I have to give a talk about how methods of particle physics are applied to cancer therapy. Is the title "Particle physics in cancer treatment" or "Particle physics for cancer treatment" correct ?
You would use "Particle physics in cancer treatment". The
in here indicates that you're describing the field that the subject is being discussed in.
For brings strange connotations that don't quite match. It indicates a purpose, and using it would indicate that you're discussing physics that are intended and specific to cancer treatment, which likely isn't what you want to say, because the laws of physics don't have intention/purpose or or work differently in different professions.
Conclusion- Use "Particle physics in cancer treatment".
Particle physics in cancer treatment.
in means that particle physics is a component part of cancer treatment. Cancer treatment includes this and other activities.
Particle physics for cancer treatment.
for indicates purpose. Particle physics exists for the purpose of treating cancer.
But as written this is worded strangely. Better would be:
Usage of particle physics for treating cancer.
A better example where to works might be:
We are accepting donations for cancer treatment.