Ms Mowlam, one of Tony Blair's close allies, has been receiving daily radiotherapy and steroids which caused the 44-year-old MP for Redcar to put on weight. She has also been wearing a wig to conceal near total hair loss.
But Ms Mowlam, one of the most popular figures in the shadow Cabinet, has been told she can expect to make a full recovery and now says she is feeling "fit and well".
The tumour was diagnosed on 15 January and, after what she described as a "harrowing week" of further tests she was informed that it was not cancerous.
She was, however, offered a course of radiotherapy treatment as a precautionary measure, which took place at an NHS hospital between 28 January and 18 March. Tony Blair was told of the situation in January.
In the meantime, Ms Mowlam kept up many public appearances and the increase in her weight, caused by the steroids, provoked some critical comment. At the time this was put down to her decision to give up smoking as a New Year resolution. Her change of hairstyle also raised eyebrows, particularly during Labour's glitzy manifesto launch when the shadow Cabinet were on parade.
On 5 April, the Mail reported: "As she wins the war of the weed, she is losing the battle of the bulge. When she sat next to Jack Straw at the Labour Party's manifesto launch some speculated that Mr Blair had found himself a new frontbencher."
A senior Labour source described the newspaper reports as "unpleasant", but said that as a seasoned politician she was not easily upset by media coverage.
However, the shadow minister was said to be relieved that friends and constituents would know the truth and thought it unfair to go into an election without voters knowing the situation.
Yesterday Ms Mowlam, who thanked the NHS for the treatment she received, said that having been offered radiotherapy, she "decided to get it out of the way before the election, rather than waiting until the end of the year".
In today's Sunday Mirror she says "the only time I was frightened was after the check-up when they told me something was wrong but I didn't know what it was". And she urges "other people, particularly women, to be positive. It is the only way to fight it".
Paying tribute to his colleague yesterday, Mr Blair said: "I have huge admiration for her courage and the determined way in which she has kept on working despite having this cloud hanging over her... She will be a big player in a Labour government."Reuse content