RTH-12

Wedge factor

WF = D_wedge / D_open on the central axis.

Reading speed

OutputKhan wedge

Governing equation

\mathrm{WF}=D_{\mathrm{wedge}}/D_{\mathrm{open}}

$D_w \in \left[0.2, 4\right]$

where

D_w: Wedged dose (Gy)
D_o: Open dose (Gy)
WF: Wedge factor (—)

The figure, part by part

Each numbered marker wears the color of its card. Hover a card to light the marker. Tap a marker to jump to that card. The slider walks a red point along the curve; Animate drawing traces it like a pen.

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①

Governing law — red

\mathrm{WF}=D_{\mathrm{wedge}}/D_{\mathrm{open}}

$D_w \in \left[0.2, 4\right]$

$D_w = 0.2$ $WF = 0.1$

Along the curve

②

Domain of the variable — blue

D_w_{\min} = 0.2,\quad D_w_{\max} = 4

$D_w \in \left[0.2, 4\right]$

$D_w = 0.2$ $D_w = 0.2$

Along the curve

③

Response curve — green

\mathrm{WF}=D_{\mathrm{wedge}}/D_{\mathrm{open}}

$D_w \in \left[0.2, 4\right]$

$D_w = 0.2$ $WF = 0.1$

Along the curve

④

Rate of change — amber

\frac{\mathrm{d}WF}{\mathrm{d}D_w}

$D_w \in \left[0.2, 4\right]$

$D_w = 0.224$ $WF^{\prime} = 0.5$

Along the curve

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-12 — Wedge factor) is the form associated with Khan wedge. Working symbols: $D_w$ , $D_o$ $\rightarrow$ $WF$ . Physical wedges attenuate 20–70 %. Dynamic/EDW wedges change WF with field and angle.

Purpose

Purpose: compute

WF

from

D_w

D_o

in Radiotherapy via

\mathrm{WF}=D_{\mathrm{wedge}}/D_{\mathrm{open}}

WF = D_wedge / D_open on the central axis. Use it when a real radiotherapy question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

D_w = 1.600\,\mathrm{Gy}

D_o = 2.000\,\mathrm{Gy}

, the governing relation

\mathrm{WF}=D_{\mathrm{wedge}}/D_{\mathrm{open}}

yields

WF = 0.8000\,\mathrm{—}

. An open beam and a wedged beam, two CAX doses. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Wedged dose D_w1.6 GyOpen dose D_o2 Gy

Outputs

Wedge factor WF0.8000 —

Reading speed

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Narration of this film

An open beam and a wedged beam, two CAX doses.

Physical wedges attenuate 20–70 %. Dynamic/EDW wedges change WF with field and angle.

Reading speed

Watch on YouTube