He underwent a bone marrow transplant surgery.

Participation is optional.

He chose that job of his own free will.

Participation is optional.

He chose to act on his own free will.

A brain tumor was found in his medulla oblongata.

When the myelin sheath of nerve cells is damaged, nerve transmission slows down.

I think seeing a rainbow on the first day of the New Year is an auspicious sign.

He has lived without going against the fate that was decided by his birth.

That auspicious sign was a very good omen for us.