read_attach.php-1.pdf

Isodose Distributions (I)

藍仁鴻

中國醫藥大學放射技術系

2004/10/20

What are isodose curves?

•They are lines passing through points of

equal dose.

•They are drawn at regular intervals of

absorbed dose and expressed as a

percentage of the dose at a reference

point.

•It represents levels of absorbed in the

same manner. ( z.B. isotherms or isobars)

Isodose chart

•A family of isodose curves

•In equal increments of PDD

•The variation in dose as a function of

depth and transverse distance from the

central axis

•It can normalized either at the point of

maximum dose on the central axis (SSD)

or at a fixed distance along the central axis

in the irradiated medium (SAD).

Isodose chart

General properties for isodose charts

•The dose at any depth is greatest on the central axis of the

central axis of the beam and gradually decreases toward

the edges of the beams. ( horns in some LINACs 

overcompensate ).

•The dose rate decreases rapidly as a function of lateral

distance from the beam axis in the penumbra region. (

geometric penumbra with side scatter ↓)

•It could be defined a physical penumbra as the lateral

distance between two specified isodose curves at a

specified depth. ( z.B. lateral distance between 90%  10%

or 80  20%)

•Therapeutic housing/source housing: lateral scatter from

the medium and leakage from the head of the machine