First Year Exam: Special Physics Flashcards
What is the α/β for the following…
Normal Tissue
Tumors
L Spine
Cauda
T Spine
C Spine
Prostate
Normal Tissue - 3
Tumors - 10
L Spine - 4
Cauda - 4
T Spine - 2
C Spine - 2
Prostate - 1.5
What is the equation for EQD2?
EQD2 = nd( (d+α/β) / (2+α/β) )
What is the equation for EQD2 using BED?
EQD2 = BED / (1+2/α/β)
For Chen, how are low, intermediate and high risk determined?
Low Risk - Cumulative Dmax < 95 Gy EQD2 and treatment interval > 2 years
Intermediate Risk - Only one of the following is true, the other is false: Cumulative Dmax > 95 Gy EQD2 or treatment interval < 2 years
High Risk - Cumulative Dmax > 95 Gy EQD2 AND treatment interval < 2 years
For Nieder, how are low, intermediate and high risk determined?
Low Risk - cumulative max dose < 150 Gy2 BED and interval > 6 months
Intermediate Risk - cumulative max dose > 150 Gy2 BED and < 170 Gy2 BED or interval or one course contributes > 102 Gy2 BED by itself
High Risk - cumulative max dose > 170 Gy2 BED.
What is the equation for BED?
BED = nd( 1 + d/α/β )
What does BED stand for?
Biological effective dose
What α/β corresponds to late responding tissues? Which correspond to early?
Lower α/β corresponds to late responding tissues
Higher α/β corresponds to early responding tissues
What are the four (arguably five) R’s of radiobiology?
Repair
Repopulation
Reoxygenation
Redistribution
Radiosensitivity (debated)
Would you use a higher or lower dose per fraction for late responding targets?
Higher dose per fraction
During which time period is the fetus most sensitive to radiation damage?
2 - 15 weeks post conception
What are associated abnormalities with irradiation during organogenesis?
Decreasing head size
Mental retardation
Congenital malformations
What is the largest component of scatter radiation at larger distances from the field?
Leakage
What is the largest component of scatter near field edges?
Scatter from within the patient
What three points would you put your dosimeters if you wanted to do in-vivo dosimetry for a pregnant patient?
Fundus, Symphysis pubis, umbilicus
What shielding is required for fetus in pregnant women?
Bridge over patient with 4-5 HVLs of lead
Above how much dose is there significant risk to the fetus?
10-50 cGy
Above what dose is there very high risk to fetus in any trimester?
And above what dose is there a 50% chance of killing fetus?
50 cGy
At 100 cGy there is a 50% probability of killing fetus
At about what distance from the field edge is the contribution of patient scatter and head leakage approximately equal?
20 cm (per TG 203)
What is the approximate % dose 10 cm away from field edge?
1% of central axis dose
What is the approximate % dose 30 cm from field edge?
0.2% of central axis dose
True or False
NCRP considers biological risk to fetus from photoneutrons to be negligible, therefore it is fine to treat with energies > 10 MV in pregnant patients
False
NCRP does consider the risk to be negligible, HOWEVER, it is still recommended to use 10 MV or less to avoid even the negligible contribution
What 5 planning recommendations would you give dosimetry for pregnant patients?
- Modify field size and angle to avoid irradiating direction of fetus
- Avoid wedges (decreases scatter)
- Use conventional 3D-CRT instead of IMRT (less leakage)
- Select a lower energy (easier to shield and less internal scatter at distance)
- Shield using bridge
What considerations/practices do you need to implement for planning hip prostheses?
- Make sure extended HU is turned on
- Use Acuros
- Call vendor to figure out what the prosthesis material is, and assign it to the plan
- Do not treat fields that enter or exit through the prostheses
- Manually correct streak artifacts
Why does the percent depth dose through a prostheses spike after the beam exits the prostheses for 18X fields, but not for 6X fields?
The attenuation ratio of prostheses/water is lower for 6X than it is for 18X due to pair production becoming more prevelant in 18X
What do the percent depth dose curves look like for 6X and 18X photon fields through a prosthesis?
How do you classify low, intermediate and high risk pacemaker patients?
Low risk - Cumulative dose < 2 Gy and patient not device dependent
Intermediate risk - Cumulative dose 2-5 Gy and/or patient is device dependent
High risk - Cumulative dose > 5 Gy and/or energy > 10 MV
What is the protocol for estimating dose to the pacemaker for 3D coplanar beams?
If the pacemaker is < 3 cm from the 50% isodose line, use TPS
If the pacemaker is > 3 cm and < 10 cm from the 50% isodose line, use in-vivo dosimetry
If the pacemaker is > 10 cm from the 50% isodose line, no dose estimate required
What is the protocol for estimating dose to the pacemaker for 3D noncoplanar beams?
Always use in-vivo dosimetry
What is the protocol for estimating dose to the pacemaker for IMRT coplanar beams?
If the pacemaker is < 3 cm from the 5% isodose line, use TPS (NOTE, THIS TIME IT IS THE 5% LINE, NOT 50%)
If the pacemaker is > 3 cm and < 10 cm from the 50% isodose line, use in-vivo dosimetry
If the pacemaker is > 10 cm from the 50% isodose line, no dose estimate required
What are some recommendations you would make in planning with pacemakers?
Avoid wedges and beam modifies that increase scatter dose
Avoid energy > 10 MV
Avoid entering or exiting beams through pacemaker
Avoid coplanar fields
3D is preferred over IMRT due to less leakage
Contour pacemaker for an approximate dose distribution
What type of devices are pacemakers typically?
Metal oxide semiconductors
True or false
Radiation damage to pacemakers is impossible below 1 Gy
False
Damage to pacemaker is stochastic. Below 2 Gy it is unlikely, but still possible
What is the proposed mechanism of radiation damage to pacemakers (not neutrons)?
Radiation induced unwanted charge accumulation in the gates between source and drain
Charges in the drain or source can be trapped by the accumullated holes in the radiation region, resulting in decreased sensitivity
What is the proposed mechanism of neutron damage of high LET damage to pacemakers? (3 possible mechanisms)
Change in electron state of RAM (0 or 1) due to change in accumulated local charge deposition
and/or
Changes to microprocessor circuitry
and/or
Abnormal high current state in a device
What two cases are adaptive radiotherapy commonly concerns for?
H&N where anatomy can change drastically due to weight loss
Small cell lung where target can change drastically over the 33 fractions
Why would you not want to calculate any dose of the day on a CBCT?
Not only is an HU curve typically not defined for CBCT in your TPS, but the image quality of a CBCT is typically so bad you wouldn’t want to calc on the scan anyway
What two things do we have to guarantee in a CBCT before we can use it for adaptive planning?
No brass mesh bolus is in the image (causes artifact which ruins the DIR)
Large FOV (particularly in HN) is used such that the entirety of the shoulders are captured, in addition to any other areas where a beam may directly enter or exit
What is the process for doing adaptive planning in our clinic?
- Use a CBCT with no brass mesh bolus and a large FOV
- Deform CBCT to planning CT (expands CBCT and gives DVF with instructions to go from CBCT to CT in the deformed region)
- Use reverse process of DVF to go from CT to CBCT (only gets applied to region present in current CBCT. The rest of the region is copy and pasted from the original sim)
- Not you have a CT deformed onto CBCT that has a known HU curve and high enough image quality to calc dose on in eclipse
- Evaluate dose to the adapted volume
What are the three types of dosimeters used for In-Vivo dosimetry?
(Hint: think of the measurement methods)
Which is the least commonly used? Which is most commonly used?
- Surface dosimeters (most commonly used)
- Implantable dosimeters (least commonly used)
- Transmission dosimeters
What is the point of measurement for skin measurements for in-vivo dosimetry?
0.5 mm below surface
What are the two possible setups for transmission dosimeters?
Mount on gantry downstream from MLC to measure entrance dose to patient
Mount on machine (such as EPID) behind patient to measure exit dose
Why is it disadvantageous to measure transmission data for in-vivo?
Because you measure a combination of setup error and machine delivery error, so it’s hard to distinguish between the two
For TBI, what is one In-Vivo dosimetry concern that we normally wouldn’t consider for other procedures?
Temperature dependence
The diode stays on the patient for a while during treatment, so the patient’s body temperature will slowly heat up the diode throughout treatment
Because of this we need an additional calibration factor just for TBI
How would you determine if a hip prosthesis is hollow or not (besides calling vendor)?
Take an MV image
True or False
According to TG-63, neutron production from a hip prosthesis should be a concern during 18 MV treatments
False
(contribution is only 0.5% of CAX dose to around the prosthesis)
In IMRT QA, what does “True Composite” mean?
Setup in which actual treatment parameters are being employes during the mock plan delivery for QA
This kind of setup is recommended by TG-218 since it best mimics treatment conditions. Our ArcCheck, MapCheck and Lucy are all examples of “True Composite”. Portal Dosimetry would not be.
A True Composite phantom setup is recommended for IMRT QA per TG-218. What are the pros and cons to the setup? 3 pros, 2 cons
Pros
- Best mimics delivery
- Analysis is quicker since you’re analyzing all fields combined
- You are capable of evaluating multiple components of the treatment delivery that you wouldn’t be able to with other methods (Ex. gantry movement, couch sag, MLC leaf positions vs gravity, etc)
Cons
- Cannot distinguish between errors caused by individual fields. Which can be fine since the treatment as a whole is what you’re really most concerned about
- Some 2D planar arrays may not receive a meaningful portion of the fields
What is one instance in which a perpendicular field-by-field or composite method may be preferred over a true composite method?
If your measurement array has angular dependence
What is the TG-218 recommended action level gamma analysis criteria?
3% / 2 mm - 90% passing
Below this, action is required
If you are going to use a ion chamber for point dose measurements, what is the TG-218 recommendation on dose variation?
Max and Min doses need to be within 5% of the mean chamber dose
What is the difference between acute and chronic effects of radiation?
Acute effects manifest themselves shortly after exposure (Ex. inflammation, erthema)
Chronic effects manifest themselves after a delay period. They’re sometimes called “late” effects (Ex. Fibrosis, Ulceration)
When measuring pacemaker dose with nanodots, you realize your measurement is much higher than what you expected. What are three reasons that may contribute to this?
- Surface dose increase due to stray electrons from out of field, this is the build-down effect where the shallower you measure, the higher the dose
- Lower average energy out of treatment field may lead to OSLD over-response (5-30% over-response)
- The TPS in general under-estimates dose to begin with, even if you were to measure exactly at the pacemaker with an ion chamber, it would still be underestimating
What three materials do hip prosthesis usually come in? Which has the highest density? Which has the lowest?
Cobalt alloy
Titanium (lowest density)
Stainless Steel (highest density)
Approximately what percentage of CIED patients are considered device dependent?
10%
What is the threshold dose for accumulated charge failure mode in pacemaker? What is associated failure rate?
Threshold is 5 Gy
5% failure rate
Approximately how deep are CIEDs located?
1 - 3 cm from surface
True or False
For pacemakers, you only need to be concerned with dose to the main body of the pacemaker
False
For the majority of pacemakers, this is true, only the main body is radiosensitive. For some pacemakers however, the ends of the leads located near the heart may also be sensisitive
What are the three things that vary for recommendations/protocol of patient management vs risk level?
- Frequency of interrogations
- Crash cart use (external pacing/defibrillation)
- EKG monitoring
What is the gamma analysis equation?
Gamma = sqrt (r2/ 𐤃d2 + δ2/𐤃D2)
Where does the highest out of field dose typically occur?
On skin (due to electron contamination)
What are the three types of registrations?
Rigid (6DoF, body does not morph)
Affine (parallel lines remain parallel, but stretching and shrinking may occur)
Deformable (distortion of anatomy can occur in any direction)
