Final answer:
Medicare Part A covers Billy's inpatient radiation therapy, as it covers hospital stays and associated services, with beneficiaries responsible for deductibles and copayments.
Step-by-step explanation:
Billy received radiation therapy when he was an inpatient, and this type of service is generally covered by Medicare Part A. Medicare Part A pays for inpatient hospital services, which include a hospital stay where radiation therapy might be provided. This coverage is particularly beneficial for those receiving radiation treatments like external beam radiation therapy (EBT) or brachytherapy, which might require a period of hospitalization. Medicare Part A funding comes from payroll deductions and employer contributions, and individuals are typically responsible for deductibles and copayments related to their hospital stay. However, Medicare has set no limits on the total costs that a person can incur, so all necessary inpatient treatments should be covered under this plan, after deductibles and copayments are paid.