Introduction: Survivors of childhood brain tumors (SCBT) are an emerging group of cancer survivors that has an increased risk of cardiovascular disease, stroke, and type 2 diabetes. SCBT have equivalent obesity rates but excess fat mass (adiposity) when compared to the general population. As adiposity is an important and potentially modifiable risk factor for cardiometabolic outcomes in the general population, its measurement may allow for early stratification of adverse health outcomes in SCBT so that they can be targeted with prevention and treatment strategies designed to improve outcomes. However, measuring adiposity often requires specialized equipment that is not always readily available, and a clinical measure is needed to facilitate these measurements in a feasible fashion. Tri-ponderal Mass Index (TMI; kg/m3) is a superior measure of adiposity compared to Body Mass Index (BMI) z-score in healthy children. However, it has not been assessed in SCBT. The aim of this thesis was to validate TMI as an adiposity measure in SCBT compared to non-cancer controls. Methods: A cross-sectional analysis was completed from a cohort study sample including 44 SCBT (n=20 female) and 137 (n=64 female) healthy controls between 5-17 years of age. Total adiposity was determined by fat mass percentage (%FM) using bioelectrical impedance analysis and central adiposity was assessed by waist-to-hip (WHR) and waist-to-height (WHtR) ratios. Results: TMI demonstrated equally strong correlations with total adiposity and stronger association with WHtR compared to BMI z-score in SCBT and healthy control children. Conclusions: TMI may serve as a reliable and feasible clinical measure of adiposity in both SCBT and healthy children. The availability of TMI may allow for early stratification of survivors at risk of excess adiposity to allow early targeting with interventions to improve health outcomes. / Thesis / Master of Science in Medical Sciences (MSMS) / Over the last thirty years, childhood cancer survival rates have greatly improved. However, these rates decrease over the lifespan of survivors. Certain groups within the survivors of childhood cancer population, including survivors of childhood brain tumors (SCBT), are at a higher risk of obesity, heart disease and type 2 diabetes. Excess body fat is a major contributor to the development of these adverse health outcomes in the general population and may represent an entry point to prevent and treat these conditions in SCBT. However, measuring fat mass in the clinical setting requires specialized equipment that can be expensive, time-consuming and not readily available in all settings. Therefore, this thesis aims to explore measures of fat mass that are both feasible and reliable in a clinical setting in SCBT compared to the general pediatric population. We have identified the tri-ponderal mass index (TMI), defined as weight divided by height cubed (kg/m3) as a valid measure of the fat mass in both SCBT and healthy children. We conclude that TMI may serve as a reliable and feasible measure of adiposity in both SCBT and healthy children in clinical settings and assist in the early identification of survivors at risk of obesity and cardiometabolic outcomes to prioritize early interventions to improve outcomes.