Field Size and Shape Flashcards
Field size is defined where?
at isocenter
the PHYSICAL RADIATION FIELD SIZE is defined where?
at the 50% isodose line
modern LINACS have ______ and _____ jaws
asymmetric and independent jaws
there are a total of how many jaws?
4
X1, X2
Y1, Y2
what is the transmission factor for our collimating jaws?
< 0.5%
Jaws ___do/do not____ affect scatter and thus patient dose
jaws DO affect scatter and thus patient dose
explain why/how jaws effect patient dose and scatter?
as the jaws open, there is more surface area for the beam to interact with jaws - which would create more scatter to contribute to patient dose
- we compensate for this by reducing the MUs
how do we compensate for jaws increasing patient dose and scatter?
reducing the MU
when field size is closed to zero (0), there is no ___________
there is no beam divergence
when field size is closed to zero (0) what is that technique called?
Half Beam Block
Half Beam Block technique is beneficial when?
when matching adjacent fields or avoiding crit structures
-ex. 3-fld or 4-fld breast (picture supraclav and tangent fields)
we can use Half Beam Block techniques instead of what?
couch rotations/kicks
label the axis X1, X2, Y1, Y2
A = X1
B = Y2
C = X2
D = Y1
larger field sizes require less ____ because of scatter
MU
most modern LINACs have a max FS of ____x_____ at iso that is calibrated to 100 cm
40 x 40 cm
at iso that is calibrated to 100 cm
when a FS is larger than 40cm what is needed/happens ?
extended distance !
there are around ____-____ MLCs in most modern LINACS
52-160 leaves
MLCs stands for -
Multileaf Collimators
what are MLCs made of?
Tungsten or Tungsten alloys
MLCs are driven by _________
computers
T/F
MLCs can be individually controlled?
TRUE
what is the width of MLCs?
.4 - 2.0 cm
what is the height of MLCs?
6 - 7.5 cm
MLCs have a transmission factor of what?
< 2%
for varians -
MLCs are located where?
AFTER the collimating jaws
for varians -
The MLCs are in line with which jaws? X or Y
the X jaws
