Particle accelerators are used for two types of cancer treatment: for radiotherapy (which is used in 50% of all cancer cases) and for proton therapy (or hadron therapy). The one I’ve worked on is proton therapy, where the particles give a much more accurate dose to the tumour with less damage to healthy tissue than radiotherapy.
One of the best applications for proton therapy are cancers which are deep within the body and hard to treat with radiotherapy. It is also a great treatment choice if there are organs nearby which you don’t want to irradiate because they are very sensitive, because it’s accuracy means you can spare those organs. So in the case of stomach cancer, yes, it would be very effective as it’s quite a sensitive area deep inside the body.
Keep in mind though that every cancer is different and it takes a medical expert to diagnose and to prescribe treatment, and I’m not a medical doctor!
We still have to catch cancers early (before they spread) and any one type of treatment is not guaranteed to work – we’ve come a long way but we still have an awfully long way to go!