Final Exam - CT Sim & Sim Design Flashcards
What is the goal of Radiation Therapy? How does Simulation fit into this goal.
- 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
What is true about the isocenter with regards to sim?
- 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
What is the difference between SAD and SSD treatments?
- 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.
What are the three main components of the simulator gantry head and what are their purpose?
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
How can you use the image intensifier to reduce magnification during flouroscopy?
We can move it as close to the patient as possible
What is the difference between primary and secondary walls?
- Primary walls receive direct beam radiation from teh machine
- Secondary walls only receive scatter radiation
What are the factors that get taken into consideration when buidling shielding walls?
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
AP
Anterior to posterior
CA (CAX)
central axis
IFD
inner field distance
SAD
surface to a axis distance
SFD
surface to film distance
SSD
skin to source distance
SSN
super sternal notch
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?
92.5 cm
CT numbers are known as what? What is the CT # for dense bone, water, and air?
CT Units = Hounsfield Units
Dense Bone –> 1000
Water–> 0
Air –>-1000
Which term best describes the largest volume of tissue in the patient planning process
PTV (Planning Target Volume)
PTV(CTV(GTV))
T or F
Presimluation planning includes the localization of disease using radiography and flouroscopy?
False
can’t localize until it has been filmed
What does an effective immobilization devide do for use in terms of treatment?
- 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
The level of noise on a CT image is affected by which of the following factors?
mAs, kVp, filtration, pixel size, slice thickness, detector efficiency and patient dose
increase in FOV and slice thickness decreases noise without increase mA
What must CT simulators specific for RT have?
- must have lasers
- must have large aperature so immoblization devices can fit
- higher-preformance scanners
Benefits of CT simulation?
- CT sim allows us to do virtual planning so the patient doesn’t have to be in the room
- The HU’s help us to identify treatment volumes
- We can better locate critical structures
- Higher quality images and can overlay w/ PET or turn into DRR
- Allows treatment machine to be used for real purpose and not clogging up schedule for linear accel
Smaller pixel size = _____________resolution
greater
Low mAs = High/ Low dose
low
Discuss the importance of contrast media in CT simulation
- 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
What is respiratory gating and what is it used for?
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
What are common artifacts in a CT scan?
BEAM HARDENING ARTIFACT
PARTIAL VOLUME ARTIFACTS
STAR ARTIFACTS
RING ARTIFACTS
miscalibrated detector element
MOTION ARTIFACTS
