Very sad news. Elisabeth Sladen and Tom Baker were the first characters I watched on Dr. Who in the early 1980's on PBS. I was shocked when she left the show not knowing companions were always leaving the Doctor.
Then I was floored when Tom Baker's Doctor regenerated into Peter Davison's Doctor having never seen that. Then the happy surprise of Ms. Sladen returning for the Five Doctors and the rest is history.
R.I.P. Ms. Sladen, and thank you for the memories.