Field Shaping and Craniospinal Irradiation Flashcards
What is the shape of a field that utilized just x-rays jaws alone for collimation?
Rectangular
How can fields of irregular shaped be created?
Secondary Blocking or MLC’s
What are some device examples of secondary blocking?
Lead or Cerrobend Block
What is the material utilized for custom blocks?
Lipowitz Metal (Cerrobend)
What is the composition of Lipowitz Metal/Cerrobend?
Bismuth, Lead, Tin and Cadmium
What is the main advantage of Cerrobend?
Low melting point as opposed to Lead
What is the toxic element in cerrobend?
Cadmium
What is the ratio of Cerrobend to Lead?
1 cm of lead = 1.21 cm of Cerrobend
Positive Block
The Blocks are put in place to block out a structure within the inner field (Island block, lung blocks)
Negative Blocks
Blocks that are utilized to conform to a structure/shape (MLC conforming)
What are Multi-Leaf Collimators?
system of large amount of collimator blocks or leaves that are automatically and independently to generate a field of any shape
What is the typical amount of MLCs?
60-80 Pairs
What is size of the individual leafs?
0.5 - 1 cm leaves
What is the Collimation system for Varian?
Upper Jaw, Lower jaw and MLC
What is the Collimation system for Elekta?
MLC, Lower Jaw
What is the Collimation system for Siemens?
Upper Jaw, MLC
Size of the physical pneumbra is impacted by _________ from the target
MLC Distance
Does a shorter distance from the target to the MLC leafs create greater or lesser pneumbra?
Greater
What is the interleaf transmission less than?
3%
What is intraleaf leakage?
Leakage through the leaf itself
What is interleaf leakage?
Leakage between the leafs
What are the two important factors for MLC leaves?
- Leaves must overlap to prevent any interleaf transmission
2. Leave are focusing in the orthogonal plane and incorporate divergence into design
