Radiation Physics and Equipment
Radiation physics and equipment tests how photons and electrons are produced, shaped, measured, and delivered by a linear accelerator. Expect calculations involving inverse square, attenuation, half-value layer, percentage depth dose, and output factors, plus equipment function.
Production and beam quality
In a linac, electrons from a gun are accelerated in a waveguide driven by a magnetron or klystron. A bending magnet directs them to the target for photons; electrons can also be extracted for electron therapy. Photons arise as bremsstrahlung plus characteristic X-rays. Flattening filters produce a uniform photon field; MLCs shape it. Electron beams are scattered and have a high surface dose, with practical range related to energy. Photon Dmax deepens with energy and field size; surface dose rises with field size and beam obliquity.
Core rules
Inverse square: dose rate changes as (d1/d2)^2 for a point source in air. Attenuation: I=I0 e^-μx; HVL=0.693/μ. Output factor compares dose at a reference depth for a given field to the reference field. PDD depends on energy, depth, field size, and SSD; it is not transferable between SSDs without a Mayneord-type correction. Isocenter is the intersection of gantry, collimator, and couch axes; mechanical and radiation isocenter should agree within tolerance.
Worked example
A 6 MV beam gives 1.00 Gy/min at 100 cm SSD in air. What is the approximate dose rate at 150 cm, ignoring attenuation and scatter? Ratio=(100/150)^2=0.444; dose rate=0.444 Gy/min. Reasoning: inverse square applies to the divergence of photons from the focal spot. It does not describe dose inside a patient, where attenuation, scatter, and inverse square combine.
Traps
Do not apply inverse square to a full patient plan as if tissue were air. Do not assume Dmax is fixed at 1.5 cm for all energies. Higher photon energy usually lowers surface dose but increases penetration; electron energy changes range much more than surface dose. A larger field raises output and scatter, so output factors and PDDs are field-size dependent. Do not confuse HVL, an attenuation measure, with Dmax, a dose-deposition measure.