You can get insurance to just cover accidental injuries. Thai banks have debit cards that provide accident insurance. The cards cost around 1.5K/year. They pay about 20K per incidence. There is an age limit to coverage though, I think 70 for Thais.
They also have accident insurance through Chubb which will cover the same things but with higher limits(like around 80K). I think it was around 6K/year when I was doing it for my wife.
It is also possible to get policies just for cancer too.
Thai insurance companies will want her to go to a hospital and get an examination and blood tests done before issuing the final policy. Their goal is to find as many preexisting conditions as possible. To limit what they cover. Also when you do have a claim they will ask for permission to check your records. Then if the doctor had asked your wife about previous problems and she gave information to the doctor the insurance company will add more exclusions or cancel the policy. They also ask you what medications you are taking and will do exclusions based on that. Like if you take any cholesterol or blood pressure medications they will exclude anything to do with heart attacks and strokes.