<s>[INST] My dad (68 yrs old) just recently went through radiation for tounge cancer.  He finished his tx about a month ago, which was after he had the very small mass removed. Last night he noticed a small, pea sized, non-painful, not raised lump just below the skin in his right armpit.  Also the same side he had the radiation on.  It doesnt feel deep, just below the skin. No pain at all, he only noticied it when he was washing up.  He had a negative pet scan just before his operation.  Please tell me this isnt anything to worry about!!!  It seems like we just finished going through this horrific diagnosis and I feel we are starting over at day one. [/INST] Welcome to Chat Doctor .1. The lesion in the armpit / axillary region, non-painful lump after the treatment for the carcinoma tongue - most possible cause could be lymph nodal enlargement.2. Lymph nodal enlargement is seen after any infection / inflammation / malignant causes and any systemic diseases - treat them if you are aware of anything.3. At this moment I would suggest a contrast enhanced computer tomography to see the postoperative site to confirm there is no recurrence.4. An ultrasound to confirm the lump in the axillary region, its source and nature of the lesion solid / cystic, FNAC - fine needle aspiration cytology - to evaluate the kind of cells.5. At this point i would recommend to folio the steps - and a regular followup. Good luck. Hope it helps you. Wish you a good health. Anything to ask ? Do not hesitate. Thank you. </s>