It’s not the mark. There is no beast yet. You know, there are those that believe that when the bible says in Rev 13 about demanding the mark, somehow up until that time it was just voluntary but that the mark would be available as soon as the tribulation starts…but for the first 3.5 years its just voluntary. 
Rev 13:5-18
And it was given to him to give breath to the image of the beast, so that the image of the beast would even [k]speak and cause all who do not worship the image of the beast to be killed. 16 And he causes all, the small and the great, the rich and the poor, and the free and the slaves, [l]to be given a mark on their right hands or on their foreheads, 17 and he decrees that no one will be able to buy or to sell, except the one who has the mark, either the name of the beast or the number of his name. 18 Here is wisdom. Let him who has understanding calculate the number of the beast, for the number is that of a [m]man; and his number is [n]six hundred and sixty-six.
It will be such a great day when what the number of the beast is is finally revealed and on more guessing 
Notice there is no mention of a mark before that. I do not believe it is in the character of God to permit “the mark” to be voluntary. The scripture does not say so, and to think so, as far as I know, is adding to scripture (saying that for some who spread this kind of thinking–not you just trying to encourage you )
What i see these days is a hyper interest in knowing deception which doth reminding me of the garden incident. We all fall pray to it. Heck i found out about end timed finding out that Nibiru was ending the world every day for a week. At that point I decided to move on from that. That was four years ago.
Right now knowledge doubles every 12 to 18 months. If we get the Internet of Things up and running it will double every 12 to 24 hours. If we have that up for just 1 month that means that we will have over 1 billion the times of info we have now in one month. Knowledge used to double every 10 years. Prior to that it was like every 100 years. So the thing to ask I believe is this: “Will we have any idea of what the number of the beast technology could be if the world when it comes out will be a billions times more knowledgeable than we are today?”
The character of God would not deceive us with the mark of the beast. If we are alive at the time of the mid tribulation and we have not repented to trust in Him for salvation, then God would permit us to indeed believe a lie. But that is after refusing to believe after an angel himself from heaven warns (Rev chapt 14). We can tend to get fixated on fear porn. I don’t believe God is about either of those two things.
You mentioned you have / had brain cancer? Was that before, or how might I pray for you?