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The occurrence of Heart cancer (similar, but not the same as Rhabdomyosarcoma) is extremely rare, about 1 per year according to MayoClinic. The reason for this rarity is explained to be the post-mitotic nature of cardiac muscles. Since they stop dividing early in the life cycle of humans and grow only by increasing in volume, they are very less prone to oncogenic activation events caused due to mutation as a result of an error in DNA replication (since they do not replicate their DNA as they do not divide).

Although I am not an expert in tumours and cancers, I think that malignant tumours in nervous tissue, i.e. brain tumours are fairly common, at least more common than heart cancer by several orders of magnitude. But neurons constituting these tissues are also permanently in interphase and hence do not divide. Moreover, secondary malignancies in Brain and nervous tissue is also much more common than secondaries in heart (rare) though metastatic secondary tumours, I believe, do not require oncogenic transformations in the host tissue. A metastatic fragment of a primary malignancy transported by blood is enough to cause secondary tumours.

How then these tumours (primary and metastatic-secondary) are more common in nervous tissue than heart, though both are post mitotic, i.e. permanently in interphase?

Here and here are a few links which give some basics of cardiac malignancies, though they don't address the question quite directly. The study here provides possible explanation of lack of cancer due to high oxygen content.