05 Biology
Relative biological effectiveness
Reference dose over test-radiation dose for the same effect.
Listen
Listen · English
Simulation
Relative biological effectiveness — Change the numbers; the scene follows.
Where it works
Proton gantry
Patient / Bragg peak
At the Bragg peak in the patient: clinical proton RBE is applied to physical dose here.
Open this machineFormula
Variables
Results
RBE
Relative biological effectiveness
2.5
Explanation
What it means
Relative biological effectiveness is the photon (or reference) dose divided by the test-radiation dose that produces the same effect. Protons are assigned RBE 1.1 in the clinic; carbon ions and neutrons are higher and vary with LET, dose, and endpoint. This is a working relation in Radiobiology.
Where it is used
Clinically it sits on the Proton gantry — Patient / Bragg peak. At the Bragg peak in the patient: clinical proton RBE is applied to physical dose here. Radiobiology sits between the prescription and the organ-at-risk: LQ, BED, EQD2, and why 2 Gy is not 2 Gy if the fraction size changed. Use it to compare regimens, not to invent one.
Proton gantry · Open this machineHow to use it
Enter the two isoeffective doses. RBE is endpoint-specific (clonogenic survival ≠ late fibrosis). For protons this calculator is a definition, not the 1.1 clinical convention. Change one input and watch the curve and the simulation follow.
Symbols
- D_refReference dose (photons)2 Gy
- D_testTest radiation dose0.8 Gy
Worked example
A typical case from the default values: D_ref = 2 Gy (Reference dose (photons)); D_test = 0.8 Gy (Test radiation dose). Substituting into the relation gives RBE = 2.5. These are teaching numbers — align them with your machine.
Typical values give
- RBE = 2.5
Where it comes from
The displayed formula is the working relation. Reference dose over test-radiation dose for the same effect. Usual reference: ICRU / Hall. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: ICRU / Hall
Assumptions & limits
Same effect, same fractionation, same oxygenation. Clinical proton RBE 1.1 is a consensus factor, not a measured ratio for your beam.
Pitfalls
α/β is a model parameter, not a measured organ. BED from incomplete repair or a changed overall time is not the simple n·d·(1+d/(α/β)). Never EQD2 a stereotactic dose with an α/β you did not state. Same effect, same fractionation, same oxygenation. Clinical proton RBE 1.1 is a consensus factor, not a measured ratio for your beam.
Keep this
Always write α/β and the fraction size next to a BED or EQD2. Same effect, same fractionation, same oxygenation. Clinical proton RBE 1.1 is a consensus factor, not a measured ratio for your beam.
In this specialty