practice test 4
The effective treatment depth (80%) for tissues using a 12 MEV electron beam is approximately:
4 cm (12 mev/3 = 4cm)
If a patient is to receive 300 rads/day from a single ap SSD port to a depth of 6 cm using 4 mv photons, 6 x 15 field size. What is the d max dose? TAR=.763
393 rads 300/76.3 x 100 = 393
An error in looking up the %DD was made. The incorrect value was 86%. The correct value should have been 80%. What is the magnitude of this error (%)?
7.5% A/P = 86/80 = 1.075 = 7.5%
What is the dose rate at the center of a tumor if the dose rate at the same point in free space is 100 rads/min and the TAR at the depth of the tumor is 0.70?
70 rads/min 100 rads/min x .70 = 70
What is the wedge angle with a 30 degree hinge angle?
75 degrees 90 - hinge/2 90 - (30/2) = 90-15 = 75 degrees
A minimum unacceptable value for a high dose (hot spot) regions when wedge pair techniques are employed is greater than:
10%
A Co60 unit is set for a 10 x 10 field at 80 cm SSD. What is the field size at 100 SSD?
12.5 x 12.5 (direct proportion) 10/x = 80/100
The use of a universal wedge is most often employed to modify the distribution of: Cobalt teletherapy beams electron beams orthovoltage beams Linac photon beams
linac photon beams
The rationale behind using lower than the 100% isodose curve for chest wall therapies is: - lower lung exposure - all of the above - reduced bolus use - increasing skin sparing
lower lung exposure only
The application of single treatment fields may be employed when: - RBE is low - tumor dose distribution is not uniform - the structure receives more than the tolerance dose - none of the above
none of the above
Which of the following is not an advantage of a source axis distance SAD treatment compared to an SSD treatment: - patient is required to move - laser location is possible - TAR is improved - field size delineation
patient is required to move
With low energy photons, the absorbed dose per roentgen exposure is greater in bone than in soft tissue because:
photoelectric interaction (photoelectric increases with increase in atomic number)
The use of shielding blocks will require dose calculations involving:
unblocked field blocked field depth of target
The tilting of an isodose curve from its normal position is best accomplished by the use of:
wedge filters
All of the following parameters affect the shape of isodose curves, except: source size SAD SSD SDD
SAD (beam quality, beam collimation, SSD, SDD, source size, field size)
When treating with a shaped irregular field, one must correct the dose delivered as compared with an unblocked field by doing:
SAR calcs (irregular fields)
Which one of the following does not influence backscatter? - depth of the irradiated part - SSD (This is the correct answer, though I personally like answer "C") - beam quality - area and shape of field
SSD
Which of the following does not influence tissue air ratio?
SSD (goes for TMR too) (does beam quality, area of field, tumor depth)
Which of the following parameters effects the amount of penumbra---1. SAD; 2. SSD; 3. source size
SSD and source size (energy, SSD, SDD, source size)
%DD improves when:
SSD is increased (dose rate at further distances is slower than at shorter distances)
Calculate the treatment time to deliver 2.0 Gray to midplane on a Co60 80cm SSD unit. The output is 150 R/min, fields size is 10 x 12 open port appa, f-factor is .957, patient separation=18, %DD=86
0.81 min 200/2 *midplane means 2 fields 100 rads per field/150*.957*.86 =.81
The output of a Co60 teletherapy unit is 175 rad/min. Calculate the time necessary to deliver a dose of 200 rads at a PDD of 63 and a 1.02 BSF.
1 min 47 sec 200/175*.63*1.02 .77*60 sec= 1 min 47 secs
The TAR at the depth of maximum electron buildup (d-max) is 1. always less than or equal to 1; 2. dependent on the field size; 3. dependent on the SSD; 4. the backscatter factor
1. always less than or equal to 1 2. dependent on the field size
A patient is treated with two adjacent Co60 fields. One field is 8cm in length and one is 12 cm in length. Both fields are applied with an SSD os 80 cm. To give a uniform junction dose at a depth of 8cm, the gap between the fields on the skin should be:
1.0 cm Gap formula
A spine is to be treated at 100 cm SSD with two adjacent 26 cm field lengths. If the fields match at 7cm depth, the skin gap should be:
1.82 cm Gap formula
A patient is treated with two adjacent cobalt fields 12cm and 18 cm in length respectively at 80 SSD. Calculate the gap on the skin necessary to give a uniform junction at a depth of 10cm.
1.87 Gap formula
Calculate the equivalent square field size for a 6 x 38 rectangular field
10.36 cm^2 (6x38x2/6+38)
To treat an area of 40cm x 40 cm on a treatment unit with a maximum field size of 32 x 32 cm at an 80 cm SSD/TSD, what is the extended SSD/TSD required?
100 cm (direct proportion) 32/40 = 80/x
What is the hinge angle for a 40 degree wedge?
100 degrees (180 - wedge x 2)
The multiple for the prefix "centi" is:
10^-2
10^47 x 10^53 =
10^100
Calculate the monitor units required to deliver 100 rads to a depth of 6 cm for a 6 MV accelerator. The TAR is .888 and the AOF is 1.02.
110 MU 100/.888*1.02
What will be the finishing angle of an arc set-up if the starting angle is 270 degrees, the rotation direction is clockwise, mu/degree=2.16 and mu setting=302?
130 degrees * This is no longer on the registry exam, so the answer is 130°
What is the blocked equivalent for a 22 x 12 cm rectangular field with a 6 x 10 cm block in the field?
14.28 cm
Calculate the exit dose from the following information: given dose is 300 rads, tumor dose is 200 rads, %DD at exit point is 48%
144 rads (100%/48% = 300/x = 144 rads)
Wedge filters used in linear accelerators commonly vary in angle from:
15-60 degrees
A treatment field is 15 x 25 cm. The equivalent square of this field is:
18.75 15x25x2 / 15+25 = 18.75
Calculate the equivalent square for a 16 x 23 cm rectangular field.
19x19 (16x23x2 / 16+23 = 18.87 cm)
A Co60 unit has a source size of 1.5 cm in diameter. The SAD is 80 , the TFD is 112 cm and the source collimator distance is 48cm. What is the penumbra size on the film?
2 cm Penumbra formula s(SSD+d-SDD)/SDD
A (Cobalt) teletherapy machine has an output of 183 rads/min to a small mass of tissue 80 cm from the source for a 10 x 10 cm square field. Calculate the treatment time needed to deliver a dose of 300 rads to a point 6 cm deep on the central axis through a single field. A compensating filter is used which absorbs 10% of the incident radiation and the PDD at 6 cm is 0.756, BSF=1.113
2.16 300/183*.90*.756*1.113 = 2.16 (absorbs 10% = .90 for equation*)
Using the information in the previous 2 problems, what is the true total daily dose:
215 (1.075% percent error x og daily dose) 200*1.075
The collimator setting to treat a 40 cm field at a 130 cm SSD using an 80 cm SAD Co60 unit is:
24.6 cm x/40 = 80/130 (80*40/130=24.6)
A narrow beam of monoenergenic radiation from a point source strikes a water phantom at an SSD of 100 cm, The HVL for this beam in water is 20 cm. At a depth of 40 cm, what percentage of the surface dose is being received?
25% e^(-.693/20)*40 e^-1.38 = .25 = 25% e^(-.693/HVL * depth)
Calculate the mu's for a patient receiving 200 rads/day for a single ap port to a depth of 6cm. The TAR is .783 and the output factor is 1.02
250 200/.783*1.02
A lesion is treated using a 90 degree arc and the midpoint of the tumor volume is 5 cm below the skin surface. The isocenter should be placed at least __________ cm past isocenter
3.5 cm - isocenter 3-4 cm past isocenter *past pointing*
A patient was to have received 200 rads daily in 20 fractions at 80cm SSD. What is the % error if all treatments were received at a 70 cm SSD?
30% overdose inverse square (1.30 = 30% overdose)
A Co60 treatment is to be 4.35 minutes. Calculate the time required for a timer that can only be set in minutes and seconds.
4 time 21 seconds (60*.35= 21 secs)
A dose of 5000 rads is to be delivered at a depth of 12 cm at a PDD of 64%. What is the dose to an underlying organ at 16 cm at a PDD of 52%?
4062 rads (5000/x = 64/52)
THIS ONE ACTUALLY DOESNT GIVE YOU ENOUGH INFO TO COMPLETE, SO JUST SELECT: 5261 rads Calculate the total dmax dose (Entrance + Exit dose). DATA: 6 mv x-rays, 100 cm SSD, 13 x 14 field size open appa, separation=20cm, 5000 rads/25 fractions to midplane, TAR=.95
5261 rads
A treatment designed to be given at 120 cm SSD is mistakenly given at 100 cm SSD. What is the error in dose delivered?
44% overdose (P/A)^2 = 1.44 = 44% overdose
Two treatment fields on a patient with cancer of maxillary sinus have a hinge angle of 90 degrees. Calculate the wedge angle.
45 degrees (90- 90/2)
The wedge isodose angle is located at what isodose line?
50%
It is discovered that a patient receiving 200 rads daily has been given 25 fractions at 77 cm SSD instead of 80 cm SSD. Instead of the prescribed 5000 rads, the patient actually received __________ rads.
5395 rads inverse square
If an output of 100 rads was measured at 80 cm, what would the output be at 100 cm?
64 rads (inverse square) 100/x = 100^2 / 80^
The %dd for a 12 x 12 field size Cobalt beam, 8 cm depth and 80 cm SSD is 61.5. Calculate the percent depth dose for the same field size and depth for 100 cm SSD.
63.52 (Mayneord F factor)
An electron beam therapy is being used for irradiation of the chest wall. The most likely isodose curve chosen is:
80%
In electron beam therapy treatment of the breast, the energy most often chosen will correspond to a/an _________ depth dose at the chest wall-lung interface:
80%
If the dose rate in air at 80.5 SSD is 125 rads/min, what is the dose rate in air at 100.5 cm SSD?
80.2 rad/min inverse square (I1/I2=D2^2/D1^2)
The percent depth dose for a 12 x 12 field, 4 MV beam, 5 cm depth and 80 cm SSD is 82.8. Calculate the percent depth dose for the same field size and depth for 100 cm SSD.
84.8% Mayneord F factor
The most commonly used axial image database for a 3-D treatment planning computer is:
CT
Which of the following beams produce the least backscatter? - 250 kv x-ray - 6 mev electrons - 100 kv x-ray - Co 60
Co 60 (Increased energy results in increased forward scatter, so you would typically look for the highest energy beam. But, electron beams of all types have a lot of backscatter because electrons bounce back and are much less likely to transmit forward through tissue - which is why the dose goes to zero very rapidly when using electron beams)
The formula commonly employed for calculation dmax dose is:
Dd/PDD x 100
In order compensate for a change in distance with respect to percentage dose, a ______ is often employed:
Mayneord f-factor
When using isodose curves, one need not consider: - beam quality - field size - SAD or SSD - OER
OER
Hot and cold radiation spots are common problems associated with treatments involving: TNI fields superficial fields mantle fields none of the above
TNI fields
Which of the following does not modify an x-ray beam? - filters - a shadow tray (this is the best answer:) (fyi: this is the tray that the old custom blocks would sit on, probably an outdated term for the current ARRT exam) - gantry angle - all of these modify an x-ray beam - bolus
a shadow tray (does not modify x-ray beam)
The depth of maximum ionization is most dependent on:
beam energy
The backscatter factor (BSF) is dependent on:
beam quality and field size (also filtration)
The hinge angle is defined as:
between the central axis of 2 angle fields
An isodose curve can be employed to obtain measurements of depth dose percentage as a function of transverse distance from the:
central axis
For sloping skin surfaces, the skin sparing effect is often maintained by the use of:
compensating filter
Which of the following beams will result in the least amount of skin sparing, all other factors remaining unchanged?
curve 1
All of the curves in the diagram represent different quality beams at a 100 cm SSD and a 10 x 10 field size. The beam possessing the lowest energy is represented by:
curve 2
Pertaining to the curves in the diagram, which of the following beams would have a depth dose of 90% approximately 7 cm below the skin surface?
curve 3
Which of the following curves represent the beam possessing the highest energy
curve 3
5100 rads dose is to be delivered 5 days/week in 6 weeks through a single open port. If the TAR is .580 at tumor depth and the in-air factor is 1.10 for the treatment field, then: the daily tumor dose is 154 rad the daily mu setting is 255 mu's the daily mu setting is 322 mu's the daily mu setting is 293 mu's the daily given dose is 187 rad
daily mu setting is 255 mu's (really 266 but best answer) 170/.580*1.10=266
The dose build up region decreases as the energy of the beam:
decreases
Central axis depth dose distribution depends on beam energy. Which of the following is true?
depth of a given isodose curve increases with beam quality
The Mayneord f-factor that compensates for change in central axis depth dose includes corrections for:
distance/SSD - change in PDD with change in distance
An increase in the field size will increase the: electron emission from the collimator all of the above dose in the buildup region electron emission from the air
electron emission from the collimator dose in the buildup region electron emission from the air
Which of the following is not an advantage of parallel opposed fields? - excessive dose to surrounding normal tissue - homogenous tumor dose - set up reproducibility - less chance of geometric miss
excessive dose to surrounding normal tissue
The surface backscatter depends of the : - time of the exposure - source skin distance - mA - filtration of the x-ray beam
filtration of the x-ray beam
Integral dose is expressed as one of the following:
gram/rad (Kg/Gy)
An area outside the target area which receives a higher dose than the specified target dose is termed a :
hot spot
Which of the following combinations will result in the largest percent depth dose from a beam of x-rays generated at 250 kV?
large fields, long FSD, HVL 1.5 mm Cu (increase in filtration increased average energy)
A correction factor of about 15% is applied to the dose when using Cobalt therapy to the lungs because of the:
increase in dose due to the difference in density of lung (increase in dose with lungs, decrease with bone)
The concept of scatter air ratio (SAR) is most commonly employed for calculations involving ________________ irradiation.
irregular field
The SAR is a useful factor in the determination of the scatter produced in:
irregular fields
Transit dose is:
is measured at the central ray at a point beyond the patient ("transit dose" is another name for "remnant dose" - which is the dose leaving the patient (also called exit dose). Just as is the case with dose that is typically described to the tumor, is measured along the central ray)
The representation of volumetric variations in absorbed dose at different depths are depicted by means of:
isodose curves
The tumor that is enclosed by an isodose line is called the:
mean target volume
The presence of inhomogeneities in the body changes the dose distribution:
more with Co60 than with MV x-rays
Which of the following has no influence on the heterogeneity of an x-ray beam? - fluctuations of kilovoltage - output - characteristic radiation from the filter - multiple collisions
output
Which of the following does not influence %DD? - quality of the beam - size and shape of the filed - output - function of depth
output (does quality of beam, size and shape of field, function of depth, SSD)
The reduction of electron contamination in a photon beam can be reduced by all of the following except: - employing tin filters - reduce the angle of incidence - decreasing field size - increase shadow tray to skin distance
reduce the angle of incidence (can by: employing tin filters, decreasing field size, increase shadow tray to skin distance)
The BSF is effected by all of the following except: quantity of radiation treatment field size radiation quality
quantity of radiation (dependent quality and field size)
Variables not represented by inverted ratios are: exponents inversely square similar inversely proportional
similar
An isocentric treatment system means that: the 100% value is taken at the tumor depths the SSD is constant the 100% value is taken at the skin the field size is defined on the skin
the 100% value is taken at the tumor depths
The integral dose is:
total energy absorbed in a specific region (product of dose and mass Kg/Gy)
Which one of the filters below is not a compensating filter? - wedge filter - transmission filter - bolus - wax mold
transmission filter
