While most brains were natural, the Rani once constructed a giant brain to absorb the mental energies of various individuals and compute the equations for a time manipulator. (TV: Time and the Rani)
In certain circumstances, the brain could be transplanted out of the biological entity into another vessel. This was done with Morbius' brain after his body was destroyed. His ally, Mehendri Solon, had to find a head large enough to fit the brain and only found the right match when he met the Fourth Doctor. (TV: The Brain of Morbius) On Pete's World, the Cybermen there were created by having the brain transplanted from the human form into the cybernetic suit. (TV: Rise of the Cybermen)
Lady Cassandra O'Brien.Δ17 was able to survive the destruction of the sheet of skin which formed her "body" because her brain was undamaged, allowing her to constitute a new skin body with another skin sheet from her old human body - this one taken from her backside. (TV: The End of the World, New Earth)
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