Cancer can come from genetics but it can also come from toxins as well. Do you get your water from a well? Benzene causes cancer and gasoline is up to 10% benzene, if any was spilled on the ground it could seep into your water supply if you are on a well. Lead or other heavy metals can cause cancer, any chance of contamination? Lead paint? What bedding do you use? What food are you using? Any toxic vapors in the air? Do you keep your rats near cans of gasoline or other solvents?
One or two tumors in a colony of 41 tubs with over 1000 babies isn't too bad... I've never had cancer in my rats but I did have it in some of my other farm animals and in a couple dogs, it happens.