Radiation Therapy Physics and Protection
Core concepts
Understand external beam therapy (linacs, photon/electron beams), brachytherapy (HDR/LDR, sources such as Ir-192, I-125), and the concepts of absorbed dose prescription, fractionation, and treatment planning. Know the difference between teletherapy and brachytherapy, and the role of the medical physicist in commissioning, QA, and shielding. Understand that high dose rates require robust interlocks and area monitoring. Know the principles of dose verification and in-vivo dosimetry.
Traps
Questions test that therapy doses (Gy) are orders of magnitude above diagnostic doses and that errors can be catastrophic-hence rigorous QA. Don't confuse HDR (high dose rate, remote afterloading) with LDR. Watch for source decay corrections (Ir-192 T½ ≈ 74 d). Shielding design for therapy vaults uses workload, use factor, and occupancy factor. A common trap is that patient dose in therapy is intentional and not governed by the same limits as occupational dose.
Worked example / rule of thumb
An Ir-192 HDR source with T½ ≈ 74 d decays to half activity in ~74 days; after 148 days, 25% remains. Rule of thumb: for therapy sources, always decay-correct to the treatment date.