Radiation therapy? I'd get a second opinion from somebody in parisitology. I would try all sorts of antibiotics first, also Rifaximin, which treats bacterial overgrowth.
The Mayo website says " metronidazole (Flagyl), the combination medication sulfamethoxazole and trimethoprim (Bactrim, Septra, others), and the antiprotozoal medication iodoquinol (Yodoxin, others)."
I have a relative who contracted something like this on a trip. Colonoscopy was useless, stool cultures showed nothing, but blastocystis was a possibility. It took nearly a year for the condition to clear up with Rifaximin, which stopped it, but had to be repeated for a week or so every couple of weeks.
Medscape article: "Several studies conducted during the past 5 years indicate, however, that metronidazole 800 mg three times daily for 510 days is effective in some patients, although the paucity of information means that the accurate prediction of a response is difficult in individual patients. Co-trimoxazole in standard doses (sulfamethoxazole 800 mg and trimethoprim 160 mg, twice daily for 7 days) is, however, reported to eradicate the organism in more than 90% of infected, symptomatic individuals. Numerous other antiprotozoan componds have been tested, with variable results, but perhaps the most promising new drug is nitazoxanide. A placebo-controlled trial of nitazoxanide 500 mg twice daily for 3 days reported a clinical and parasitological cure rate of 86%."