RTH-16

Geometric penumbra

P = s (SSD − SDD)/SDD. Source size s seen through the diaphragm.

Reading speed

ProfilesGeometric penumbra

Governing equation

P=s\,(\mathrm{SSD}-\mathrm{SDD})/\mathrm{SDD}

where

s: Source size (mm)
SSD: SSD (cm)
SDD: Source–diaphragm (cm)
P: Geometric penumbra (mm)

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-16 — Geometric penumbra) is the form associated with Geometric penumbra. Working symbols: $s$ , $SSD$ , $SDD$ $\rightarrow$ $P$ . Physical penumbra adds transmission through the leaf tip and lateral scatter. 80/20 width is the usual report.

Purpose

Purpose: compute

P

from

s

SSD

SDD

in Radiotherapy via

P=s\,(\mathrm{SSD}-\mathrm{SDD})/\mathrm{SDD}

P = s (SSD − SDD)/SDD. Source size s seen through the diaphragm. Use it when a real radiotherapy question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

s = 2.000\,\mathrm{mm}

SSD = 100.000\,\mathrm{cm}

SDD = 50.000\,\mathrm{cm}

, the governing relation

P=s\,(\mathrm{SSD}-\mathrm{SDD})/\mathrm{SDD}

yields

P = 2.00\,\mathrm{mm}

. A finite source, jaws, a blurred field edge. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Source size s2 mmSSD SSD100 cmSource–diaphragm SDD50 cm

Outputs

Geometric penumbra P2.00 mm

Reading speed

Watch on YouTube

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Narration of this film

A finite source, jaws, a blurred field edge.

Physical penumbra adds transmission through the leaf tip and lateral scatter. 80/20 width is the usual report.

Reading speed

Watch on YouTube