RAPHEX VII Flashcards
which of the following brachy isotopes cannot be produced in a reactor?
90Sr
103Pd
137Cs
192Ir
103Pd
it is proton rich and thus cannot be produced from borbadment of neutrons in a reactor
Isotop 228A goes through alpha and beta decay. Which of the following is not a possible daguther product?
228W
224X
222Y
220Z
222 Y
With alpha, would lose 4 nucleons, with beta don’t lose any nucleons
if scattering foil is left out, CAX dose will be?
> 50% too high
pencil beam
Does 6 MV or 15 MV require higher electron current to produce same dose rate?
6 MV requires more
because Bremss more efficient at high E, so more current is required at low energy
why does photon mode require more current than electron mode?
Bremss efficiency is always <1 and many photons are attenuated
-much more current needed
For electron beam where does most of the xray contamination come from?
jaws and electron cones
(made of high Z and much thicker than scattering foil)
A 10 MeV photon undergoes a Compton interaction and the scattered photon is emitted perpendicular to the direction of the incident photon. What is the approximate energy of the scattered photon?
0.511 MeV
-backscattered photons approach half of this value
Do FFF beams deliver higher dose rate than conventional beams for all FS?
No, only for small FS
-dose is higher at CAX but not higher than conventional on beam edges
If the FF is not centered properly, the beam will:
-be unflat, asymmetrical, and have slightly different PDD due to differences in beam hardening
absorbed dose in fat vs muscle vs bone at 6 MV and 20 MV
6 MV- Compton dominates- bone- muscle fat (fat has highest electron density)
20 MV- PP- fat-muscle-bone (bone has high Z)
portal dosimetry using EPIDs refers to all of the following except:
a.dose profile measurement
c. IMRT QA
b. patient position verification
d. MLC alignment verification
e. fluence distribution map
c- this is not portal dosimetry
which of the following is not a measure of plan quality?
isodose distributions conforming to PTV
meeting clinical dose constraints for normal tissues
beam arragement
total number of monitor units
MU is not quality
not sure I agree..
The relationshio between HU and electron density is approximately linear for most tissues except:
bone
lung
air cavities
muscle
bone
CT images are largely dependent on PE whereas therapy dose calcs are based on Compton
-first order CT numbers of most tissue scale linearly with density, but because of high calcium content, bone will deviate from this trend, giving higher CT values than appropriate for use in RT planning
-most TPS do this conversion automatically
All of the following are advantages of a dynamic wedge over a physical wedge, except:
a. approximately same CAX as open beam
b. field height is not limited
c. therapists do not have to lift a heavy wedge
d. less dose outside the field
e. wedge transmission factor is independent of field width
e
for dynamic wedge, the effective wedge transmission decreases with increasing field size since more beam modulation and hence more monitor units are required. This is similar to needing a thicker physical wedge with lower transmission factor for larger field widths
for the same delivered dose, which statement is false?
a. dynamic wedge are higher than for an open field
b. physical wedge are higher than for open field
c. universal wedge are higher than for open field
d. dynamic wedge are higher than for physical wedge
d
wedged field is always more MU than open field
MUs fur dynamic wedges are lower than for either a physical or universal wedge because the latter 2 objects are both thick metal objects placed in the beam
when treating a 3 field chestwall using mono-isocente,r which of the following is true:
a. collimator rotation is needed
b. couch kick awat from gantry needed
c. couch kick toward gantry needed
d. beam splitter needed
d
beam splitter eliminates beam divergence- doesn’t require couch or collimator rotation to achieve field match
A prostate cancer has a defibrillator/pacemaker. They decide to use 6 MV instead of 18 MV. Which statement best describes the effects of using the 6 MV beam?
a.total MUS are lower for 6 MV plan
b. in-field hot spot close to skin will be lower for 6 MV plan
c. 6 MV high isodoses will conform better to shape of prostate
d. none of the above
c
electrons have shorter range, thus it is easier to control shape of isodose lines
when treating s 0.6x0.4 cm2 tumor with 6 MV electrons, which of the following is true?
a. Pb mold should be shaped tightly to lesion size without margin
b. no bolus required
c. isodose lines will lie closer to patient surface than for a larger field
d. MU will be lower than for an unblocked field
c- isodose line constriction
the skin dose for a 6 MeV electron beam is lower than for a 15 MeV beam because for the 6 MeV beam there is..
less range straggling
-when normalized total electron fluence, the skin dose is actually the same for the 2 energies. But for the lower energy beam there is less range straggling, meaning that dmax dose is higher. Therefore, dose relative to dmax appears lower for 6 MeV beam
an electron depth of ___ is most suitable for treating PTV extending to 3 cm below the surface
they picked 9-12 MeV over 12 or 16 MeV
therefore assuming 80-90% prescription isodose line occurs at MeV/3
how many mm of melting alloy per MeV to shield electron beam?
0.5 mm ish
TG142 recommends picket fence how often?
weekly
TG142 recomments that geometric alignment of kV with MV is checked how often?
daily
most useful parameter directly measureable in PET scan
SUV
Standard uptake value
ratio of isotope absrobed by tumor versus background
All of the following studies have demonstrated an increased risk of radiation-induced cancers except:
a. atomic bomb survivors
b.chest fluoro for tuberculosis
c. diagnostic bone scans
d. radium dial painters
e. radation therapy
c
no evidence of this
according to NCRP, patients receiving therapeutic amounts of radioactive material may be released from the facility if the total dose to a member of the public is maintained below:
1 mSv
what requires lens and mirror system?
“old-fashioned” image intensifier
-newer ones have pixilated technology
for a non-gated lung treatment, a CBCT image can be acquired to assist with patient setup. The CBCT will be most similar to the:
a. inhale phase if 4DCT
b. exhale phase of 4DCT
c. average image of 4DCT scan
d. maximum intensity projection image
c
CBCT scan dose in air
10-50 mGy
dose from single kV projection
0.1- 0.5 mGy
Continuous tracking of prostate motion during fixed gantry IMRT delivery can be accomplished by:
a. CBCT with kV
b. MV CT
c. stereoscopic optical camera system
d. stereoscopic infrared camera system
e. implanted RF becon localization
e
the first 2 would have to rotate, and the other 2 only track the patient surface
typical range of acceptable table shifts after performing CBCT 3D/3D matching or kV 2D/2D matching?
2-7 mm
in which layer of the EPID do most xray interactions occur?
metal screen
which of the following is not an advantage of 3T MRI relative to 1,5 T?
a. lower distortions
b. increased SNR
c. increased spectral separation
d. increased susceptibility effects
a- distortions get worse
remember that susceptibility effects can be advantageous
in commerical a-Si flat panel imaging systems, which of the following is the primary means of detection in the pixelated array layer?
a. conversion of xrays into electron-hole pairs
b.conversion of electrons into visible light
c.conversion of xrays into high energy electrons
d.conversion of visible light into electrical charge
d
size of each pixel for 30 cm FOV CT
30 cm/512 = 0.6 mm
how many fducials are needed to determine x,y,z coordinates and the rotation?
3
3 pts determine a plane and thus the rotation
what energy to use for MV imaging?
lower energy image will have better contrast and sharper penumbra
why is MU lower for VMAT than IMRT?
more beam directions = less need tor intensity modulation (MLC leafe openings are larger on average for VMAT than IMRT)
cause of tongue and groove effect
“adjacent leaves that extend into field by very different amounts”
-beam may pass trough tongue or groove rather than full thickness- results in different beam transmission
which factor will increase the MU for IMRT field?
a. switxhing from 1 cm to 5 mm leaf widths
b. switching from sliding window to step and shoot
c. adding more OARs in optimization
d. using leaf transmission factor that is too high
c
to avoid structures, will need more modulation
minimum recommended total arc length for VMAT
360
what should dose rate for TBI be
< 0.2 Gy/min to minimize side effects
how is transit dose affectd by source decay?
decreases as source decays
because dose rate decays while transit time is constant
according to AAPM, QA of multi-channel intracavitary HDR applicators for breast includes:
a. daily CT scan
b. daily check of balloon volume and consistency of applicator position
c. check of each channel length prior to treatment
d. all of the above
d
type of electromagnetic energy used for hyperthermia treatment
microwave and RF
typical desired treatment temp for hyperthermia
45 degrees celsius
RBE of protons
says 1.1 but isn’t it 2????
