Supplemental Table 1. Level of evidence and conclusion of studies.

Authors	Study Year	LOE (1a-5)	Study Design	Subjects	Authors’ Conclusions
Tandon20	2019	4	CR	1	Patients with current malignancies in addition to cardiac symptoms and/or ECG changes should warrant investigation for possible cardiac metastasis. Different imaging modalities such as CMRI, PET-CT, and echocardiography play an important role in diagnosis and management.
Nagata29	2012	4	CR	1	Cardiac metastasis from LSCC is typically a difficult diagnosis due to the lack of clinical findings in the early stages. However, as the metastases become more advanced, clinical signs may occur, resulting in a variety of cardiac pathologies. Many of the cardiac sequelae result in ECG changes, emphasizing the importance of ECG testing in these patients.
Onwuchekwa30	2012	4	CS	2	These cases show the importance of evaluating the heart with echocardiography to assess for cardiac metastasis in patients with head and neck cancer and confirmed metastatic disease.
Browning31	2015	4	CR	1	It may be necessary to evaluate patients with lingual malignancies with imaging to rule out metastases. Lingual squamous cell carcinoma metastases are commonly asymptomatic, and thus, imaging is important in establishing the diagnosis.
McKeag32	2013	4	CR	1	This case shows that cardiac metastasis from LSCC can present without symptoms or characteristic metastasis examination findings such as lymphadenopathy.
Hans33	2009	4	CR	1	Clinicians may consider the potential for cardiac metastases in patients with a history of head and neck malignancies who display new-onset cardiovascular symptoms. These patients typically have a very poor prognosis.
Kumar34	2018	4	CR	1	Clinicians should consider cardiac metastasis and hypercalcemia-induced rhythm disturbances in patients with a history of LSCC malignancy or current LSCC malignancy and concurrent ECG changes. It may be necessary to check calcium levels and treat the patient if levels are abnormal.
Nanda35	2019	4	CR	1	This case shows the importance of an initial echocardiogram in addition to the integration of PET and CT scans to detect LSCC cardiac metastasis.
Werbel36	1985	4	CR	1	This case demonstrates the importance of using echocardiography in patients with LSCC and atypical angina and/or a new heart murmur. Echocardiography is a good initial step to look for cardiac metastases.
Puranik37	2014	4	CS	2	In asymptomatic patients with a history of cancer, whole body dual imaging with PET-CT can show undiagnosed metastatic sites.
Makhija38	2009	4	CR	1	This case report stresses the lack of appropriate screening protocols for patients with LSCC and possible cardiac metastases. The report recommends a transesophageal echocardiogram as the best initial screen, as it is a sensitive test.
Wadler39	1985	4	CR	1	This case report emphasizes that there are many other factors besides morphology and anatomic extent that can be used to determine LSCC prognosis.
Shafiq40	2019	4	CR	1	This case report shows that the lack of symptoms in cardiac metastasis from LSCC can result in delayed cardiac imaging and subsequently delayed diagnosis.
Dredla41	2014	4	CR	1	Patients with a history of cancer who have a cerebrovascular accident should have cardiac imaging to rule out a cardiac cause, such as cardiac metastases.
Yadav42	2014	4	CR	1	This case highlights the importance of an ECG evaluation in patients with a cancer diagnosis. This can provide diagnostic clues for potential cardiac metastases. Diagnostic confirmation can be done using multiple imaging modalities, such as echocardiography, CT scan, and CMRI.
Zatuchni43	1981	4	CS	2	LSCC cardiac metastasis can result in myocardial or pericardial injury, shown by ST-elevations on ECG. ST-elevation may be a potential sign of cardiac metastasis.
Shafiq44	2019	4	CR	1	This case emphasizes the importance of the clinician maintaining a high index of suspicion to identify patients with metastatic LSCC. A high risk patient may need surveillance imaging with echocardiography, CMRI, and/or PET-CT.
Kim45	2019	4	CR	1	LSCC patients with cardiac symptoms may benefit from a multimodal approach consisting of PET-CT, CMRI, echocardiography, and ECG. These imaging modalities are used to confirm the diagnosis and establish the location and extent of disease, which can guide management.
Demir46	2018	4	CR	1	This case shows the importance of keeping myocardial metastases on the differential diagnosis when patients with known malignancies present with ECG changes. Cardiac metastases can sometimes present like an acute myocardial infarction on ECG testing.
Chua47	2017	4	CR	1	LSCC is more common in Asian countries. It is always important to consider metastasis of LSCC to the heart and other organs, especially if patients with known LSCC present with cardiac symptoms.
Duband48	2011	4	CR	1	Clinicians may consider an initial cardiac evaluation and serial follow-up ECGs in patients with SCC of the base of the tongue.
Ito49	2007	4	CR	1	Sudden death in patients with LSCC should prompt clinicians to consider the potential of cardiac metastases.
Rivkin50	1999	4	CR	1	Patients with cancer who develop new cardiac symptoms should prompt the clinician to evaluate for potential cardiac metastases. Evaluation should consist of echocardiography and/or CMRI. Treatments are typically only palliative, as cardiac metastases possess a poor prognosis.
Matsuyama51	1963	4	CR	1	This case reports a rare finding of widespread LSCC metastasis with cardiac metastases isolated to the myocardium. This is a rare finding since most metastases affect the pericardium.
Malekzadeh19	2017	4	CR	1	Patients with a history of head and neck malignancy who present with cardiac symptoms may prompt the clinician to evaluate for cardiac metastases.
Cabot52	1971	4	CR	1	There are often no clinical signs of primary LSCC or metastatic LSCC to the heart, making the ante-mortem diagnosis very difficult.
Reddy53	2014	4	CR	1	Management of tumor-induced acute coronary occlusion is similar to acute coronary syndrome occlusion, utilizing coronary angiography and percutaneous coronary intervention.
Ewald54	1997	4	CR	1	Patients with a history of malignancy and unexplained arrhythmias or ECG changes may prompt the clinician to consider cardiac metastases on the differential diagnosis. Cardiac metastasis may present similarly to many types of cardiac pathology.

Abbreviations: CMRI = cardiac magnetic resonance imaging; CR = case report; CS = case series; CT = computed tomography; ECG = electrocardiogram; LSCC = lingual squamous cell carcinoma; PET = positron emission tomography; PET-CT = positron emission tomography-computed tomography; SCC = squamous cell carcinoma.