Radiation Oncology
Core Concepts
Radiation oncology questions cover the principles of radiation therapy, including fractionation, radiobiology, treatment planning, and common indications for radiation in various cancers. You must understand the 4 R’s of radiobiology (repair, redistribution, repopulation, reoxygenation), the difference between teletherapy and brachytherapy, and the concepts of total dose, fraction size, and biologically effective dose (BED). Also, know the common side effects of radiation (e.g., mucositis, radiation pneumonitis, proctitis) and the principles of palliative vs. curative intent.
Common Traps
A frequent trap is confusing the intent of treatment: for example, radiation for bone metastases is palliative, while radiation for early-stage prostate cancer is curative. Another trap is the timing of side effects: acute effects occur during or shortly after treatment (e.g., mucositis), while late effects occur months to years later (e.g., radiation myelopathy). Also, beware of dose constraints for organs at risk (e.g., spinal cord max dose).
Worked Example
A 65-year-old man with painful bone metastases from prostate cancer. The question asks for the best management. A single 8 Gy fraction is as effective as 30 Gy in 10 fractions for palliation. Rule of thumb: for bone metastases, single-fraction radiation is convenient and effective.
Key Points
- 4 R’s: repair, redistribution, repopulation, reoxygenation.
- Brachytherapy: radioactive source placed inside or near tumor.
- Acute vs. late effects: mucositis (acute), myelopathy (late).
- Palliative radiation for bone metastases: single 8 Gy fraction.
- Dose constraints: spinal cord max 45-50 Gy.