All Physics Quizzes 3 (Starting from Quiz 19)
1. Data on dose distribution is typically measured in what media and why?
Water because: - It's universally available - Has similar effective Z number and *electron density* as soft tissue
17. An ortho unit has a beam quality of 1 mm Cu. It will be used to deliver 200 cGy at 1 cm depth in a patient. What is the beam on time if the exposure rate is 114 R/min at 40 cm, field size is 10 x 10 cm, SSD = 40 cm, PDD = 83%, BSF = 1.15, and rad/R = 0.95
When you have an *exposure rate or dose rate* and are asked to solve beam on time, you're solving for MINUTES, not MU like normal.
5. If 3500 cGy is delivered at midplane (separation 20 cm) with parallel opposed 6x6 cm fields at 80 cm SSD, the max tissue dose is ______ cGy: Co-60 PDDs at 80 SSD for a 6x6 cm: *Depth*, PDD: *0.5*, 100.0 *1.0*, 97.7 *2.0*, 92.6 *3.0*, 87.0 *4.0*, 81.6 *10.0*, 52.5 *19.5*, 24.1
When you see "midplane" and "parallel opposed" and you're asked to get Dmax... That's your clue that you need the entrance and the exit dose.
21. A 16 cm thick patient will be setup to a 12x12 cm field at 100 cm SSD. The 15 MV x-ray beam is calibrated to 1 cGy/mu at Dmax. The MD wants to deliver 300 cGy per day to a depth of 8 cm and a total dose of 7500 cGy. a. What is the beam on time? b. What is the Dmax dose? c. The MD changes the setup to parallel opposed fields. What is the combined entrance and exit dose? d. Which way is better to treat this patient?
When you're solving for entrance dose, you would normally just take the Dmax that you solved in the previous part. HOWEVER this question states that the MD changed to PARALLEL OPPOSED, so therefore you have to get a new Dmax using the new dose of 150/PDD.
2. When does the Mayneord F factor work *well*?
When you're working under normal conditions: - Energy isn't too low - Field size isn't too large - Depth isn't too great - SSD change isn't too great
4. A dose distribution is normalized to 100% at the isocenter. The dose maximum in the target volume is 109%. If a total dose of 70 Gy is prescribed to the 90% isodose level, what is the max dose in Gy? A. 70 B. 82.4 C. 84.8 D. 98.8 E. 102
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5. A patient is 20 cm thick. We will be delivering 200 cGy per day mid plane from parallel opposed fields using 6 MV x-rays. The field size is 15x15 and the patient is set up to 100 cm SSD for each port. The OPF = 1.065 for 15 cm equivalent square, 1.077 for 16.5 equivalent square and the calibration is 1 cGy/MU at Dmax. The PDD at 10 cm = 69.2% and 18.5 cm = 44.6%. The TMR at 10 cm = 0.809 and 18.5 cm = 0.604. A. What is the beam on time for each field B. What is the Dmax dose for each field? C. What is the beam on time if we picked the 90% isodose line after treatment planning was completed? D. For the 100% isodose line, what is the beam on time if we set the patient up isocentrically?
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8. A patient will be treated 100 cm SSD with a 32x40 cm field using 6 MV x-rays with a solid tray to hold the blocks and a 30 degree wedge. The patient will receive 90 cGy per field to a depth of 10 cm. The output is 1 cGy/MU in H20. A. What is the beam on time? B. What is the beam on time if we switch to an SAD set up? C. What is Dmax dose for 8a?
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TMR
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TPR
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7. What happens to TARs with: a. Increasing SSD b. Increasing field size c. Increasing beam energy d. Increasing blocking of a field e. Increasing depth
a. Increasing SSD: *Independent* of SSD b. Increasing field size: *Increase TAR* c. Increasing beam energy: *Increase TAR* d. Increasing blocking of a field: *Decrease TAR* e. Increasing depth: *Decrease TAR*
1. A patient is simulated with AP/PA fields set up at 100 cm SAD at 12 cm depth. The collimator setting is 17x17 cm. The setup is then changed to 130 cm SSD. The collimator setting should be _______ cm. A. 8 B. 9.2 C. 12 D. 14.4 E. 15.7
C. 12
6. If the patient receives 125 cGy at a depth of 10 cm and the Dmax dose is 192 cGy, what is the *percentage* depth dose?
PDD = Dd/Dmax PDD = 125/192 x 100% = *65.1%*
Homogenous tissue equivalent phantom
Phantom made of material that is close in Z to real tissue
Isocenter
Point of interception between rotation of the gantry and rotation of the collimator
Tissue air ratio
TAR = Dd/Dfs
3. Define TAR and why was it developed?
TAR = Dd/Dfs It was developed for: - Rotational therapy - Irregular fields - Isocentric setups
Dmax
The point of peak dose inside the material at CAX. Also the point of electronic equilibrium
Quiz #22
This was the throw away quiz... So I'm not sure if some of these answers are correct or not
Geometric field size
Your field light at a determined distance
Extra Credit D. What is the Dmax dose for 8b?
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6. What happens to PDD and why with: a. Increasing SSD b. Increasing field size c. Increasing beam energy d. Increasing blocking of field e. Increasing depth
a. Increasing SSD: *Increase PDD* due to *inverse square law* effect b. Increasing field size: *Increase PDD* due to *more scatter* c. Increasing beam energy: *Increase PDD* due to a more *penetrating* beam d. Increasing blocking of field: *Decrease PDD* due to *less scatter* e. Increasing depth: *Decrease PDD* due to *more attenuation*
2. When making measurements of a megavoltage beam, why is the electron density of the phantom of concern?
Because Compton is the predominant interaction with MV beams
4. Why is field size dependence of PDD less pronounced for higher energy beams than for lower energy beams?
Because at higher energies, photons are scattered in a more forward direction, thus, the field size dependence of PDD is less pronounced for higher-energy than for the lower energy beams.
3. When determining TMRs or %DD, why are the ion chambers preferred over TLDs, diodes, and film?
Because they are more precise and have *smaller energy dependence*
17. What is the final result of the absorption of ionizing radiation by an animate or inanimate object?
Both will experience damage. The inanimate object due to increased *heat* energy of the radiation and the animate object due to DNA damage.
20. The dose rate for a 10x10 cm Co-60 field is 176 cGy/min measured in air, at 80 cm SAD. The BSF = 1.035. The dose rate in tissue at depth = Dmax, 80 SSD is _______ cGy/min. A. 151.7 B. 173.8 C. 179.9 D. 182.6 E. 161.5
C. 179.9
2. A single, direct spine field, 6x20 cm is treated on a Co-60 unit at 80 cm SSD. The dose is prescribed at 4 cm depth. In order to calculate the timer setting in the most direct way, one would need all of the following data except: Pick an answer from below and then explain why you chose it. A. Relative output factor for 9x9 cm. equivalent square B. PDD (9x9 cm, D4cm) C. TMR (9x9 cm, D4cm) D. Dose rate in tissue at Dmax, 80 cm SSD for a 10x10 cm field E. Prescribed dose per fraction at D = 4 cm
C. TMR (9x9 cm, D4cm) With TMR, you need to factor in inverse square correction, which is not a direct way.
11. Kerma and absorbed dose are approximately equal at: A. the surface B. a depth of 1 attenuation length C. the first point of electronic equilibrium D. none of the above
C. the first point of electronic equilibrium
9. A patient is to recieve 200 cGy at isocenter by full 360 degree rotational therapy, using 18 MV x-rays and a 12x12 field. The unit is calibrated for 1 cGy/MU at Dmax, 100 cm SSD. How many MU are needed for this treatment?
He took this one off the quiz, but I think you just do: Rx / Calibration(TMR)(OPF) He just gave us 18 MV and we don't have a chart for that, so... That's probably why he took it off the quiz, idk.
#2...
I emailed him about what the correct answer was, but he kind of just asked me questions and never gave me a solid 10 point answer, so... Maybe ask someone if they got a 10 point answer?
3. Percent depth dose is a function of what parameters?
It's a function of beam energy and field size. It increases with an increase in beam energy (greater penetration), field size (increased scatter), and SSD (inverse square law effect).
***Maximum and minimum values for BSF and at what energies they occur a:
Maximum: - Can be as high as 1.5 for beams with HVLs of 0.6-0.8 mm Cu depending on field size Minimum: - For beams above 8 MV it can be 1
Percentage depth dose
P = Dd/Dmax x 100% The percentage of dose given to a reference dose
Anthropomorphic Phantom
Rando A phantom that simulates human anatomy
SAD
Source to axis distance
SSD
Source to skin distance
16. Four 5 mCi I-125 seeds are arranged at the corners of a 1 cm square. Neglecting tissue attenuation, the dose rate in tissue at the center of the square is: Exposure rate constant = 1.46 Rcm^2/mCi hr; fmed = 0.9 A. 315 cGy/hr B. 13.14 cGy/hr C. 18.58 cGy/hr D. 52.56 cGy/hr E. 74.33 cGy/hr
Steps for these kind of problems: 1. Multiply what you're given (5 mCi, 1.46, 0.9) 2. Do the Pythagorean theorem 3. Inverse square law 4. Multiply by the # of seeds
8. From a patient treatment viewpoint why was TAR develop?
To get rid of dependence on SSD. Useful for: - Rotational therapy - Isocentric techniques - Irregular fields
Isodose curve
A line connecting points of equal dose
12. A single posterior spine field is treated at 130 cm SSD. Compared with treatment at 80 cm SSD, the exit dose will be: Explain your answer. A. Greater B. Smaller C. The same
A. Greater Because as SSD increases, PDD also increases.
18. Is the dose response curve linear or non-linear with increasing dose for: A. Ion chambers B. Calorimetry C. Film D. Fricke dosimetry E. TLDs
A. Ion chambers: *Linear* B. Calorimetry: *Linear* C. Film: *Non-linear* D. Fricke dosimetry: *Linear* E. TLDs: *Non-linear*
19. A 20 MV x-ray beam is incident on a water phantom. The absorbed dose is: A. Less than kerma at the surface B. Equal to kerma in the buildup region C. Greater than kerma beyond the depth of max dose D. Less than kerma at all depths
A. Less than kerma at the surface C. Greater than kerma beyond the depth of max dose
5. The SSD of a linac field is increased from 100 cm to 140 cm. Which of the following is true? A. The output at dmax will decrease B. The BSF for the same collimator setting will decrease C. The PDD will increase for the same depth in tissue D. The TAR for the same equivalent square and depth will decrease
A. The output at dmax will decrease C. The PDD will increase for the same depth in tissue
9. A patient treated with a complete 360 arc rotation using an isocentric linac. For a typical transverse pelvic contour the following is true about the absorbed dose to the isocenter (ignoring attenuation through the treatment couch): A. cGy/degree at the isocenter is not equal for the entire rotation B. Entrance dose in cGy/degree is not constant for the complete rotation C. Total dose at Dmax (cGy/degree) is constant around the entire contour D. All of the above E. None of the above
A. cGy/degree at the isocenter is not equal for the entire rotation B. Entrance dose in cGy/degree is not constant for the complete rotation
7. If primary transmission through a lead block if 5%, how can we have 20% of the dose under the block compared to the open field?
Because 5% is due to transmission and the other 15% is due to scatter... (He gave me 8 points and wrote "scatter from where?" so I assume I should have said "scatter from the collimator"?)
5. Calculate the equivalent square of a 7x23 cm field size using an equation.
Equivalent square = 2(7 x 23)/(7 + 23) = *10.7*
2. A patient is treated by a complete 360 degree arc rotation using an isocentric linac. For a typical transverse pelvic contour all the following are true. Explain why for each answer (ignore attenuation through the treatment couch): a. cGy/degree at the isocenter is not equal for the entire rotation b. Entrance dose in cGy/degree is not constant for the complete rotation c. Total dose at Dmax (cGy/degree) is not constant around the entire contour
*a. cGy/degree at the isocenter is not equal for the entire rotation* The body has different thicknesses; thus, some segments may exhibit more attenuation than other segments before it reaches isocenter. *b. Entrance dose in cGy/degree is not constant for the complete rotation* Entrance dose is focusing on the skin... So if the angle is large and is skimming more of the skin, then the entrance dose will be greater *c. Total dose at Dmax (cGy/degree) is not constant around the entire contour* There will be different depths at different angles, which means that for some segments, the beam will have to travel further to reach Dmax; thus varying the dose to Dmax with different angles
8. What steps do you follow to calculate the beam on time for a 360 degree rotation for treatment of the esophagus or prostate?
- Get a contour of the pt. using lead solder wire. Measure the AP and LAT dimensions of the wire to get the contour - Divide the contour into 20 segments - Get the mean depth from those segments - Get the mean TAR to figure out beam on time
2. How and why does TMR change as a function of depth, field size, SSD, and beam quality?
- TMR is *independent of SSD* because inverse square correction is *already factored in* TMRs. - TMR *increases with increased field size* due to *more scatter* - TMR *decreases with increased beam quality* due to *more forward scatter* - TMR *decreases with increased depth* due to more *attenuation*
1. Define
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1. Define the following using equations
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1. Define: If using equations, define all variables
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4. Define the following (using equations when appropriate)
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Quiz 19
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Quiz 20
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Quiz 21
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Quiz 22 (I think this should have been 23, but he also called this one 22... We took this one after the last throw-away quiz)
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Quiz 23
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Quiz 24 (Ch. 11 and 4)
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Use the following data for questions 4 and 5 Co-60 PDDs at 80 SSD for a 6x6 cm: *Depth*, PDD: *0.5*, 100.0 *1.0*, 97.7 *2.0*, 92.6 *3.0*, 87.0 *4.0*, 81.6 *10.0*, 52.5 *19.5*, 24.1
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7. A patient receives 250 cGy at a depth of 6.5 cm. If the PDD is 81%, what is the given dose?
0.81 = 250/x x = *308.6 cGy*
Match the backscatter factor for a 10x10 cm field with the energy of the radiation: 10 MV x-rays 20 MV x-rays Superficial, 2.5 mm Al HVL Co-60 A. 1.00 B. 1.02 C. 1.035 D. 1.26
10 MV x-rays: *1.02* 20 MV x-rays: *1.00* Superficial, 2.5 mm Al HVL: *1.26* Co-60: *1.035*
3. Parallel opposed fields are set up on a Co-60 unit at 80 cm SAD. The patient's AP thickness is 28 cm, and the field size at midplane is 18x18 cm. The field size on the skin is ______ cm. A. 10.8 B. 12.8 C. 13.8 D. 14.9 E. 17.6
18:80 as x:66 x = 14.9
3. What are off-axis factors and how are they used in treatment planning?
An off axis factor, such as POAR, is a ratio of the dose at a given point relative to a point off of CAX. We use them when using independent jaws and are treating a point off axis (e.g. breast) (This was a 9 point answer)
10. How does the initial build up change with beam quality and why?
As energy increases, the e- become more energetic and therefore travel farther, which means Dmax will be reached at a farther depth.
6. The fraction of dose due to scatter is greatest for which of the following Co-60 fields? A. 5x5 cm field at Dmax B. 10x10 cm field at 10 cm depth C. 5x5 field at 10 cm depth D. All of the above have an equal fraction of scatter
B. 10x10 cm field at 10 cm depth
7. The fraction of dose due to scatter is greatest for which of the following Co-60 fields? A. 5x5 cm field at Dmax B. 10x10 cm field at 10 cm depth C. 5x5 cm field at 10 cm depth D. All of the above have an equal fraction of scatter
B. 10x10 cm field at 10 cm depth
19. The dose rate at a patient's midplane is found to be 212 cGy/min at 100 cm SAD. A protocol stipulates that the dose rate must be no more than 100 cGy/min. The minimum SAD at which this dose rate is achieved is: A. 68.7 cm B. 146 cm C. 158 cm D. 250 cm E. 14. 56 cm
B. 146 cm
11. In the calculation of the timer setting for a mantle field, using the Clarkson method, TAR0 presents: A. The scatter component of the dose on the beam axis B. The primary component of the dose on the beam axis C. The backscatter factor for the blocked field D. The TAR of the equivalent square of the blocked field E. The tissue air ratio for the open, unblocked field
B. The primary component of the dose on the beam axis
3. In the calculation of the timer setting for a mantle field, using the Clarkson method, TAR0 presents: A. The scatter component of the dose on the beam axis B. The primary component of the dose on the beam axis C. The backscatter factor for the blocked field D. The TAR for the equivalent square of the blocked field E. The tissue-air ratio for the open, unblocked field
B. The primary component of the dose on the beam axis
Backscatter Factor
BSF = Dmax/Dfs
16. How does BSF vary as a function of energy, SSD and field size?
BSF is independent of SSD, no changes. As field size increases, BSF also increases. As energy increases, BSF decreases
4. When calculating the dose to a point off the central ray, what are the two man corrections you must apply to estimate the dose to the off-axis point?
Corrections for inverse square and depth
13. For an isocentric treatment setup, calculate the effective TAR, for a 10x10 cm beam of a Co-60 teletherapy unit, at the depth of 10 cm, using the data and tables provided below. TAR0 = 0.534 Radius#______Length______SAR 1__________________5.0___________0.161 2__________________5.1___________0.163 3__________________5.3__________0.167 4__________________5.8__________0.176 5__________________6.6__________0.192 6__________________6.6__________0.192 7__________________5.8__________0.176 8__________________5.3__________0.167 9__________________5.1___________0.163 A. 0.534 B. 0.361 C. 0.173 D. 0.707 E. 0.389
D. 0.707
5. Parallel opposed fields are set-up on a Co-60 unit at 80 cm SAD. The patient's AP thickness is 18 cm, and the field size at midplane is 22x22 cm. The field size on the skin is _______ cm. A. 24.8 B. 12.8 C. 13.8 D. 19.5 E. 22.0
D. 19.5
20. If only 1 of every 100 photons incident on an exposed film is not absorbed by the film, what is the OD? A. 0.01 B. 0.1 C. 1 D. 2 E. 100
D. 2 OD = (100/1)log OD = 2
11. Which is not true for backscatter factor A. Increase with field size B. Is energy dependent C. Can be as large as 1.4 D. Always increases with increasing energy
D. Always increases with increasing energy
10. As a quick check of the MU setting for a 360 degree rotation plan you would need all of the following except: A. TAR (or TMR) tables B. Average AP/PA and lateral depth of isocenter C. Output in air (or at max) for collimator setting used D. An isodose curve at standard SSD E. Prescribed dose per fraction at the isocenter
D. An isodose curve at standard SSD
9. The TAR for a 10x10 cm field at 100 cm SAD for 4 MV photons is 0.8 at a depth of 7 cm. What change would you expect in the TAR by extending the SSD from 93 cm to 193 cm (200 cm SAD)? A. 5% increases in TAR B. 10% increase C. 15% increase D. No change in TAR
D. No change in TAR
10. TAR at Dmax can be calculated from the BSF by: A. Multiplying by the SAR B. Dividing BSF by the collimator output factor C. Applying inverse square correction to the BSF D. No need to calculate; they are the same at Dmax
D. No need to calculate; they are the same at Dmax
