L 10: TP I Isodose distribution Flashcards
Parameters of Isodose curves
- Beam quality
- Source size
- Field size
- SSD
- SDD
- Beam collimation/ FFilters
Depth of Isodose curve depends on
Beam Quality
Shape of the Isodose curve depend on
- SSD
- SDD
- Source size
due to penumbra
Beam modifying devices
Wedge filter
Collimators
Things that give shape and size to the beam
- Collimator blocks
- MLC
- Wedges
- Flattening filter
Greatest influence for a Megavoltage x ray beams
Flattening Filter
* They curve the isodose lines, without the filter the isodose lines would be conical in shape.
* Makes the beam intensity and distribution uniform.
* Filter is thickest in the middle and thin in the edges.
* Avg energy of the beam is lower in the periphery and higher in the center.
Wedge Isodose angle
The angle through which an isodose curve is titled at the central ray of a beam at a specified depth”
Wedge Transmission Factor
This effect is characterized by the wedge transmission factor
(or simply wedge factor), defined as the ratio of doses with and without the wedge, at a point in phantom along the central axis of the beam.
Parallel opposed fields
- Pair of fields directed along the same axis from opp sidesof the treatment volume.
- The advantages of the parallel opposed fields are the simplicity
and reproducibility of setup, homogeneous dose to the tumor, and less chance of geometric miss (compared with angled beams), given that the field size is large enough to provide adequate lateral coverage of the tumor volume. - A disadvantage is the excessive dose to normal tissues and
critical organs above and below the tumor.
Isocentric techniques
- The isocentric technique of irradiation consists of placing the isocenter of the machine at a depth within the patient and directing the beams from different directions.
- The distance of the source from the isocenter, or the SAD, remains constant irrespective of the beam direction.
- However, the SSD in this case may change, depending on the beam direction and the shape of the patient contour.
- SSD = SAD-d
Past pointing
- For partial arcs, the mas dose is displaced towards the irradiated sector.
Tumor dose specification for EBRT
Factors that effect the physical penumbra
Dose distribution in the penumbra zone is governed by geometric penumbra, transmission penumbra, and the lateral scatter of photons and electrons. The composite of these effects is represented by what is called as the physical penumbra.
