Background: Patient-centered cancer care requires a clear understanding of what patients and caregivers find most confusing, frightening, or difficult, many of which may remained unnoticed in clinic visits because of time pressure, stigma, or uncertainty. Online cancer forums offer a space where people can describe their experiences more freely. By analyzing these posts, we can uncover explicit questions and “latent” informational needs for future supportive resources development. Methods: We conducted a content analysis of posts from the r/cancer subreddit (August–November 2025). OpenMed disease NER was used to identified posts with a specific cancer diagnosis and grouped them into broader cancer categories, focusing on the three most frequently mentioned cancers. A rule-based pipeline applied regular expressions and keyword lists to identify challenges in seven core domains and parallel information-need themes. Posts were coded as having explicit information needs if they contained question marks or advice-seeking phrases. Results: Among 1006 posts with an assigned cancer diagnosis, 397 concerned lymphoma, breast, or colorectal cancer. Diagnostic and staging challenges were common across these cancers (76.7%, 84.8%, and 87.1%), followed by treatment decision/burden challenges (64.7%, 59.8%, and 78.0%) and side-effect/symptom challenges (33.1%, 35.6%, and 37.1%). Overall, 71% of posts contained at least one explicit information need, yet theme-specific information seeking was lower than the corresponding challenge burden. Diagnostic/staging questions appeared in 28.6%, 39.4%, and 56.1% of lymphoma, breast, and colorectal posts, treatment decision questions in only 2.3%, 1.5%, and 6.1%, and side-effect management questions in a small minority of posts despite frequent reporting of side-effect burden. Conclusions: Even when patients and caregivers do not explicitly ask for information, their posts describe confusion and distress about diagnosis, staging, treatment choices, and side-effect management. These findings highlight informational needs and underscore the importance of patient-centered communication to address common challenges outside the clinic.