Radiation Treatment Delivery
Core concepts
Delivery questions test the sequence and verification of treatment. Before beam-on: confirm patient identity and laterality, verify the prescription and plan match the record-and-verify system, confirm accessories (bolus, blocks, wedges, electron cutouts, bite block), perform the time-out, position and immobilize, set gantry/collimator/couch, and verify imaging/IGRT registration. During treatment: maintain continuous audio-visual observation, monitor for motion, and respond to machine faults or patient distress. After: document delivered parameters, MU, and any deviations; report variances.
Know field-size and SSD/SAD relationships, MLC and blocking, electron energy selection and cutout orientation, arc and gated delivery, motion management (DIBH, abdominal compression, tracking), and the requirement to re-verify setup after any interruption or repositioning.
Traps
- Reversing electron cutout orientation or treating the wrong side - always verify laterality explicitly.
- Assuming a beam interruption resumes automatically; re-verify position and dose delivered first.
- Choosing electron energy from the prescribed dose rather than the target depth.
- Forgetting that with an isocentric technique SSD varies with gantry angle even though the isocenter is fixed.
Worked example / rule of thumb
Electron rule of thumb: the 90% depth (cm) ≈ energy (MeV) ÷ 4, and practical range ≈ energy ÷ 2. A chest-wall electron field needing coverage to 2 cm depth suggests ~8 MeV (8/4 = 2 cm). Confirm with the measured PDD for your machine rather than trusting the shortcut alone.