Cardiac Myxoma

Overview

Primary heart tumours rare (~0.04% autopsies). Benign (decreasing frequency): myxoma, lipoma, fibroelastoma, rhabdomyoma, haemangioma, lymphangioma. Malignant (rarer): rhabdomyosarcoma, angiosarcoma, malignant mesothelioma. Only myxoma gets full treatment.

Myxoma = commonest primary cardiac tumour (~50% of all primary cardiac tumours). Age 30-60. Any chamber/valve, but 90% LEFT ATRIUM.

Nature of the lesion

Debated: true neoplasm vs organising mural thrombus that persists tumour-like. Morphology (myxoid stroma, sparse cellularity, haemorrhage, haemosiderin) is genuinely compatible with either — directly explains the debate.

Morphology

Gross: usually single, <1-10cm, POLYPOID/PEDUNCULATED/spherical/soft/haemorrhagic — resembles organising mural thrombus. Pedunculated form in LA = structural basis for its main danger (mobile mass on stalk → intermittent flow obstruction or embolisation).

Micro: abundant myxoid/mucoid mucin+ stroma. SPARSE cellularity — stellate/spindled/polyhedral cells scattered, occasional multinucleate giant cells. Numerous capillary-sized vessels (cells aggregate around them). Scattered lymphocytes/plasma cells/macrophages. Frequent haemorrhage foci + haemosiderin.

Secondary (metastatic) tumours

MORE COMMON than primary. ~10% of disseminated cancer patients have cardiac mets (haematogenous/lymphatic spread). Order: lung ca > breast ca > lymphoma > leukaemia > melanoma. Direct extension also possible (e.g. lung ca into pericardium/chambers).

Clinical relevance

Neoplasm-vs-thrombus debate isn’t academic — directly follows from genuinely ambiguous morphology (mucoid stroma+sparse cells+haemorrhage+haemosiderin fits both). Useful example of gross/micro findings leaving a lesion’s nature unsettled even after full workup. 90% LA preference + pedunculated/mobile shape = outsized clinical importance despite bland histology — mobile mass at that location structurally positioned to obstruct mitral orifice or embolise, consequence of location/shape not malignant behaviour. Cardiac tumours far more often SECONDARY than primary (10% of disseminated cancer vs 0.04% autopsy primary rate) — useful base-rate correction: cardiac mass + known malignancy → metastasis more likely than primary tumour.

Overview

Primary tumours of the heart are rare (~0.04% of autopsies). In decreasing frequency, the benign primary tumours are: myxoma, lipoma, fibroelastoma, rhabdomyoma, haemangioma, lymphangioma; malignant primary tumours are rarer still (rhabdomyosarcoma, angiosarcoma, malignant mesothelioma). Myxoma is the only primary cardiac tumour with enough independent content to warrant full treatment here.

Myxoma is the most common primary cardiac tumour, accounting for ~50% of all primary cardiac tumours. Occurs mostly between 30–60 years. Can arise in any cardiac chamber or on the valves, but 90% occur in the left atrium.

Pathophysiology / Nature of the Lesion

The tumour’s gross resemblance to an organising mural thrombus is not incidental — it is genuinely debated whether myxoma represents a true neoplasm or is instead an organising mural thrombus that has taken on a persistent, tumour-like growth pattern; some investigators favour the latter interpretation over classifying it as a conventional neoplasm. This uncertainty is directly reflected in, and explained by, the morphology below (myxoid stroma, sparse true tumour cellularity, haemorrhage, haemosiderin — findings equally compatible with an organising thrombus as with a distinct neoplastic process).

Morphology

Gross: usually single, occasionally multiple; size ranges from under 1 cm to 10 cm; polypoid, pedunculated, spherical, soft, and haemorrhagic — an appearance that closely resembles an organising mural thrombus (the basis for the nature-of-the-lesion debate above). Its pedunculated form in a chamber (usually the left atrium) is the structural feature that underlies its main clinical danger — a mobile mass on a stalk within a blood-filled chamber is positioned to intermittently obstruct flow or embolise.

Microscopy:

Abundant myxoid/mucoid intercellular stroma, mucin-positive.
Sparse cellularity — tumour cells are stellate, spindled, or polyhedral, scattered through the stroma; occasional multinucleate tumour giant cells.
Numerous capillary-sized blood vessels, with tumour cells often aggregated around them.
Scattered lymphocytes, plasma cells, macrophages.
Frequent foci of haemorrhage and haemosiderin deposits.

Secondary (Metastatic) Tumours of the Heart

More common than primary cardiac tumours — cardiac metastases occur in about 10% of patients with disseminated cancer, reaching the heart mainly by haematogenous or lymphatic spread. In descending order of frequency, primary sites are: carcinoma of the lung, carcinoma of the breast, malignant lymphoma, leukaemia, malignant melanoma. Direct extension into the pericardium/cardiac chambers can also occur from an adjacent intrathoracic primary, such as lung carcinoma.

Clinical Relevance

The debate over whether myxoma is a true neoplasm or an organised mural thrombus is not just academic — it is the direct consequence of its own morphology (mucoid stroma with sparse cellularity, haemorrhage, haemosiderin) being genuinely ambiguous between the two processes, making this a useful example of how gross and microscopic findings can leave a lesion’s fundamental nature unsettled even after full examination.
Myxoma’s overwhelming left-atrial preference (90%) combined with its pedunculated, mobile gross form is what gives it its outsized clinical importance despite being histologically bland — a mobile intracavitary mass at that specific location is structurally positioned to intermittently obstruct the mitral orifice or to embolise systemically, consequences that follow directly from its location and shape rather than from any malignant behaviour.
Cardiac tumours being far more often secondary than primary (metastases roughly 10% of disseminated cancer cases, versus primary tumours in only 0.04% of autopsies) is a useful base-rate correction — when a cardiac mass is found in a patient with known malignancy, metastasis is statistically the more likely explanation than a primary cardiac tumour.