04 Protection
Head leakage dose
Leakage is limited to 0.1% of useful-beam dose rate at 1 m.
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Simulation
Head leakage dose — Change the numbers; the scene follows.
Where it works
Linear accelerator
Treatment head
Inside the treatment head: monitor chambers measure output; wedges and leakage live here.
Open this machineFormula
Variables
Results
Ḋ_L
Leakage dose rate
0.006Gy/min
Ḋ_L
Leakage dose rate
6mGy/min
Explanation
What it means
IEC and NCRP cap linac head leakage at 0.1% of the useful-beam dose rate at 1 m from the source. Leakage loads secondary barriers and the maze. Inverse square from the target still applies. This is a working relation in Radiation protection.
Where it is used
Clinically it sits on the Linear accelerator — Treatment head. Inside the treatment head: monitor chambers measure output; wedges and leakage live here. Protection equations are the wall, the occupancy factor, and the badge: time, distance, shielding, and WUT. They turn a room into a legal design.
Linear accelerator · Open this machineHow to use it
Enter isocentre dose rate (Gy/min) and distance from the source (not from isocentre if they differ). 6 Gy/min at iso → 6 mGy/min leakage at 1 m if the head is at the IEC limit. Change one input and watch the curve and the simulation follow.
Symbols
- Ḋ_isoIsocenter dose rate6 Gy/min
- dDistance from source1 m
- fLeakage fraction0.001
Worked example
A typical case from the default values: Ḋ_iso = 6 Gy/min (Isocenter dose rate); d = 1 m (Distance from source); f = 0.001 (Leakage fraction). Substituting into the relation gives Ḋ_L = 0.006 Gy/min; Ḋ_L = 6 mGy/min. These are teaching numbers — align them with your machine.
Typical values give
- Ḋ_L = 0.006Gy/min
- Ḋ_L = 6mGy/min
Where it comes from
The displayed formula is the working relation. Leakage is limited to 0.1% of useful-beam dose rate at 1 m. Usual reference: IEC 60601-2-1 / NCRP 151. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: IEC 60601-2-1 / NCRP 151
Assumptions & limits
Isotropic leakage of exactly f = 0.001. Real heads are closer to 0.01–0.05% in many directions and worse along some cables/waveguides. Patient is not a shield for leakage.
Pitfalls
Tenth-value layers are for the broad beam in that material and that energy — not a photocopy from another bunker. Occupancy T is not a guess; it is a use pattern. Inverse-square fails against a large scatter source. Isotropic leakage of exactly f = 0.001. Real heads are closer to 0.01–0.05% in many directions and worse along some cables/waveguides. Patient is not a shield for leakage.
Keep this
Time, distance, shielding — in that order — then calculate the wall. Isotropic leakage of exactly f = 0.001. Real heads are closer to 0.01–0.05% in many directions and worse along some cables/waveguides. Patient is not a shield for leakage.
In this specialty