01 Therapy

Output factor S_c,p

Total scatter factor as collimator × phantom scatter.

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Simulation

Output factor S_c,p — Change the numbers; the scene follows.

Where it works

Linear accelerator

Collimator / MLC

At the jaws / MLC: field size on the collimator sets equivalent square, penumbra, and Sc,p.

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Formula

S_{c,p}=S_c(r_c)\,S_p(r_d),\quad \dot{D}=\dot{D}_{\mathrm{cal}}S_{c,p}

Variables

S_c · Collimator scatterS_p · Phantom scatterḊ_cal · Calibration output 10×10cGy/MU

Results

S_{c,p}
Output factor
1.0302
Ḋ
Output
1.0302cGy/MU

Explanation

S_{c,p}=S_c(r_c)\,S_p(r_d),\quad \dot{D}=\dot{D}_{\mathrm{cal}}S_{c,p}

What it means

The total scatter factor S_c,p factors into collimator scatter S_c (jaws, from in-air measurements) and phantom scatter S_p (from in-phantom). Output relative to 10×10 is S_c × S_p. Small fields have S_c,p well below 1. This is a working relation in Radiotherapy.

Where it is used

Clinically it sits on the Linear accelerator — Collimator / MLC. At the jaws / MLC: field size on the collimator sets equivalent square, penumbra, and Sc,p. 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

Look up S_c from the jaw setting (even if blocked) and S_p from the equivalent square at the phantom. Multiply by calibration output for cGy/MU at dmax. Change one input and watch the curve and the simulation follow.

Symbols

S_cCollimator scatter1.02
S_pPhantom scatter1.01
Ḋ_calCalibration output 10×101 cGy/MU

Worked example

A typical case from the default values: S_c = 1.02 (Collimator scatter); S_p = 1.01 (Phantom scatter); Ḋ_cal = 1 cGy/MU (Calibration output 10×10). Substituting into the relation gives S_{c,p} = 1.0302; Ḋ = 1.0302 cGy/MU. These are teaching numbers — align them with your machine.

Typical values give

S_{c,p} = 1.0302
Ḋ = 1.0302cGy/MU

Where it comes from

The displayed formula is the working relation. Total scatter factor as collimator × phantom scatter. Usual reference: Khan. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan

Assumptions & limits

Does not include tray, wedge, or off-axis factors. Very small fields (< 3 cm) need a detector with small averaging volume — S_c,p is then detector-dependent.

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. Does not include tray, wedge, or off-axis factors. Very small fields (< 3 cm) need a detector with small averaging volume — S_c,p is then detector-dependent.

Keep this

Name the SSD, energy, and field size with every PDD or TMR you quote. Does not include tray, wedge, or off-axis factors. Very small fields (< 3 cm) need a detector with small averaging volume — S_c,p is then detector-dependent.

In this specialty

Radiotherapy