RAPHEX VI Flashcards
In which of the following is the microwave power absorbed at the end of the waveguide?
1.travelling waveguide
2. standing waveguide
3.superficial therapy unit
4. proton cyclotron
travelling waveguide
what range of angle can a compton electron be emitted at?
0-90 degrees
how does the HVL in Pb behave as xray energy increases from 1 to 20 MV?
HVL first increases, then decreases
u initially decreases with increased energy due to compton, then increases at higher energy due to PP.
As u decreases, the HVL increases
where is kerma greater than absorbed dose?
in build-up region
in diagnostic xray beams, filters are used to harden the beam. This process is mainly due to?
photoelectric effect
what happens if bias voltage on ion chamber is too low?
reading will be low
for ortho unit without measured data, what 2 factors are necessary to select the correct PDD table from published data?
HVL and SSD
The primary beam of a linac is set to 40x40 cm at iso (100 cm from source). What is field size at wall, which is 4 m from iso?
200x200 cm
remember have to go from source NOT iso
which of the following is not true when treating at extended SSD?
1. PDD will be greater
2.output of dmax will be IS that at shorter SSD
3. exit dose will be greater
4. surface dose will be slightly greater
surface dose will be slightly less, not greater
exit dose is greater because PDD is greater
a plan is calculated to deliver 200 cGy to the isocenter and normalized to 100% at this point. The hot spot is 105%. The physician decides to treat to 95% isodose and re-normalizes. All of the following are true except?
a. MU for each field will be increased by 5 %
b. isodose at isocenter now 105%
c. hot spots are now 110%
d. dose to OAR is decreased by 5 %
d
dose to OAR increases by 5 %
all isodose values and MUs will be increased by 5 %
A patient is treated with POP. Separation changes from 24 cm to 28 cm. If uncorrected, the dose will be___ at isocenter.
dose decreases by abut 3.5%/cm
2 cm each beam- 7 %
I really think this should have been 15% because it comes from both sides…
In a 3 field pelvis plan with post beam and lateral POPs, all of the following are true except:
a. wedges on the lateral fields compensate for dose gradient
b.45 and 60 degree wedge can both give homogeneous dose over PTV
c.thick ends of wedges will be anterior
d. field weights will depend on wedge angle used
thick ends of wedges point towards third field
c is wrong
dynamic wedges may have all the following limitations except:
a. there is minimum field size in wedge direction
b.there is min collimator setting at thin end of wedge
c.there is a minimum MU
d.there is a minimum field size in the non wedge direction
e.the wedge orientation may not be compatible with the wedge direction for some blocked fields
d
because the wedging is done with a jaw, no limit in the non-wedge direction
this is a pro over physical wedges
breast tangents are angled so that their posterior borders are aligned. Field width = 18 cm at 100 cm SAD. If LAO is 60 degrees, what is RPO angle?
divergence = tan^-1(9/100) = 5 degrees for each field
To eliminate divergence, RPO must be at 180+ 60 -2(divergence) = 230 degrees
in a lung treatment with 40 Gy, how close can fields be to pacemaker?
want pacemaker below 2 Gy
this is 5% of field and occurs about 2 cm away
remember penumbra are about 15 mm for 95/5 (actually about 30 mm in lung)
is electron or ortho field easier to shape?
ortho
but electrons have faster output, spare underlying tissue, some skin sparing, no increased dose to bone
what info does DICOM RT include?
all info in DICOM (i.e. slice thickness, HU, patient identifying info)
plus also treatment planning strructures, beams etc.
think DICOM RD has dose…
main purpose of combinate metal plate/phosphor screen in EPID is to:
a.convert incident xrays into visible light
b. filter out contaminant electrons exiting patient and couch
c.prevent backscatter of xrays into EPID
d. attenuate compton scattered xrays which improves image quality
a
how long to deliver 100 MU field for duty cycle of 25% and dose rate of 400 MU/min
60 s
dose for kV CBCT vs kV FBCT
similar
both 2 D and 3 D kV:
a.require higher resolution image detector plates than MV
b. are excellent for determining rotational setup errors
c.are valuable when used for patients with small implanted metal fiducials
d.allow visualization of soft-tissue anatomy
c
d is only for 3D apparently..
why do we use half-fan CBCT?
Increase reconstruction volume in axial plane
which of the following can provide real-time imaging during tx?
a.gantry mounted kV source and imaging system
b.dual room mounted (floor or ceiling) kV imaging systems
c.MV imaging system
d.tomographic imaging system
e. in room CT on rails
b
2 MV images are taken daily for 40 fractions. Dose to isocenter from images
each image is 2-3 MU and assume about 65% dose at isocenter depth
2 MU *2 * 40 * 0.65 = 104 ish cGy
resolution of DRRs compared to radiographs of conventional simulator
DRRs have poorer resolution in all directions compared to conventional simulation films, especially in sup-inf direction because of CT slice thickness
CI= volume covered by prescription isodose line/volume of target). A CI of 1 means…
a. target perfectly covered
b. part of target is under-covered
c. some normal tissue receives RX dose
d. target may be completely missed
e. any of the above
e
SRS cone has diameter of 7 cm. What is the largest jaw field setting that can be used?
7x7 cm2
-projection of jaw must fall completely within circle, or irradiation will leak through the aluminum to the patient
why are the cones ~ 10 cm long for SRS?
blocks beam outside the aperrture and reduces penumbra
once SRS cone is attached to linac, what should be performed?
-WL using film large enough to detect radiation outside of cone
-verify appropriate jaw size has been set
-disable jaw motion
For SRS treatment of AVM, CI is 1.3. DVH indicates D(14 Gy) is 95 %. The AVM is 23.5 cc. What volume of brain tissue receives at least 14 Gy?
target volume receiving 14 Gy is 0.95 * 23.5 cc = 22.3 cc
brain tissue receiving at least 14 Gy is 22.3 cc * 1.3 = 29 cc
since CI = volume of tissue receiving dose/volume of target receiving dose
a mammosite is planned with ortho images. Magnification of 1.4 is used instead of 1.39. What happens to dose?
dose is 15% too low because the balloon radius is assumed to be too small
(1.29/1.4)^2
tandem and ovoid plan done with MRI- do we need to do additional CT scan?
no because inhomogeneity corrections are not done for tandem and ovoid plan
need MRI compatible applicators; titanium is MR compatible
An HDR vaginal cylinder plan is optimized to deliver a uniform dose 0.5 cm beyond the cylinder surface. Which of the following is true regarding the dose distribution at different distances?
a. at shorter distances, the ends will be hotter than the center, whereas at longer distances the center will be hotter
b.at shorter distances the ends will be colder than the center whereas at longer distances the center will be colder
c. the dose will be almost uniform up to 5 cm
a
remember the peanut shape and how it dips in at the ends
most prevalent I-131 gamma ray energy
364 keV
for the same dose rate, required activity of I-125 vs Ir-192
I125 activity will be higher as it has a lower U
why use larger ovoids for tandem and ovoid in brachy?
due to decreasing dependence of IS law, larger ovoid yields less mucosal dose and better dose distribution
dose to bladder and rectum not affected
dose limiting organ for treating thyroid cancer with I-131
bone marrow
I-131 crosses stomach wall and enters bloodstream
half-life of I-131 in blood is several hours so marrow is continuously irradiated
whole body radiation dose to patient who has PET scan with 10 mCi FDG?
1-4 cGy
hyperthermia is in what temperature range and for how long?
41-45 celcisus for 30-60 min
thermal enhancement ratio
RT dose without heat/RT dose for equivalent effect with heat
all of the following effects will increase neutron contamination in a proton beam except:
a.moving range shifter closer to patient
b.using a scanning beam instead of a scatterer
c.placing the field shaping collimator closer to patient
d.increasing the primary photon energy
b
