01 Therapy
Proton RBE-weighted dose
Clinical proton dose: D_RBE = 1.1 D_phys (ICRU 78 constant RBE).
Listen
Listen · English
Simulation
Proton RBE-weighted dose — Change the numbers; the scene follows.
Where it works
Proton gantry
Patient / Bragg peak
Along the proton path in tissue, from the snout to the distal Bragg peak.
Open this machineFormula
Variables
Results
D_RBE
RBE-weighted dose
1.804Gy(RBE)
1.1 D
At standard 1.1
1.804Gy(RBE)
Explanation
What it means
Proton centres prescribe in Gy(RBE) using a constant factor 1.1 on physical dose. That hides a real distal-end RBE rise (LET climbs as the proton slows, variable RBE models predict 1.1 → ~1.3–1.6 in the last millimetres). Planning practice is to keep serial organs just beyond the distal fall-off or to use a variable-RBE model when available. This is a working relation in Radiotherapy.
Where it is used
Clinically it sits on the Proton gantry — Patient / Bragg peak. Along the proton path in tissue, from the snout to the distal Bragg peak. Radiotherapy equations sit at the console and in the bunker: output, depth dose, equivalent square, and the monitor units that treat the patient. Hand-calc them beside the TPS, never instead of a commissioned plan.
Proton gantry · Open this machineHow to use it
Enter physical dose in Gy. Read Gy(RBE) at 1.1 and at a user LET-based RBE if you override the 1.1. A 1.8 Gy(RBE) fraction is 1.64 Gy physical. Change one input and watch the curve and the simulation follow.
Symbols
- D_physPhysical dose1.64 Gy
- RBERBE (default 1.1)1.1
Worked example
A typical case from the default values: D_phys = 1.64 Gy (Physical dose); RBE = 1.1 (RBE (default 1.1)). Substituting into the relation gives D_RBE = 1.804 Gy(RBE); 1.1 D = 1.804 Gy(RBE). These are teaching numbers — align them with your machine.
Typical values give
- D_RBE = 1.804Gy(RBE)
- 1.1 D = 1.804Gy(RBE)
Where it comes from
The displayed formula is the working relation. Clinical proton dose: D_RBE = 1.1 D_phys (ICRU 78 constant RBE). Usual reference: ICRU 78 / IAEA TRS-398. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: ICRU 78 / IAEA TRS-398
Assumptions & limits
Constant 1.1 is a consensus, not a measurement at every point. Variable RBE depends on LET_d, α/β, dose/fraction and the model (McNamara, Wedenberg, Carabe). Carbon ions use a completely different RBE (NIRS/LEM, 1.5–3).
Pitfalls
Never mix PDD from one SSD with TMR from another without converting. Field size at the surface is not the size at isocentre. A hand MU is a check, not a treatment. Constant 1.1 is a consensus, not a measurement at every point. Variable RBE depends on LET_d, α/β, dose/fraction and the model (McNamara, Wedenberg, Carabe). Carbon ions use a completely different RBE (NIRS/LEM, 1.5–3).
Keep this
Name the SSD, energy, and field size with every PDD or TMR you quote. Constant 1.1 is a consensus, not a measurement at every point. Variable RBE depends on LET_d, α/β, dose/fraction and the model (McNamara, Wedenberg, Carabe). Carbon ions use a completely different RBE (NIRS/LEM, 1.5–3).
In this specialty