Hi,
I don't have advice for you in this situation beyond saying that you should speak to a lawyer who regularly deals with medical inadmissibility cases as soon as possible. What I know is that IRCC decides if it's likely that the applicant will cost the health care system $20,000 a year or so over the next five years (or sometimes ten), and then base their decision on that. Someone having cancer is a real question mark, so it would be great if you could have someone help you who knows what they're doing.
Whatever you decide, don't leave IRCC without an answer when the deadline to respond comes. One possibility is that the immigration doctor himself will decide to wait for the additional scan before filing a final report, but I don't know this for sure.