The purpose of Tumor Boards is for multidisciplinary healthcare specialists to integrate perspectives and devise the best evidenced-based clinical treatment program for cancer patients, but the treatment decision-making process does not always follow a linear path. While few studies have criticized Tumor boards, questioning their effectiveness, none have deconstructed the treatment decision-making process to understand how social-organizational structure and patient case ambiguity affects the way patient information is processed. Through ethnographic exploration of 44 Tumor Boards in the United States and the United Kingdom, this inductive qualitative study investigates how hierarchical and collaborative structures influence the treatment decision-making process for cancer patients under various conditions of case complexity. Tumor board observation and interview data were gathered over three years at seven research hospitals in the United States and United Kingdom. Results show that the Narrative Construction Process of Sensemaking, previously based on a single-actor model, is transferable and applicable to decision-making in multi-actor settings, such as those that make up Tumor Boards. Moreover, using the Narrative Construction Process as a guide, this study uncovered that the Narrative Construction Process can be influenced depending on social organizational structure and level of uncertainty. Keywords: Narrative Construction Process, Sensemaking, Decision-making, Tumor Boards