I don't know, you might be able to make it a comedy depending on the character's personality. I work for a surgeon and there is a lady who comes in and she is pretty funny. She went through some other serious trials and the cancer is a "walk in the park" compared to her car accident she had some ten years ago when she nearly lost both her feet. They were ripped off and were only hanging by little ligaments. Her ankles were completely shattered! And yet she's a hoot. I love her.
I have severe ulcerative colitis (soon to have surgery for it), and I've been pretty much chained to a bathroom for the last 14 years. We finally got the pictures back from my brother's wedding and the photographer was getting photos of the bridesmaids and groomsmen going down the aisle. So she gets a picture of me and my partner, right? In the background you can see the porta potties with RESTROOMS clear and big written across them. I saw the picture and was like "UH, figures the photographer would get the restrooms in the background with me. Not anybody else, though! The restrooms have to be in my picture. Uh. Story of my life." My family cracked up laughing. It was situational humor and something that perhaps only my family could truly find funny, but I meant for my response to be taken in a light-hearted way. Sometimes when you deal with something for so long you just have to laugh it off. It's probably some type of coping mechanism.
I'd say it's plausible to have a cancer-comedy if the character's personality lends itself to that. One of the fun things about going through medical trials is being able to exaggerate the stories with highly descriptive words just to see the horror on other people's faces. . . . At least, that's one of the things that *I* find fun.