Final Exam - CT Sim & Sim Design

Question 1

What is the goal of Radiation Therapy? How does Simulation fit into this goal.

Answer 1

• Goal: maximize the dose delivered to affected cancerous tissue while minimizing the does delivered to healthy surrounding tissue
• Helps doctor and dosimetrist see how the plan may fight and what limitations they might have to work around
• allows us to maintain high level of patient safety
• localize and visualize the tumors location
• create comfortable and reproducable design

Question 2

What is true about the isocenter with regards to sim?

Answer 2

• it is a single point in space
• The gantry rotates around the isocenter
• The axis of gantry rotation, collimator rotation, and treatment couch rotation meet
• The lasers are aligned at the isocenter

Question 3

What is the difference between SAD and SSD treatments?

Answer 3

• SAD is placing the isocenter inside the patient at 100 cm.
• SSD treatments can place the isocenter both inside and outside of the body. SSD measurement can vary in it’s cm measurement.
• SSD depends upon the patient IFD and positioning on the couch.

Question 4

What are the three main components of the simulator gantry head and what are their purpose?

Answer 4

BLADES - these define the x-ray field and light field

WIRES - define the treatment field as defined by doctor

ODI (optical distance indicator)- this houses scale and light and are what allow us to read an SSD

Question 5

How can you use the image intensifier to reduce magnification during flouroscopy?

Answer 5

We can move it as close to the patient as possible

Question 6

What is the difference between primary and secondary walls?

Answer 6

• Primary walls receive direct beam radiation from teh machine
• Secondary walls only receive scatter radiation

Question 7

What are the factors that get taken into consideration when buidling shielding walls?

Answer 7

U - use factor - time the machine is normally aimed at a wall or ceiling

STANDARD USE FACTOR - simulators primary walls and ceiling is 1/4, floors are usually 1

T - occupancy factor - considers how an area on the other side of an irradiatedd wall is going to be used

W -workload - current (mA) x Time (min) a department expectes to run the machine in a normal week

P - weekly permissable dose - 10x higher for people who have chosen a career in radiation field compared with the general public. uncontrolled areas will hvae a lower p value

B=pd^2/WUT

Question 8

AP

Answer 8

Anterior to posterior

Question 9

CA (CAX)

Answer 9

central axis

Question 10

IFD

Answer 10

inner field distance

Question 11

SAD

Answer 11

surface to a axis distance

Question 12

SFD

Answer 12

surface to film distance

Question 13

SSD

Answer 13

skin to source distance

Question 14

SSN

Answer 14

super sternal notch

Question 15

We have simulated a patient for whole brain treatment. Their IFD is 15, the CA is located 6 cm ^ TT (table top) @ m/l and 1 cm inf to helix. What would their lateral SSD be?

Answer 15

92.5 cm

Question 16

CT numbers are known as what? What is the CT # for dense bone, water, and air?

Answer 16

CT Units = Hounsfield Units

Dense Bone –> 1000
Water–> 0
Air –>-1000

Question 17

Which term best describes the largest volume of tissue in the patient planning process

Answer 17

PTV (Planning Target Volume)

PTV(CTV(GTV))

Question 18

T or F

Presimluation planning includes the localization of disease using radiography and flouroscopy?

Answer 18

False

can’t localize until it has been filmed

Question 19

What does an effective immobilization devide do for use in terms of treatment?

Answer 19

• force teh patient to remain still so that the treatment can be delivered in a consisten fashion each time
• must also consider the patients comfort

if the patient can’t wear it for long enough why bother

takes patients limits into consideration

Question 20

The level of noise on a CT image is affected by which of the following factors?

Answer 20

mAs, kVp, filtration, pixel size, slice thickness, detector efficiency and patient dose

increase in FOV and slice thickness decreases noise without increase mA

Question 21

What must CT simulators specific for RT have?

Answer 21

1. must have lasers
2. must have large aperature so immoblization devices can fit
3. higher-preformance scanners

Question 22

Benefits of CT simulation?

Answer 22

1. CT sim allows us to do virtual planning so the patient doesn’t have to be in the room
2. The HU’s help us to identify treatment volumes
3. We can better locate critical structures
4. Higher quality images and can overlay w/ PET or turn into DRR
5. Allows treatment machine to be used for real purpose and not clogging up schedule for linear accel

Question 23

Smaller pixel size = _____________resolution

Answer 23

greater

Question 24

Low mAs = High/ Low dose

Answer 24

low

Question 25

Discuss the importance of contrast media in CT simulation

Answer 25

• allows for better definition of critical structures in CT scan.
• It also makes it easier to see normal surrounding tissues as well
• The more defined the boarders are, the more accurate our planning can be

great for head & neck, abdominal, pelvic

Question 26

What is respiratory gating and what is it used for?

Answer 26

It is used to lessen breathing motion artifacts

• Used for 4-D scanning
• Spirometer is used to measure intakes
• Machine uses measurments to know when to shut off

Question 27

What are common artifacts in a CT scan?

Answer 27

BEAM HARDENING ARTIFACT

PARTIAL VOLUME ARTIFACTS

STAR ARTIFACTS

RING ARTIFACTS
miscalibrated detector element

MOTION ARTIFACTS