Is radiation therapy a last resort?

My husband was diagnosed for stage II tongue cancer recently.  The cancer was large 2cmx4cm but did not spread.  He had a surgery a month ago and it was successful.  The biopsy of the removed lymph nodes was negative.  They said they had a clean edge of the removal site.  A biopsy performed during surgery suggested presence of carsinoma in stu.  The surgeon said these may not become malignant and he would just observe it instead of using radiation therapy.  He said he wants to reserve radiation for when cancer returns.  At his post op visit, I asked about the carcinoma in stu, the doctor said at the final section, the pathologist did not even mention carcinoma in stu.  We were thrilled and felt all is well.  Now, not even a month later, my husband discovered a small bump on his tongue.  He will get this checked out in a few days.  My question is,  first, can cancer return this soon after an apparently successful surgery?   Second, why reserve radiation therapy as a last resort?  Can it not be used again if cancer returns in the future?  Third, how effective is chemo in oral cancer?  Thank you for any info.