Usually eating curry during a course of radiotherapy causes no problems, and there is no need to adjust your normal diet.

However, if you are having radiotherapy to the mouth, the back of the throat, the neck, or the centre of the chest (where the gullet passes through, joining the throat to the stomach), then these tissues will get inflamed as the course of treatment goes on, and will stay inflamed for a few weeks after treatment is finished.  This inflammation can often make eating and drinking uncomfortable, or even painful.  So during this time it is best to avoid very hot, or very spicy foods as these will irritate the already inflamed linings of the mouth, throat or gullet, increasing the discomfort, and slowing the healing process. Adjusting your diet to milder curries, with plenty of cool soothing yoghurt, would be a good idea.

Sometimes if you are having radiotherapy to the belly, certain foods can cause either sickness (nausea and vomiting) or loose bowel movements (diarrhoea).  This will vary from person to person, something that will upset one person will be perfectly okay for someone else.  So if you do find your normal diet is causing problems,  changing to milder, blander curries might be worth trying.

The side effects you might get from a course of radiotherapy can vary greatly, depending on which bit of your body is being treated, the doses of radiation being given and the length of the course of treatment.  It is always worthwhile just asking your doctor, or the radiographers who are giving you the treatment, whether any change in your normal diet would be a wise precaution.