What causes Ursula's cancer in the book, All Our Wrong Todays: A Novel?

Ursula has an arrangement with Goettreider in which they see each other once or twice a year while she stays married to Jerome. After Goettreider invents time-travel though, they see each other multiple times a week because Goettreider can travel through time and space to visit her whenever she has a moment of privacy. Ultimately, it is this repeated time-travel that causes cancer to take root in her brain, ultimately killing her.