CPT code 96521 represents the refill of a portable pump with a chemotherapy drug and captures a discrete, billable service in oncology infusion management. Nationally, this code matters because it documents clinician activity to maintain ambulatory chemotherapy delivery systems, supports accurate resource accounting for infusion services, and influences claim adjudication and provider reimbursement for outpatient oncology care. Key payers in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise explanation of the clinical purpose of CPT code 96521, typical sites of service, and the operational context for use in outpatient infusion and oncology clinic settings. The publication also summarizes payer coverage patterns and benchmark considerations where available, reviews common modifier practice affecting claim processing, and outlines areas of claim denials and documentation focus relevant to this service. Where input data is incomplete, the report notes missing fields as provided. The goal is to give clinicians, billing professionals, and policy analysts a clear, practical reference for CPT code 96521 in the national outpatient oncology billing environment.