Megan, played by Irish actress Brenda Fricker, was one of the BBC medical drama's original characters, working as a nurse in Holby between 1986 and 1990. Viewers will see her make a surprise return to the hospital as a patient later this year.
Speaking to DS about the comeback plot, Kent teased: "Megan arrives suddenly in an episode where she's admitted suffering from side-effects of cancer treatment.
"Charlie realises that she's very, very ill and the knowledge that his best friend is terminally ill is very difficult for him to cope with. He turns to Tess to help him through it and together they have to make some very difficult decisions.
"The story climaxes towards the end of the series with an almost three-hander, which is the best episode of Casualty I've ever seen. When I read the script, it made me cry so much that I couldn't see it. And when I watched it, it was the first time I'd watched an episode and not had a single note to give. Brenda's just incredible."
Fricker, who won an Academy Award for her role in 1989 film My Left Foot, will be back in Casualty for four episodes. She previously returned to the part in 1998 and 2007.