Introduction: Online breast cancer communities provide users with both information and emotional support. Members of these communities include both patients and caregivers. We aimed to characterize the differences in content between patients' and caregivers' first posts. Methods: 20189 posts were downloaded from a public online breast cancer community. Posts were sorted by user name and date to identify unique posters and coded for user demographics and content. Descriptive statistics (e.g. chi-square test) characterized differences between patients and caregivers in terms of topics and content. Results: Of the 1827 unique posters, 83% were identified as the patient (n=1277, 70%) and 13% were identified as a caregiver (n=230). 835 (69%) patients had been previously diagnosed with breast cancer. Chemotherapy was the most common topic among cancer patients. Patients tended to seek information about physical concerns more often than psychosocial concerns. In contrast, caregivers used the community significantly more often as a source of emotional support (p<.001), although they also sought information on how to best support patients. Breast Cancer Patients' Topics of ConversationTopicN(%)ExplanationExampleChemotherapy296 (36)Patients discussing the decision for chemotherapy, or their experiences with it.“I was allergic to some of the chemo meds”Radiation255 (31)Patients discussing the decision for radiation, or their experiences with it.“I wanted to answer your query about radiation…”Estrogen Modulating Therapy156 (19)Patients discussing the decision to take estrogen modulating medications (e.g. Tamoxifen) or their experiences with it.“I don't know if I'm going to take this with all the side effects I read.”Surgery55 (7)Patients discussing surgical options for breast cancer treatment (not reconstruction) or experiences with the procedures.“I don't want them to leave half of my breast with a chance for a new diagnosis”Reconstruction49 (6)Patients discussing reconstruction options or experiences with the procedures.“I recently had a TE recon and it went fine but a week later i developed an infection…” Thematic Content of Posts: Patients v. CaregiversDomainPatients n(%)Caregivers n(%)Chi-Square p-valueExplanationExampleDiscussing Physical Issues234 (29)25 (11)<.001Discussing physical symptoms as a result of cancer or treatment.“I was allergic to some of the chemo meds”Seeking Emotional Support113 (14)34 (15)<.001Poster discusses feeling upset, or directly asks for support.“Thanks for listening to my pity party...”Discussing Psychosocial Issues89 (11)21 (9).165Poster mentions psychological issues including depression or anxiety, or with social sequelae of cancer such as marital difficulties.“I also found help through the use of an antidepressant…I feel like a fog has lifted.” Conclusion: First post content varied depending on if the poster was a patient or a caregiver. Patients' first posts were often information-seeking about physical concerns, whereas caregivers tended to use the forum as a source of emotional support. By analyzing the frequency of topics and content of posts, clinicians may better understand the educational needs of patients and caregivers. Citation Format: Marmor RA, Ward EP, Epstein E, Reghunathan M, Schoenbrunner A, Unkart JT, Blair SL, Huh J. First posts: A content analysis of an online breast cancer community user's initial postings [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P4-17-06.

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