Matters of the Heart: Why Are Cardiac Tumors So Rare?
Do a Google or PubMed search for “cardiac tumor,” and your results will be littered with individual case reports and little more. There are no clinical trial results, meta-analyses, or treatment guidelines.
Because, although the heart may be the ultimate emblem of love, compassion, and chocolate-themed holidays, it also has another distinction: a near immunity to cancer. And given the heart’s importance in the body, that’s a fortunate fact of life.
However, it does raise the question: Why is this large and infinitely important component of anatomy such an inhospitable host to the leading cause of mortality for those aged 85 and younger? The answer, it appears, can be found in the highly specialized and most abundant cell in this muscle-laden organ, the cardiac myocyte.
Few, but Deadly
Cardiac primary tumors, those originating in the heart itself, are extremely rare. In published autopsy series, the high-end incidence of such tumors is about one quarter of one percent. The majority of diagnosed cardiac tumors are benign. In adults, a somewhat mushy, gelatinous type of tumor called a myxoma is the most common; in infants and children, rhabdomyomas predominate, typically associated with the syndrome tuberous sclerosis.
According to Dr. Robert J. Cusimano, a cardiac surgeon at Toronto General Hospital, malignant heart tumors are most often metastases from primary tumors in nearby organs, such as the kidneys or lungs.
“If there are metastases to the heart, the prognosis is pretty bad,” said Dr. Cusimano, who lightheartedly refers to himself as a “cardiac oncologist,” because patients with these tumors are often referred to him. Even then, his entire division may see only 12 benign tumors in a given year. He is personally involved with about 5 to 10 cardiac cancer cases per year, of which one or two are primary malignancies.
Angiosarcomas are the most common malignant primary cardiac tumor. According to published case reports, including one co-authored by Dr. Cusimano, using chemotherapy and/or radiotherapy to shrink the primary tumor and eliminate any micrometastases followed by surgery to remove the primary tumor has had some success.
The Tell-Tale Cell
Unlike other damaged organs, the heart seems mostly incapable of mending injured tissue. And that, according to leading cardiac researchers, is because the cells that compose the muscle itself, cardiac myocytes, are terminally differentiated.
In other words, these cells reach a point very early on in a person’s life where they permanently exit the cell cycle and stop dividing. After that, further growth occurs by expansion in cell size, not through cell division. This differs, for instance, from the epithelial cells that line other organs, which, in response to certain stimuli, actively divide and, when necessary, grow in number.
This “very tight cell cycle control of cardiomyocytes” acts as a double-edged sword, explained Dr. Deepak Srivastava, director of the Gladstone Institute of Cardiovascular Disease at the University of California, San Francisco. Not only does it “prevent them from ever re-entering the cell cycle to proliferate and repair damaged tissue,” but it may also explain why “they are so resistant to tumor formation,” he said.
With so little proliferative activity, added Dr. John E. Tomaszewski, from the Department of Pathology and Laboratory Medicine at the University of Pennsylvania Medical School, “the opportunity for abnormalities in cell cycle kinetics which characterizes tumors in so many other body sites is just not an issue in the heart.”
Given their extreme rarity, there is little in the way of any organized effort to further research on how and why cardiac tumors arise or how best to treat them. For now, Dr. Cusimano believes, the wisest avenue to help improve outcomes is to funnel patients to centers that have experience treating patients with cardiac tumors.
Routinely sending patients to these hospitals will ensure that “surgeons and oncologists can get more experience with these cases,” Dr. Cusimano said. “That’s the only way we can move forward with these types of cancers.”