Exercises Flashcards
Increase the weight of the beam
% and absolute dose at the isocenter increases.
Number of MU’s increase.
Decrease the weight of the beam
% and absolute at isocentric decreases.
Number of MU’s decreases.
Increase depth
Dose at isocenter doesn’t change.
Number of MU’s increase.
Decrease depth
Dose at iso does change.
Number of MU’s decreases.
Increase prescription
% dose at the isocenter doesn’t change as dose is applied to 100%.
Absolute dose @ isocenter increases.
Number of MU’s increases.
Decrease prescription
% dose at isocenter doesn’t change
Absolute dose @ isocenter decreases
Number of MU’s decreases
Increase energy
Dose at isocenter doesn’t change.
Number of MU’s decreases.
Decrease energy
Dose at isocenter does not change.
Number of MU’s increases.
Having lower energy means you need more MU’s to push dose through.
Increase field size
Dose at isocenter doesn’t change.
Number of MU’s decreases (due to increased scatter contribution)
Decrease field size
Dose at isocenter doesn’t change.
Number of MU’s increases.
Adding a wedge/Increasing wedge angle
Dose at isocenter does not change.
The number of MU’s increases.
Relationship between beam weights and normalization values
If the plans are normalized to the same point and the ratio of weights is the same the number of MU’s will not change.
However, the normalization will change by the same ratio as the beam weights.
Relationship between prescription and normalization values
The location of the 100% and the overall dose to the patient will change with changing normalization.
Effect of normalizing to a point
When a plan is normalized to a point this point will not become 100%
If the % dose at the isocenter was 97% then the plan will be scaled by a factor of 100/97. The monitor units will also be scaled by the same amount.
If the % dose at the isocenter was greater than 100% in an unnormalized plan both the plan and the number of units will be scaled down.
