RTH-09

Mayneord F-factor

F = [((SSD2+dm)/(SSD1+dm)) ((SSD1+d)/(SSD2+d))]². Converts PDD between SSDs.

Reading speed

Beam dataMayneord

Governing equation

F=\left(\dfrac{\mathrm{SSD}_2+d_m}{\mathrm{SSD}_1+d_m}\cdot\dfrac{\mathrm{SSD}_1+d}{\mathrm{SSD}_2+d}\right)^{2}

where

SSD_1: Original SSD (cm)
SSD_2: New SSD (cm)
d: Depth (cm)
d_m: dmax (cm)
F: Mayneord F (—)

Lecture brief

Historical brief

The linear-quadratic model, BED, tissue-maximum ratio and Clarkson scatter turned radiotherapy into fraction arithmetic. MU calculation on these sheets is that clinical closed form. This sheet (RTH-09 — Mayneord F-factor) is the form associated with Mayneord. Working symbols: $SSD_1$ , $SSD_2$ , $d$ , $d_m$ $\rightarrow$ $F$ . Pure inverse-square; scatter change with SSD is ignored. Good near dmax, poorer at depth.

Purpose

Purpose: compute

F

from

SSD_1

SSD_2

d

d_m

in Radiotherapy via

F=\left(\dfrac{\mathrm{SSD}_2+d_m}{\mathrm{SSD}_1+d_m}\cdot\dfrac{\mathrm{SSD}_1+d}{\mathrm{SSD}_2+d}\right)^{2}

F = [((SSD2+dm)/(SSD1+dm)) ((SSD1+d)/(SSD2+d))]². Converts PDD between SSDs. Use it when a real radiotherapy question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

SSD_1 = 100.000\,\mathrm{cm}

SSD_2 = 120.000\,\mathrm{cm}

d = 10.000\,\mathrm{cm}

d_m = 1.500\,\mathrm{cm}

, the governing relation

F=\left(\dfrac{\mathrm{SSD}_2+d_m}{\mathrm{SSD}_1+d_m}\cdot\dfrac{\mathrm{SSD}_1+d}{\mathrm{SSD}_2+d}\right)^{2}

yields

F = 1.0259\,\mathrm{—}

. Two SSDs, one depth, a conversion factor. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Original SSD SSD_1100 cmNew SSD SSD_2120 cmDepth d10 cmdmax d_m1.5 cm

Outputs

Mayneord F F1.0259 —

Reading speed

Watch on YouTube

Free library

Full library

Free PDF / open book

YouTube channels

00:0 / 00:08

Narration of this film

Two SSDs, one depth, a conversion factor.

Pure inverse-square; scatter change with SSD is ignored. Good near dmax, poorer at depth.

Reading speed

Watch on YouTube