I know I personally struggled a lot with maintaining friends during my treatment. I'm not entirely sure how much of that struggle was related to them not knowing how to act around me or related to how much I was unsure how to communicate my unique situation to them. Either way, I sometimes wonder how things may have gone differently had I been given advice on the subject. In an effort to help you avoid my particular struggle, I want to share a great resource I came across recently.
The National Children's Cancer Society (NCCS) is an awesome organization that provides financial and informational resources to children with cancer and their families. They have also started to develop more and more resources for teenage cancer patients and survivors. A particularly helpful resource they have produced is called "An Educational Guide for Friends of Teens with Cancer." The guide is meant to be given in its entirety to a friend to help him/her understand what to do and what to say for you - their friend with cancer. I definitely recommend sharing it with your friends, or even just taking a look at it yourself so you can get some ideas on how to communicate with your friends about your cancer and your treatment!