C. dose calculation Flashcards
The beam energy that is used for treatment is determined by the
depth of the target volume, minimum target dose, and acceptable dose to organs at risk in path of beam
For dosimetric calculations, all rectangular treatment fields must be converted to
squares
What is equivalent square equation?
EQSQ= (2ab) / (a+B) or ( 4 x area)/ perimeter
_____fields are fields that use MLCs or blocks to change the shape of the treatment field
Blocked
A simpler approach is to take the square root of the open area minus the
blocked areaExample: the open area is 20 x 10 and the blocked area is 4x 6 First do the equation (20 x 10) — (4 x 6) Next, take the square root of the above answerClarkson integration can be used to find the area of irregularly shaped fields
The main cause of collimator scatter (sc) is from photons scattering within the
head of the gantry
Collimator scatter (Sc) Mostly occurs with the
collimating jaws and the flattening filter
Bigger field sizes creates ___ scatter
more
Smaller field size creates ___ scatter
less
Scatter that happens within the patient or phantom is called
phantom scatter (Sp)
phantom scatter (Sp) amount depends on the
volume of tissue within the treatment field
As the field size increases, the amount of phantom scatter ___
increases
Backscatter factor (BSF) and peak scatter factor (PSF) both compare the dose rate in free space to the dose rate within
a phantom at the dmax
Backscatter factor (BSF) is used for ___ beams
low-energy beams
Peak scatter factor (PSF) is used for ____ beams
high-energy beams
____ is the depth where electronic equilibrium is reached and 100 percent of the dose is delivered
dmax
The dmax for higher-energy beams is
deeper
What is dmax for 4MV beam
1 cm
What is dmax for 6 MV beam
1.5cm
What is dmax for 10MV beam
2.5 cm
What is dmax for 15MV beam
3 cm
What is dmax for 18MV beam
3.2cm
What is dmax for 20MV beam
3.5 cm
What is dmax for 25MV beam
5 cm