01 Therapy

Output factors Sc, Sp, Sc,p

Total output Sc,p = Sc(collimator) × Sp(phantom). Measured relative to 10×10.

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Simulation

Output factors Sc, Sp, Sc,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_p),\quad \mathrm{OF}=\frac{D(r)}{D(10\times10)}

Variables

S_c · Collimator scatterS_p · Phantom scatter

Results

S_{c,p}
Total output factor
1.0302
S_p=OF/S_c
Sp if OF given as Sc,p
1.01

Explanation

S_{c,p}=S_c(r_c)\,S_p(r_p),\quad \mathrm{OF}=\frac{D(r)}{D(10\times10)}

What it means

Collimator scatter Sc (or Sc, head scatter) is measured in air with a miniphantom and depends on the jaws / MLC opening. Phantom scatter Sp depends on the irradiated phantom area at the level of the detector. Their product Sc,p is the in-phantom output factor used in every MU calculation. Blocking a field with MLC changes Sp more than Sc. 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

Enter Sc and Sp (looked up from your machine tables). Read Sc,p. If you only have the measured in-phantom OF and an in-air Sc, then Sp = OF / Sc. Change one input and watch the curve and the simulation follow.

Symbols

S_cCollimator scatter1.02
S_pPhantom scatter1.01

Worked example

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

Typical values give

S_{c,p} = 1.0302
S_p=OF/S_c = 1.01

Where it comes from

The displayed formula is the working relation. Total output Sc,p = Sc(collimator) × Sp(phantom). Measured relative to 10×10. Usual reference: Khan Ch. 10 / AAPM TG-74. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan Ch. 10 / AAPM TG-74

Assumptions & limits

Normalized to 10×10 at the stated SSD/SAD. Equivalent-square interpolation is assumed. MLC leakage and tongue-and-groove are not in Sc/Sp. FFF beams have a different (often flatter) Sc,p vs size.

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. Normalized to 10×10 at the stated SSD/SAD. Equivalent-square interpolation is assumed. MLC leakage and tongue-and-groove are not in Sc/Sp. FFF beams have a different (often flatter) Sc,p vs size.

Keep this

Name the SSD, energy, and field size with every PDD or TMR you quote. Normalized to 10×10 at the stated SSD/SAD. Equivalent-square interpolation is assumed. MLC leakage and tongue-and-groove are not in Sc/Sp. FFF beams have a different (often flatter) Sc,p vs size.

In this specialty

Radiotherapy