first incarnation of Romana, the only one of the Doctor's companions to be a Time Lord (well, Time Lady) like him, during Tom Baker's term as the Fourth Doctor. She left after a year because her character had become the standard "companion that gets in trouble and needs rescuing by the Doctor", as opposed to one who could hold her own. She died of cancer today at age 62.
There's something about her death that bothers me greatly. Mary Tamm is the fourth actor from Doctor Who to die from cancer in about a year and a half. Nicholas Courtney, who played Brigadier Alistair Gordon Lethbridge-Stewart, died back in February of last year. Elisabeth Sladen, who played Sarah Jane, died last year as well, while Caroline Jane and now Mary Tamm have both died a month apart. The fact that they all died of cancer makes it hard to believe that this is mere coincidence. I know that last sentence probably comes off as conspiratorial, which is unintended. The idea that keeps bopping around in my head is that maybe it was environmental. Maybe there was some kind of chemical or material they used during filming of the old Doctor Who series that was carcinogenic. I don't know, it just seems hard to believe that four people who worked on the same TV show would ultimately die from cancer and it's all just coincidental.
Picture via the TARDIS Index File.