What Causes Oozing Pus From Surgical Site Post Buccal Mucosa Surgery?

hello, my father recently went through radiation therapy after surgery,now he had a pus coming out from surgical site and he take heavy dosage of antibiotics but they didn t affect. they did every test, result is positive. His cancer dr. says don t be panic. but i read everywhere pus formation for a long duration is not gud. he had pus formation around 2 months along neck side. he had a mouth cancer. buccal mucosa surgery was performed.