I disagree with the previous poster - if it were me, I would very much look upon it from the point of view of a woman with breast cancer, because that is precisely the context in which this previously uncharacteristic behaviour on the part of your husbnand occurred. How can you NOT consider it in context?
Right now, this is very new, and you are furiously angry with him and deeply hurt, not to mention vulnerable and frightened and weak from the breast cancer and surgery. But decisions made in the extremes of emotional upset are often not necessarily ones that are in your own best interests, or those of your children.
I'm not going to try to excuse your husband's behaviour, because it would have been an awful lot better if you hadn't had to deal with such a horrible betrayal right now, but it's important to remember that a diagnosis of breast cancer has a devastating effect on our nearest and dearest, and not just on us.
I would imagine that your husband is absolutely terrified of losing you, of what might happen next; and he's probably never had to deal with a life-threatening disease before, in any context. It's all a massive shock to the system. and people, and by people I mean men, because it's usually men, find different ways of escaping from these kind of stress levels in such dire circumstances: drink, drugs, sex with strangers, just upping sticks and leaving altogether. It's not admirable, but it happens. An awful lot of men simply leave partners with serious illnesses, while women rarely do; there are lots of statistics on this.
If you love your husband, if he's a good father and not a serial adulterer, and has simply cracked temporarily under the strain, then I wouldn't do anything in a hurry.
You're both going through a terrible time, and he has failed you once - it doesn't necessarily mean he will fail you again. Your illness is still quite new, but its legacy is going to be with you for a long time - you will need his support. You have a whole history, a family, a life together - I hope you can get though this, and come out the other side, without losing all that.
I know how very angry and hurt you are right now, but take a deep breath, and try to stay calm.
Best wishes and good luck xxx