Dosimetry and Quality Assurance
Core concepts
This domain covers calibration, calculation, and verification. Know that absorbed dose is reported in gray (Gy); 1 Gy = 100 cGy = 100 rad, and 1 cGy = 1 rad. Understand reference calibration with an ion chamber and electrometer, temperature-pressure correction, and the modern calibration protocol (TG-51). Know the QA program structure (e.g., TG-142 categories): daily output constancy, monthly checks, and annual full calibration and mechanical/imaging checks - and that QA frequency and tolerance are machine- and modality-specific.
Calculation content includes MU computation, PDD vs TMR/TPR, collimator and phantom scatter factors, inverse square correction, tray/wedge/block factors, heterogeneity corrections, and independent second checks. IMRT/VMAT patient-specific QA commonly uses gamma analysis (e.g., 3%/2 mm) plus independent dose recalculation. Brachytherapy adds source decay (A = A₀e^(−0.693t/T½)), source strength units, and wipe/survey tests.
Traps
- Unit slips: confusing Gy with cGy, or rad with rem (dose equivalent).
- Using PDD at a non-reference SSD without inverse-square correction.
- Assuming one fixed QA tolerance applies to every machine and technique.
- Forgetting decay correction for an implanted source over a multi-day course.
Worked example / rule of thumb
MU = prescribed dose ÷ (output × PDD or TMR × Sc × Sp × inverse-square × accessory factors). For decay, an Ir-192 source (T½ ≈ 74 days) falls to roughly half strength in ~2.5 months: after 74 days, activity ≈ 50% of the calibrated value.