At a minimum, how often must radiation therapy fields be verified after the first day?

The correct answer is that radiation therapy fields must be verified at least once a week after the first day. This frequency is established to ensure patient safety and treatment accuracy. Verification helps to confirm the alignment of the radiation beams with the treatment plan, accounting for any changes in the patient's anatomy, position, or other variables that may occur over the duration of treatment.

By checking the fields weekly, practitioners can make necessary adjustments promptly and address any discrepancies that may arise. This helps maintain the efficacy of the therapy while minimizing the risk of harm to healthy surrounding tissues.

Other options suggest more frequent verifications, which may be overly cautious and could strain resources without significant benefit. Weekly verification strikes a balance between patient safety and practical application in a clinical setting.