01 Therapy

Mayneord F-factor

PDD correction when SSD changes at constant surface field size.

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Simulation

Mayneord F-factor — Change the numbers; the scene follows.

Where it works

Linear accelerator

Treatment couch

On the couch: changing SSD versus SAD is a setup change, not a new beam model.

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Formula

F = \left(\frac{\mathrm{SSD}_2+d_{\max}}{\mathrm{SSD}_1+d_{\max}}\right)^2\left(\frac{\mathrm{SSD}_1+d}{\mathrm{SSD}_2+d}\right)^2

Variables

PDD₁ · PDD at SSD₁%SSD₁ · Original SSDcmSSD₂ · New SSDcmd · Depthcmdₘₐₓ · Depth of dose maximumcm

Results

F
Mayneord factor
1.0259
PDD₂
PDD at SSD₂
68.7375%

Explanation

F = \left(\frac{\mathrm{SSD}_2+d_{\max}}{\mathrm{SSD}_1+d_{\max}}\right)^2\left(\frac{\mathrm{SSD}_1+d}{\mathrm{SSD}_2+d}\right)^2

What it means

Mayneord F corrects PDD when SSD changes at constant surface field size. It is two inverse-square ratios: one at dmax and one at depth. Longer SSD slightly improves PDD (less inverse-square fall-off across the phantom). This is a working relation in Radiotherapy.

Where it is used

Clinically it sits on the Linear accelerator — Treatment couch. On the couch: changing SSD versus SAD is a setup change, not a new beam model. 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 PDD₁ at SSD₁ plus the new SSD, depth, and dmax. For 6 MV, 10 cm, going from 100 to 120 cm, F is a few percent above 1. Apply F to PDD, not to TMR. Change one input and watch the curve and the simulation follow.

Symbols

PDD₁PDD at SSD₁67 %
SSD₁Original SSD100 cm
SSD₂New SSD120 cm
dDepth10 cm
dₘₐₓDepth of dose maximum1.5 cm

Worked example

A typical case from the default values: PDD₁ = 67 % (PDD at SSD₁); SSD₁ = 100 cm (Original SSD); SSD₂ = 120 cm (New SSD); d = 10 cm (Depth); dₘₐₓ = 1.5 cm (Depth of dose maximum). Substituting into the relation gives F = 1.0259; PDD₂ = 68.7375 %. These are teaching numbers — align them with your machine.

Typical values give

F = 1.0259
PDD₂ = 68.7375%

Where it comes from

The displayed formula is the working relation. PDD correction when SSD changes at constant surface field size. Usual reference: Khan, Ch. 9. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Khan, Ch. 9

Assumptions & limits

Overestimates at low energy and large depth because scatter does not follow inverse square. Not valid if field size is specified at isocentre rather than at the surface.

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. Overestimates at low energy and large depth because scatter does not follow inverse square. Not valid if field size is specified at isocentre rather than at the surface.

Keep this

Slightly overestimates at low energy and large depth because of scatter.

In this specialty

Radiotherapy