Fundamental Dosimetry Quantities and Monitor Unit Calculations
Problem Set 1
2010
Name: ________________________
Date: ___________________
For problems below use the provided Varian 6 MV Beam Data Set
1) The PDD at a depth of 10 cm in a Varian 6MV, 100-cm SSD, 10x10 field is
66.8%. What is the PDD at this depth in a 120-cm SSD? (Assume the
10x10 field size represents the size of the field at the SSD.)
2) A Varian 6 MV beam has a collimator setting of 15x15 and a water
phantom is set beneath it at 100 cm SSD.
a. If the dose at 10 cm depth along the central axis is 100 cGy, what
is the dose at 20 cm depth (along the central axis)?
b. What is the dose at 20 cm depth at a point located 5 cm away from
the central axis? (Note: The distance “5 cm” is measured
perpendicularly to the central axis at 100 cm from the source.)
3) A Varian 6 MV beam is collimated to a 20x20 collimator setting. In a
phantom located at 100 cm SSD, the dose is 200 cGy at a depth of 20
along the central axis.
a. What is the dose at this point if the collimator setting is changed to
15x15?
b. What is the dose at this same point if the collimator setting remains
at 20x20 but the MLC is set to produce a 15x15 field?
4) A 30x30x30 cm water phantom is set such that it’s center is located at the
isocenter of a Varian accelerator. The phantom is irradiated with 6 MV x
rays with two fields: an “anterior” field (from the top), and a “posterior”
field (from the bottom). Each field is 12x12 and each field delivers the
same dose to isocenter.
a. What are the fields’ monitor-unit settings to deliver 200 cGy to
isocenter?
b. If the total dose at isocenter is 200 cGy, what is the dose at a point
(P) located 10 cm above (towards the ceiling) isocenter?
c. What is the dose at point P if a 30° enhanced dynamic wedge
(EDW) is erroneously used for the anterior field only (the monitorunit settings are unchanged)?
5) A Varian 6 MV x-ray beam has a collimator setting of 6x20. The MLC is set
at 4x10 and 200 MU are delivered.
a. What is the dose along the central axis at 20-cm depth in a
phantom set at 100 cm SSD?
b. What is the dose along the central axis at 20-cm depth in a
phantom set at 80 cm SSD?
c. What is the dose along the central axis at 20-cm depth in a
phantom set at 90 cm SSD?