PurposeWe aimed to evaluate recent Instagram and Twitter posts to identify the primary disseminators of information related to radiation therapy on social media (health care professionals vs patients), to characterize their influencer status, and to characterize the content of this information. Methods and MaterialsUsing 2 commercial hashtag analytics platforms, 1000 of the most recent eligible posts from each platform were evaluated for content, tone, and engagement, as well as user (poster) characteristics. Inclusion criteria were as follows: unique posts, written in English, relevant to human cancer treatment, and containing 1 of 11 predetermined hashtags (#radiation, #radiotherapy, #radiationtherapy, #radiationoncology, #radonc, #radiationtherapist #radiationtreatment, #medphys, #cyberknife, #radiosurgery, #protontherapy). ResultsUsers of radiation oncology content on Instagram were primarily patients/caregivers (47%), specifically adult patients (94%) with breast cancer (53%). Patient/caregiver content was focused on patient experience (79%), with approximately half specific to radiation therapy (51%), and most patient/caregiver posts demonstrated a positive tone (86%). In contrast, Twitter content was dominated by health care professionals (53%), specifically within radiation oncology (90% of unique users). Health care professional content was focused on colleague education/research dissemination (53%), with a high proportion of posts specific to radiation therapy (95%). ConclusionsGiven the disproportionate number of patients versus radiation oncology professionals active on Instagram versus Twitter, and the lack of radiation therapy-specific content on Instagram, there may be an opportunity to improve patient outreach and education by promoting the presence of radiation oncologists on Instagram.