Before radiation therapy can begin, imaging such as CT or MRI is used to plan treatment. Which option best describes this purpose?

• A. To plan the radiation fields and dose
• B. To diagnose the cancer
• C. To determine chemotherapy regimen
• D. To replace the need for surgery

Answer: (A)

Imaging before radiation therapy is used to map the tumor and surrounding anatomy in three dimensions so the treatment can be tailored. CT provides the exact anatomy and electron density needed to calculate how the radiation dose will deposit in tissues, while MRI offers superior contrast to delineate the tumor from nearby healthy tissue. By combining these images, clinicians define the radiation fields (where beams enter and their angles and shapes) and determine the dose to deliver, ensuring the tumor gets enough radiation while sparing as much normal tissue as possible. This approach focuses on planning delivery, not diagnosing cancer, choosing chemotherapy, or replacing surgery.