Why won't he tell his mother? Is he afraid she won't understand (mine has dementia, and can't understand)? If she is competent, then it is unfair for your father to withhold the information, especially with the extra burden it places on him.

Ask him his reasons, and if they aren't valid, tell him that you have to tell your grandmother - his secret is affecting your relationship with her and with him.

Well, I think the reasons are varied. She lost two sons to cancer already... One at 2 years old from stomach cancer, and one in his 20's from leukemia. Her husband just passed this October. My dad is her first born... She is fairly old and somewhat forgetful but definately not into dementia or anything of that sort. She is 89 and physically fairly healthy, but I think dad thinks otherwise. I honestly think his plan is to outlive her.
Our family is notoriously closed off and uncommunicative, this is not a topic of conversation that (at this point) is up for discussion with him.
FRUSTRATION.

I think if your dad is trying to protect his mom from more sadness (? I hope I have the relationships right here) that his not telling her is very noble actually. I'd be really proud of my dad for doing that (forgoing his own comfort,i.e. mom's pitying comfort in order to give her comfort, i.e. my eldest boy is still alive) and I would respect his opinion. I think he should be allowed to tell her when he is ready. You could just say he is battling the flu or something if she asks.

Ouch. Your grandmother has been through a lot. I understand your dad wanting to protect her. I also understand your frustration. It was several weeks after I was diagnosed before we told my dad, because we knew how devastating it would be. We wanted more information, and for the family to all be there to support him - he was 95, frail, but mentally competent. My diagnosis was quite a blow.

Under the circumstances, I would respect his wishes, and let him skip out of the holidays as "under the weather". Perhaps he needs a hair piece, or to let his mother know that he "decided" to shave his head.

My heart goes out to you, your dad and your grandmother. Cancer is a hellish burden.

As hard as it may be on you right now I have to agree with your Dad his Mother is old and fragile and just lost her husband of many years I'm sure. You will all know when the time is right to tell her and if she may pass before then God Bless her for not having to suffer the pain of loosing another child. Be strong you will follow your heart and know what the right choice is we can only give you guidance from our experience. Be strong and we are here if you need someone to talk to.