Download PHYS297 – Exam 3 December 9, 2011 Name

PHYS297 – Exam 3
December 9, 2011
Answer Key
Name: ____________________________________________________
(2 points)
Part I – Multiple Choice. Circle the letter of the best answer for each question. (3 pts each)
1. A typical width of an MLC leaf at isocenter is:
a. 1 mm
b. 2 mm
c. 1 cm
d. 2 cm
2. Dmax for a Cobalt-60 beam is
a. 0.5 cm
b. 1.0 cm
c. 1.5 cm
d. none of the above
3. Multi-leaf collimator systems may be installed as
a. an upper jaw replacement
b. a lower jaw replacement
c. a collimator add-on
d. all of the above
4. On linear accelerators, the light field coincides with the _____ isodose line.
a. 95%
b. 80%
c. 75%
d. 50%
5. One advantage of a Van de Graf accelerator over a Cobalt-60 unit is
a. Shallower Dmax
b. More compact
c. Larger field size
d. Smaller penumbra
PHYS297 Exam III
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6. A field with a 15 cm collimator setting is set up isocentrically at d=12 cm. The light field
size on the skin is ______ cm.
a. 15.0
b. 13.2
c. 12.8
d. 10.6
7. A field with a collimator setting of 10 x 10 cm is set up at 100 cm. The field projects to
___________ cm at 107 cm.
a. 9.3 x 9.3
b. 10 x 10
c. 10.7 x 10.7
d. 11.5 x 11.5
8. A patient is having isocentric fields planned. The field size display shows 10 cm x 16 cm
at 100 cm. The SSD is 89 cm. What is the field size on the patient?
a. 11 cm x 18 cm
b. 10 cm x 16 cm
c. 7.3 cm x 13.2 cm
d. 8.9 cm x 14.2 cm
9. On a Cobalt-60 unit, Timer Error corrects for:
a. Source travel
b. Operator error
c. Misaligned timer
d. Source decay
10. A patient is treated to a field 15 x 15 cm at a depth of 10 cm. SAD is 100 cm. What is the
field size on the skin?
a. 11.5 x 11.5 cm
b. 12.5 x 12.5 cm
c. 13.5 x 13.5 cm
d. 14.5 x 14.5 cm
11. The agreement between the light field and radiation field should be within::
a. +/- 1mm
b. +/- 2mm
c. +/- 3mm
d. < 5mm
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12. A Van de Graaff accelerator uses _____________ to accelerate electrons.
a. electromagnetic radiation
b. magnetic fields
c. varying electric fields
d. electrostatic energy
13. At what SSD will the maximum field size of 40 cm diverge to 56cm?
a. 116 cm
b. 128 cm
c. 140 cm
d. 156 cm
14. Betatrons and cyclotrons maintain particles in a circular path by use of:
a. Radio frequency waves
b. Magnetic fields
c. Electric fields
d. None of the above
15. Given a 10 x 10 cm field size at 100 cm SSD and an image size of 12.5 x 12.5 cm, what
distance was used to record the image?
a. 80 cm
b. 100 cm
c. 125 cm
d. 150 cm
16. If a field is 10 cm long at 100 SSD, what is its length at an extended distance of 150
SSD?
a. 6.7 cm
b. 10 cm
c. 12.5 cm
d. 15 cm
17. Emergency Off buttons on an accelerator are used to::
a. Terminate the radiation beam
b. Open the door in an emergency
c. Call the service engineer
d. None of the above
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18. Which of the following can accelerate electrons to over 40 MeV?:
a. Van de Graaff accelerator
b. Cobalt-60 unit
c. Magnetron
d. Betatron
19. The highly evacuated section of a cyclotron in which the particles are accelerated is
called the:
a. Dee
b. Stripper
c. Waveguide
d. Injection chamber
20. The penumbra of a photon beam increases with:
a. Decreasing SSD
b. Decreasing source-collimator distance
c. Decreasing depth in tissue
d. Decreasing effective source size
21. The primary beam of a linac measures 40 cm at the isocenter (100 cm from the source).
This projects to ______ cm at the wall, which is 4 m from the source.
a. 120
b. 160
c. 200
d. 240
22. Using a magnification factor of 1.4 and a depth of 10.5 cm measured on the lateral film
for the spinal cord, what is the actual depth of the cord in the patient?
a. 7.5
b. 10.4
c. 14.7
d. none of the above
23. When computing a gap on the patient’s skin, which of the following need to be taken into
account:
1. SSD
2. Field Size 3. Beam Energy
a. 3 only
b. 1 & 2 only
c. 1, 2 & 3
d. 1 only
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24. Which of the following is true regarding geometric penumbra?
a. It increases as source diameter increases
b. It decreases as SSD increases
c. It is independent of source-collimator distance
d. It decreases as depth increases
25. Which of the following machines does NOT accelerate electrons:
a. Linear accelerator
b. Betatron
c. Van de Graaff accelerator
d. Cyclotron
26. Which of the following is NOT a part of the Daily Therapist QA on an accelerator:
a. Radiation warning lights
b. Lasers
c. TG-51 calibration
d. Optical Distance indicator
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Part II – Problems. Answer each problem and SHOW YOUR WORK!
27. A patient is to be treated with an AP lung field on a Clinac 6/100 at an SSD of 89 cm.
The field size set on the machine is 10 x 15 cm. And the patient’s thickness is 22 cm.
A. Draw a diagram of the setup with triangles clearly marked (3 points)
B. What is the field size on the patient’s skin (AP)?
(7 points)
A.
89 cm
100 cm
?? X ??
11 cm
10 x 15 field
B.
X
10
=
89
100
X = (89)(10)/100
X = 8.9 cm
Y
15
=
100
89
Y = (89)(15)/100
Y = 13.35 cm
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28. A patient is to be treated with adjacent fields that need to match at a depth of 7 cm below
the skin surface. The data for each field is listed in the table below.
a. Draw and label a diagram
(5 points)
b. Compute the gap to be set on the patient’s skin.
(5 points)
Field 1
Field Size
10 x 10
SSD
85
SAD
100
Depth = 7 cm
Field 2
15 x 15
90
100
a.)
b.) g1 = L1/2 * (d/100) = (10/2) * (7/100) = 0.35 cm
g2 = L2/2 * (d/100) = (15/2) * (7/100) = 0.525 cm
Gap = g1 + g2 = 0.35cm + 0.525 cm = 0.875 cm
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29. BONUS QUESTION (Extra Credit). A patient is treated in the prone position with two
fields that match on the skin. The spinal cord is 5 cm below the posterior surface. Using
the data below, draw a diagram and compute how much spinal cord overlap is being
treated. (+10 points – full credit only)
Field 1
Field 2
Field Size
10 x 20
10 x 30
SSD
100
100
SAD
100
100
20 cm
30 cm
5.0 cm
spinal Cord
overlap area
Using similar triangles:
1) (L1/2)/100 = (x1)/d
(20/2)/100 = x1/5
x1 = 5*10/100 = 0.5 cm
2) (L2/2)/100 = (x2)/d
(30/2)/100 = x2/5
x2 = 5*15/100 = 0.75 cm
Total Overlap = X1 + x2 = 0.5 cm + 0.75 cm = 1.25 cm of cord.