how does cancer pick when or where or if it will spread?

The "seedlings" or small bits of cancer down to even single cells

spread out in the blood. Some cells find a microenvironment suitable

for continuous replication (cell division / growth). That depends on

each tumor type, but some kinds of cancer have patterns of places

where they "like" to grow. That determines sites of metastases.

It is far more complicated, but this should get you started if you

want to do further research via the internet.

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Every person with any type of lung cancer is different

from another person with the same cancer type.

In a woman with breast cancer, not all the cancer cells are the same.

No one knows why one breast cancer goes to bone while another

goes to lung and yet another goes to brain.

You are asking questions no one can answer at this time.

Depending on what kind of cancer...But normally it is random.