blood vessels and the smooth muscle elastic fibers that make up the tissue . it is also considered that the implantation of cancer cells is obstructed by the persistently high internal pressure in the blood vessels 18 . in addition , it was recently discovered that the hormone atrial natriuretic peptide ( anp ) , which is released from the atrium , acts upon the vascular endothelium and is thought to restrain the metastasis of cancer 19 . therefore , one possible reason for such a low incidence of heart and myocardium metastasis is high anp levels . because cardiac tamponade obstructs the expansion of the atria and ventricles , secretions of anp and brain natriuretic peptide were reported to be restrained under these conditions 20 . as in the case of lung cancer , it has been suggested that the restraint of anp secretion by diastolic obstacles represent a background factor in myocardial metastasis . in our case , although vital signs were stable , cardiac effusion was confirmed , thus creating an environment in which the secretion of anp was restrained . in our patient rather , this was regarded as cardiac effusion related to hypoalbuminemia . with respect to the site of metastasis for myocardial metastasis , there have been several case reports stating that lifethreatening arrhythmia can be derived from myocardial metastasis near the atrial septum 14 , 21 . for example , one report claimed that there was more anp secretion in the right atrium than in the left atrium , more anp secretion in the epicardium than in the endocardium , and anp levels were low in the atrial septum 22 . if the function of anp is to restrain the metastasis of cancer , it follows that cancer will readily be able to develop metastasis at the atrial septum because anp levels are low at this location . chemotherapy can be used to treat myocardial metastasis , and it has been reported that local radiation treatment can be used effectively in the treatment of this condition 23 . however , it has been reported that radiation treatment in the presence of an icd implant could cause permanent damage to semiconductors in the icd 24 . local radiation therapy for the myocardial metastases was not performed for the reasons given earlier . myocardial metastasis often occurs in the final stages of cancer , and the only remaining option for patients is palliative care . in our case , icd implantation was conducted , and the patient 's general condition improved to the point where we could introduce chemotherapy . we believe that there are arguments both for and against the insertion of icd in patients with myocardial metastasis of cancer . in our case , because complete atrioventricular block was confirmed with an ecg , insertion of the icd was adopted from the view point of cardiovascular disease . insertion of the icd itself involves risk ; if complete atrioventricular block is not confirmed on ecg , insertion of an icd may not be essential . in that case , the patient should be followed carefully and assessed frequently to detect any abnormalities on ecg . although it was difficult to ascertain whether insertion of an icd could lead to a good prognosis , it prevented sudden death , which has often been reported in patients with myocardial metastasis of cancer . we believe insertion of an icd would make a significant improvement to the quality of life in these patients and maintain their performance status . in conclusion , we report a rare case of myocardial metastases with lifethreatening arrhythmia originating from gingival cancer . we believe that an icd implantation could be selected as one means of palliative care . further research should be conducted to elucidate mechanisms of myocardial metastasis and to develop an effective therapeutic approach .
key clinical messagewe present a rare case of gingival cancer with pulmonary metastases that developed lifethreatening complete atrioventricular block and ventricular fibrillation as a result of myocardial metastases . 
 this case suggests that implantable cardioverter defibrillators significantly improve the quality of life in these patients and maintain their performance status .