Abstract: Active surveillance (AS) has emerged as an American Urological Association (AUA) and National Comprehensive Cancer Network (NCCN) guidelines-recommended treatment strategy for men diagnosed with low-risk and favorable intermediate-risk prostate cancer (LIPCa; defined as cT1-cT2c, cN0, cM0 with Gleason grade group [GG]≤2 ) to reduce overtreatment of indolent disease and reduce the morbidity and mortality associated with radical treatment (RT). Despite the benefits, AS initiation among elderly Medicare beneficiaries remains suboptimal, with variations across regions, physicians (i.e., 0% to 100%), and practices (i.e., 4.0% to 78.0%), raising equitable access concerns. Among AS initiators, persistence under AS has been decreasing over time, with a trend toward early transitions to RT. This dissertation fills a gap by comprehensively quantifying the factors associated with AS utilization to guide interventions aimed at improving AS utilization when clinically indicated. Aim 1 quantified the association between patient and contextual factors and AS initiation among Medicare Fee-For-Service (FFS) beneficiaries with LIPCa using a generalized linear mixed effect model. The analysis identified patient age, geographic region of residence, comorbidity burden, tumor clinical T stage, and GG, confirmatory magnetic resonance imaging-guided prostate biopsy (MRI-Bx) receipt, and year of treatment initiation as the key patient-level drivers of AS initiation. Educational disadvantage and public transport access at the Census tract level significantly influenced AS initiation. Notably, physician specialty and case volume were the most significant drivers of AS initiation. Aim 2 extended these insights by quantifying the patient and contextual factors associated with time-to-transition from AS to RT, using the Fine and Gray subdistribution hazard model. The findings revealed that alongside tumor risk group at diagnosis, marital status, specific comorbidities, and the year of AS initiation were significant patient-level driver of RT transitions. Additionally, housing insecurity and internet access in the living environment influenced transitions to RT. The study showed that the specialization of the treating physician and the practice-level case volume were associated with RT transitions. These findings offer new perspectives on how pre-index date patient and contextual factors independently influence treatment trajectories in LIPCa, potentially supporting the development of tailored interventions to optimize AS adherence. In Aim 3, the study quantified the economic impact of confirmatory MRI-Bx versus systematic biopsy (SBx) on the short-term reimbursed Medicare Costs (RMC), using propensity score inverse probability of treatment weighted generalized estimating equation model. MRI-Bx was associated with 11.4% higher adjusted Medicare reimbursement during the year following the procedure, corresponding to an incremental cost of $3,588. These findings are particularly relevant given the increasing use of MRI guidance in confirmatory biopsy and contribute new RWE to support cost-benefit assessments of emerging diagnostic technologies in LIPCa care. This dissertation research advances our understanding of multilevel factors influencing AS utilization and the cost implications of utilizing advanced MRI in confirmatory biopsy settings in LIPCa. It offers a foundation for equity-focused, cost-conscious LIPCa health care delivery within the elderly Medicare FFS population.