01 Therapy

Backscatter factor BSF

BSF = TAR(dmax, s) = dose at dmax in phantom / dose in air at the same point.

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Simulation

Backscatter factor BSF — Change the numbers; the scene follows.

Where it works

Linear accelerator

Isocenter

At isocenter, on the central axis through the patient (or a phantom in the same place).

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Formula

\mathrm{BSF}(s)=\mathrm{TAR}(d_{\max},s)=\frac{D(d_{\max})}{D_{\mathrm{air}}}

Variables

D_max · Dose at dmaxcGyD_air · In-air dosecGy

Results

BSF
Backscatter factor
1.03
BSF−1
Scatter contribution
0.03

Explanation

\mathrm{BSF}(s)=\mathrm{TAR}(d_{\max},s)=\frac{D(d_{\max})}{D_{\mathrm{air}}}

What it means

Backscatter is the extra dose at dmax coming from photons scattered back by the phantom. It rises with field size and falls with energy (MV beams have BSF ≈ 1.02–1.06; orthovoltage 5×5 to 20×20 can be 1.1–1.5). Peak scatter factor (PSF) is the same quantity. Output at dmax in phantom = output in air × BSF. This is a working relation in Radiotherapy.

Where it is used

Clinically it sits on the Linear accelerator — Isocenter. At isocenter, on the central axis through the patient (or a phantom in the same place). 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.

Linear accelerator · Open this machine

How to use it

Enter dose at dmax in phantom and the corresponding in-air dose. For 6 MV 10×10, BSF is only a few percent above 1; for 250 kV it may be 1.3. Change one input and watch the curve and the simulation follow.

Symbols

D_maxDose at dmax103 cGy
D_airIn-air dose100 cGy

Worked example

A typical case from the default values: D_max = 103 cGy (Dose at dmax); D_air = 100 cGy (In-air dose). Substituting into the relation gives BSF = 1.03; BSF−1 = 0.03. These are teaching numbers — align them with your machine.

Typical values give

BSF = 1.03
BSF−1 = 0.03

Where it comes from

The displayed formula is the working relation. BSF = TAR(dmax, s) = dose at dmax in phantom / dose in air at the same point. Usual reference: Khan / Johns & Cunningham. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan / Johns & Cunningham

Assumptions & limits

Defined at dmax of that energy and field. Electron contamination and tray scatter are not BSF. Do not apply a kV BSF table to an MV beam.

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. Defined at dmax of that energy and field. Electron contamination and tray scatter are not BSF. Do not apply a kV BSF table to an MV beam.

Keep this

Name the SSD, energy, and field size with every PDD or TMR you quote. Defined at dmax of that energy and field. Electron contamination and tray scatter are not BSF. Do not apply a kV BSF table to an MV beam.

In this specialty

Radiotherapy