Ford Flashcards

Question

activity of daughter is half life of parent is much longer than daugther

Answer 1

-activity of daughter about equal to activity of parent Activity of daughter = parent activity (half life of parent)/(half life of parent - half life of daugther)

Answer 2

(tau source/ tau Ra)* source activity

Answer 3

pt source falls off fastest, followed by 3 mm line and 5 mm line

Answer 4

half of its energy in MeV (cm)

Answer 5

30 kV photon can ionize anything but k-shell highest energy photon is L1-M1 = 4.12 keV If this energy instead goes to auger, 4.12 - 1.072 = 3,04 keV is highest energy auger electron

Answer 6

higher energy beams = higher energy electrons= longer range = bigger offset

Answer 7

they produce higher energy electrons which travel further in the medium. It thus takes longer for equilibrium to be reached

Answer 8

increases linearly with density -mass attenuation coefficient doesn't depend on density

Answer 9

(0.5)^n = 0.03 take ln of both sides and solve for n

Answer 10

yes, because electron ranges are longer in lung

Answer 11

(Z/m)^2 and acceleration ^2 (ie Srad increases with energy) this is in units of MeVcm^2/g Z is of medium and m is of particle

Answer 12

decreases in higher Z material as there are less electrons to interact with not a strong dependence on energy

Answer 13

mostly collisional loss, radiative loss very small -depends on Z of medium, z^2 of particle -inversely proportional to velocity^2 -does not depend on particle mass

Answer 14

electron undergoes "hard" collision in which substantial amt of energy is transferred to electron -delta ray can travel relatively long distance

Answer 15

The charge produced is from collisional losses. The collisional stopping powers for a 6 MeV electron in the above materials are, respectively, 1.870, 1.874, 2.474 MeV cm2/g, so methane should be the right answer. However, most of the electron collision interactions are not in the gas itself but rather in the water which then streams into the ion chamber. The readings, therefore, are approximately the same.

Answer 16

tungsten The exposure at 2 m will be from bremsstrahlung photons. In higher Z materials the production of bremsstrahlung is much larger (much larger radiative stopping power). If the block were very large, self-shielding may reduce the ux of photons from a high-Z material.

Answer 17

collisional losses of the electrons

Answer 18

skin sparing

Answer 19

-oscillating voltage is applied to accelerating structures -oscillation frequency is such that electron reaches the gap between cavities just as the voltage gradient is at its maximum -frequencies are S or X band for linacs -cavities are designed so that one wavelength of RF spans one cavity length- this means X band can be made more compact (however harder to make)

Answer 20

-magnetron is smaller, generates microwaves -klystron is amplifier- amplifies W to MW- larger, bulky, and is stationary

Answer 21

-Cathode is heated- electrons boil off of it and accelerate towards anode -magnetic field makes the electrons circle around the cathode instead of hit the anode directly -electrons accelerating around the cathode produces microwaves which are picked up by the output antenna

Answer 22

to prevent arcing -if there is no gas in system, there are no atoms to ionize and carry the current of a spark -however, microwave generation system is exception as it has SF6 at high pressure

Answer 23

cathode through which electrons are emitted

Answer 24

introduces pulses into klystron/magnetron and also into gun -electron system and RF system are switched on and off in synchrony with each other

Answer 25

-pulse repetition rate and current in peak of pulse

Answer 26

-all electrons strike target at same position for achromatic -otherwise, electrons strike target at different positions depending on their energy achromatic = magnetic field increases going out radially from center. Therefore, as higher energy electrons travel through a larger orbit, they experience a larger magnetic force and are bent back to same location -slalom magnet achieves same as achromatic but uses 3 magnet sectors- can make gantry head more compact

Answer 27

-not by flux of electrons but rather by energy of electron as it emerges from waveguide -this is controlled by switching sections of the waveguide on or off

Answer 28

The ratio of collisional stopping power in tungsten at 40 keV vs. 6 MeV is 3.17. The implication of this is that relatively more energy is dissipated in collisional losses in an X-ray tube vs. a linac. This energy goes into heating the anode. More energy is converted to bremsstrahlung photons in a linac target (e.g. 6 MeV electrons) compared to an X-ray tube (e.g. 40 keV electron).

Answer 29

c. reduces overall output The emerging photons are high energy and mostly undergo the Compton process in the copper surrounding the target. In this energy range, the Compton cross-section is roughly independent of energy so the beam does not harden or soften much, i.e. it does not preferentially lter out higher or lower energy photons. However, it does reduce the overall uence because it attenuates the beam. Note that the very softest part of the spectrum (keV) energies will be ltered

Answer 30

a- lower energy electron emerges If the electron bunches are not synchronized with the RF, they cross the gaps when the potential between cavities is low. Therefore, they are not accelerated efciently in the waveguide. The output decreases because high-energy electrons ultimately do not reach the the target

Answer 31

A larger magnetic eld would bend the electrons more (i.e. into a tighter circle), which theoretically should move the spot away from the gun.

Answer 32

W = source size (SSD-SCD)/scd SCD is source to collimator distance if penumbra is at some depth in the patient d, then SSD becomes SSD + d -if SCD = SSD (ie collimator placed at skin) then penumbra at skin would be 0

Answer 33

tongue and groove make dose fall-off less sharp than if these features were not present

Answer 34

-no FFF -85 cm SAD vs 100 cm

Answer 35

high frequency and low energy

Answer 36

-source size -shape of beam collimation device and transmission through edge of device -scatter at edges of field- higher energy electrons can scatter further and thus penumbra are smeared out

Answer 37

horns are larger for 10 MV beam

Answer 38

2 x dose for 40x40 dose at skin is similar for different energies, slightly lower for high E skin dose is increased if beam goes through couch or other device

Answer 39

decrease skin dose increase from plastic

Answer 40

PDD2 = PDD1 * ((SSD2+dmax)/(SSD2+d))/((SSD1+dmax)/(SSD1+d))

Answer 41

FS unblocked * square root (1-f) where f is fraction of field that is blocked

Answer 42

PDD is smaller than TMR so the calculated MU will be larger than it would have been if the TMR value were used. The delivered dose therefore will be larger. The PDD differs by including an inverse square factor which for depth of 10 is approximately 20%, i.e. (100/110)2

Answer 43

Sc will be for the "unblocked" field- i.e. the size of the collimator at iso

Answer 44

convolve TERMA with kernal -TERMA is total energy released in matter at each voxel. Considers attenuation, radiological pathlength -kernel is calculated using MC and describes the contribution of terma in one voxel to neighbouring voxels -kernels are also scaled based on density (ie in regions of lower density, kernel is larger because range of electrons is larger)

Answer 45

penumbra are wider in lung due to longer range of electrons and this effect is worse in high energy beam because electron range is longer

Answer 46

-beam attenuation increases in bone due to higher density -distal to bone there is increase in dose due to backscattered electrons -within bone itself dose is not much different for 6 MV; at higher MV, pair production becomes significant and dose in bone increases

Answer 47

small field high energy

Answer 48

heels together, toes apart, like in tango

Answer 49

90- (hinge angle/2) hinge angle is angle between 2 beams

Answer 50

Order (smallest to largest hot spots): c, a, d, b. The hot spots in tissue are largest for larger separation and lower energies. This is due to the PDD dropping off more for larger separations and for lower energies.

Answer 51

where dose can be accurately calculated -not in build-up, steep gradient, or tissue interface For IMRT, use dose-volume prescription and not dose to a pt since dose is not homogeneous

Answer 52

single fraction: SF = e^(-alpha D - beta D^2) multiple fractions: SF = e^(-alpha D - beta* n*d^2)

Answer 53

-uses dose efficiently whereas with leaves, much of the beam is "wasted"

Answer 54

Order is: a. 3D-CRT, d. Compensator IMRT, b. Step-and-shoot IMRT with MLC, c. Dynamic MLC IMRT

Answer 55

The 3D-CRT plan delivers more doses to the OAR at the intermediate dose levels. However, the situation is reversed at low-dose level, the VMAT plan “spreading out the dose” more.

Answer 56

With a rotation the leaves do not line up with each other. This prevents interleaf leakages from overlapping exactly in the same place as the arc rotates around.

Answer 57

higher energy electrons travel further and lower energy electrons travel less -electrons scatter with air along the intervening path - some have lost energy

Answer 58

R90 = E/3.3 R80 = E/3 R50 = E/2.33 Rp = E/2 higher energy beam has: -deeper penetration -higher surface dose -more spread out fall-off -broader region of dose max surface dose is about 75+ E % -Brems. contamination is 1-5 % of peak dose and is higher for high E electrons

Answer 59

-dmax closer to surace -surface dose increases

Answer 60

-penumbra for higher E is smaller than for lower E because higher E electrons are more forward directed -as depth increases, penumbra become wide, especially for high energy beams. Higher isodose lines constrict, lower isodose lines bulge out

Answer 61

5 cm is typical also leave 1 cm wider on each side than treatment area to account for penumbra

Answer 62

-larger penumbra - more constriction and bulging -output decreases- use virtual source for IS corrections -light field no longer accurately tracks radiation field

Answer 63

-dmax gets pulled closer to surface -doses are higher on side where angle is sharper ie dose lines pull closer to surface

Answer 64

-tissue prominence in beam center- dose under prominence is shifted up, regions on either side of prominence have more dose due to outscatter of electrons from prominence into these regions --tissue defect- isodose lines under defect are shifted away from source. Inscatter of electrons from sides into region under the defect, creating hot spots on either side just inside edge of defect

Answer 65

Lateral disequilibrium effects become important when the eld size becomes smaller than the practical range of electrons (E/2). For a 12 MeV eld this is approximately a 6 × 6 eld

Answer 66

The low-Z absorber is intended to prevent electrons from backscattering into the tissue proximal to the shield. If the orientation is reversed the lip would receive an extra dose. This dose increase could be as high as 70%.

Answer 67

along CAX b/c brems is more forward directed

Answer 68

-0.6 cc -0.125 cc -0.06 cc -0.006 cc

Answer 69

From Fig 4 (or Table I) in TG-51 the factor at 65% is 0.993 for the chamber model # NE2571. This value is higher than the one in Problem 2 by a factor 0.993/0.972 = 1.022. Calibrated output will be 2.1% too low. Because of the high kQ the corrected reading will be too high. In order to achieve 1 cGy/MU, the machine would therefore be calibrated down to a lower reference dose Kq decreases as % dd10x increases

Answer 70

The ratio of doses for eld sizes 20 × 20 vs. 10 × 10 is TMR(20X20)/TMR(10X10) = 1.043. Therefore, the measured dose will be too large by a factor of 1.043. In order to achieve 1 cGy/MU, the machine would therefore be calibrated down to a lower reference dose by a factor of 1/1.043 = 0.96.

Answer 71

pros: instant readout, sensitive, small, no bias voltage 'cons: energy dependense, dose rate dependence, temperature dependence, radiation damage, directional dependence

Answer 72

for photon diodes -provides shielding from low energy scattered photons, which diode overresponds to

Answer 73

No -neither are film or OSLDs -only chambers are primary standard

Answer 74

Al2O3:C -wavelenght of photons released is 400 nm -when radiation is absorbed, electrons are excited from valence to conduction band and become trapped. Upon exposre to light (or heat for TLD), the trapped electrons transition down and emit photons -TLD material is LiF

Answer 75

pros: small, don't need wires, sensitivie, easy to use, minimal dependence on temperature, dose rate, or beam direction cons: non-linear dose dependence (supralinear, must be careful while calibrating), fading, difficult to reuse, over-respond to low energy photons, readout is not instantaneous

Answer 76

OD = log10 (Io/I)

Answer 77

OD with no exposure -0.2

Answer 78

pros: high resolution, low cost cons: requires processor, scanner, has energy dependence

Answer 79

pros: high resolution, no processor, not energy dependent cons: scanner required, complex calibration, expensive

Answer 80

-scatter is softer fluence -OSLD thus over-responds 10-25 %

Answer 81

-slower because diode over-responds to low E photons

Answer 82

effective Z of Al2O3:C OSLD is 10- higher than tissue -OSLD can over-respond by factor of >3 for low kV energies

Answer 83

The beam spectrum is much softer on the exit side of the patient due to scattered photons which are lower energy. The diode will therefore provide an articially high reading on the exit side vs. the entrance side. This effect will depend on the thickness of the patient.

Answer 84

failure mode and effect analysis multiply severity, occurrence, and detectability

Answer 85

dose verifications (IMRT QA) in vivo QA transmission devices: mounted on head of linac and beam is transmitted through them calculation based- use log file fro patient and calculate fluence and compare to plan

Answer 86

Earthquakes can cause a movement of lasers which are mounted on the wall. All the other systems are physically integrated systems tied to the linac beam itself and are less likely to be affected.

Answer 87

This would require a measurement and that measurement would have to be performed on the patient, not on a phantom. In vivo diode measurements are typically performed on the entrance side of the patient, so anatomical changes are typically not registered. EPID dosimetry in vivo (i.e. with the patient under treatment) may identify the changes, since the transmitted dose changes.

Answer 88

This asks about the number of total points. Ranking is: a, b, c, d. Film has more points than the diode array. 2%/2mm is a stricter criterion than 3%/3mm so the pass rate will be lower.

Answer 89

Film has the highest spatial resolution and so will be more sensitive in the high-gradient situation where the change in dose per cm is large.

Answer 90

film The gamma metric is a combination of dose difference at a point and distanceto-agreement (DTA). This requires some spatially resolved measurement, i.e. more than a single point measurement from a TLD (or OSLD). There are no chambers in the mail order phantom.

Answer 91

a and d This problem requires the QA to be measured at the actual gantry angle of delivery (not at gantry angle 0). A single ion chamber only provides a point measurement which is not sensitive to this problem

Answer 92

source width * ((SID-SOD)/SOD)

Answer 93

-magnification increases but resolution decreases

Answer 94

-copper plate yields electron from photon due to compton -electron interacts in scintillator- Gadolinium oxysulfide -light are created -photons are registered in detection layer - photodiode- amorphous silicon -detector is read ouit row by row -voltages on gate lines control what row is being read out

Answer 95

N/root(N) = root(N)

Answer 96

number of optical photons produced for each xray photon that enters

Answer 97

mA kVp^2 at skin of patient mA kVp^5 transmitted through patient

Answer 98

responsiveness of detector as a function of frequency of features in image

Answer 99

yes, but does not provide any useful information

Answer 100

digital imaging and communication in medicine -communication

Answer 101

picture archiving and communications system -storage

Answer 102

D/S -D is distance table travels in one rotation S is slice thickness -pitch> 1 is undersampled pitch < 1 is oversampled

Answer 103

square root of mA s slice thickness

Answer 104

focal spot size detector resolution pixel size mA affects noise but NOT spatial resolution

Answer 105

1 min vs 1 s -more motion with CBCT -also more scatter with CBCT -because of scatter, HU from CBCT not accurate (measured signal no longer depends on attenuation but also on scattered photons)

Answer 106

faulty pixel in detector reconstructs as a ring

Answer 107

center of image appears darker than periphery -larger scatter contribution to center -beam hardening is more through center

Answer 108

due to beam hardening in projection angles that pass laterally through the high density objects - yields a hardened spectrum- detector registers this as fewer photons - gives dark streak

Answer 109

The distance the table travels per rotation is d = × = × = p S . . .1 0 1 5 1 5 mm/rotation, where p is pitch and S is slice thickness. The table therefore travels at a speed of 3.0 mm/s. For a scan length of 500 mm the required time is 167 sec

Answer 110

This results in streaks due to missing information from various projection angles.

Answer 111

gyromagnetic ratio * B/(2 pi) 1 H is 42.6 MHz/T

Answer 112

gyro = magnetic dipole moment / (spin * hbar)

Answer 113

dark on T1 weighted and bright on T2 weighted

Answer 114

shortens T1 relaxation rate -gives brighter signal on T1 weighted images

Answer 115

the spins are inverted 180 degrees and then allowed to decay. tarting the remaining sequence at a specific time nulls the signal from a particular tissue

Answer 116

-changes in local magnetic field are equivalent to moving the pixel in space

Answer 117

metal object distorts field (or really any hetereogneous object)

Answer 118

if gradient is larger than expected, the image would be compressed in that direction

Answer 119

-interface of 2 materials -signal is "mis-mapped" because it has a different frequency- this is interpreted as being mapped to a different location

Answer 120

140 keV photons

Answer 121

-radiopharmaceutical emits photons -photons interact with scintillator- convert gamma rays into optical photons -photons are registered by PMT tubes -collimator in front of scintillator for spatial localization

Answer 122

-decays to 18O and beta plus -110 min half life -18F-FDG -15-20 mCi per patient

Answer 123

-positron produced in decay wanders in tissue and annihitles with an electron, producing 2 0.511 MdV photons opposite each other -these photons are registered as events coincident in time in the crystals of the PET detector ring and a line of response can be calculated -as more decays occur, more lines of responses are acquired at different angles

Answer 124

-annihilation photons are not exactly colinear because some momentum imparted to nucleus must be balanced by momentum vector of photons in opposite direction -LOR does not intersect exactly with position of annihilation event and thus affects spatial resolution with PET

Answer 125

-4-5 mm FWHM -non-colinearity of annihilation photons -size of crystals in detector ring -energy of positrons- they travel in tissue before annihilating and thus blur image -algorithms and filters used in reconstruction

Answer 126

activity in image/(injected activity/body mass)

Answer 127

-some tissues (ex lung) attenuate less than others- would make it look like there is more activity -use CT to do attenuation correction

Answer 128

Advantages: Reduced artifacts (e.g. chemical shift or magnetic susceptibility), lower cost, allows for open design (less patient claustrophobia), lower fringe elds (i.e. low Gauss lines extending out from magnet), low SAR (specic absorption rate, i.e. energy deposition in tissue). Also less effect on electron paths when used during therapy, i.e. the “electron return effect.” Disadvantages: Lower signal-to-noise (approximately proportional to B0), longer scan times, some pulse sequences not available, more distortions due to B0 inhomogeneities since permanent magnets are often used, less enhancement with contrast agents like Gd.

Answer 129

The PET detector ring in a small animal scanner is much smaller. Therefore, the non-colinearity of photons has a much smaller effect than in a patient scanner because of less divergence in the paths of the two photons. The range of the positron is about the same in both cases which limits resolution.

Answer 130

The resolution is affected by the range of the positron. Resolution is worse with increasing range, i.e. increasing energy.

Answer 131

ultrasound can resolve features in the depth direction that are one-half the distance of a spatial pulse length. Here the spatial pulse length is three cycles, i.e. 3 lambda = 3v/f = 3*1540/(3.5*10^6)mm. Therefore, the smallest feature that can be resolved in the depth dimension is half of that or 0.66 mm. Increasing the frequency to 5 MHz would improve the depth resolution further but there would be less penetration of the ultrasound wave in depth.

Answer 132

SAR is “specic absorption rate.” It is a measure of energy deposited in tissue and has the units of W/kg. SAR is affected by many parameters including eld strength (quadratic dependence), the gradient pulses used, the repetition rates, the ip angles, and frequency. To maintain safety, the FDA limits SAR levels to 4 W/kg for a 15-minute whole body scan (other similar values are in place for specic sites). The IEC limits are similar though different “operational levels” are dened and a 6-minute scan length is considered.

Answer 133

Sensitivity increases at rst with injected activity. At very low activity, the signal is photon limited, and increasing the number of photons improves the signal-to-noise ratio. As the activity increases, however, the number of random counts registered in the detectors increases. These are event pairs that are agged as simultaneous but which are actually not from the same annihilation event. Also the number of scatter events increases. Therefore, at very large injected activity the sensitivity decreases again.

Answer 134

assumes the anatomy of the patient is completely rigid and that a translation and rotation can be performed to make the patient align perfectly with the reference scan -not always case due to changes in anatomy, neck flexing (for example)

Answer 135

-CBCT linac -tomo -halcyon -cyberknife -exactrac -US guidance -Calypso (transponders) -surface imaging- AlignRT

Answer 136

linac MRI -electrons don't travel in straight lines in magnetic field -electrons can bend back toward source and create regions of high dose (happens in lung for example- get high dose at chest wall)

Answer 137

-online (image prior to treatment) -offline (image at time of treatment but apply corrections at next treatment) real time- image continuously through treatment

Answer 138

custom acceptance test phantom

Answer 139

-use 4DCT to make ITV -gating -breath hold -compression -tracking

Answer 140

kV planar images are a single exposure and much lower dose (<1 mGy/image) while CBCT delivers approximately 2 cGy. Multiple planar images acquired over time (“uoro mode”) potentially allow for tracking of features that can be visualized. This is used, for example, in CyberKnife treatments

Answer 141

10 mm * tan ((pi/180)*3) = 5.2 mm

Answer 142

. Less uniform for 40 cm Less uniform due to an increased “cupping artifact” for larger phantom sizes. Recall that this is due to scatter in the CBCT geometry.

Answer 143

noise scales like root(N) , where N is the number of photons. Increasing the mA increases the number of photons. relative noise decreases (rootN over N)

Answer 144

Advantages: Reduces intra-fraction movement, the possibility to eliminate the iGTV and therefore have a smaller treatment volume, inates lung (if breath is held at end inspiration) which results in more normal lung sparing, moves heart away from the treatment eld potentially. Challenges: Achieving the same lung ination at each breath-hold may be a challenge (residual air in the lung at tidal exhale can confound the perceived ination volume); it may be challenging for some patients to hold their breath especially those with compromised lung function; treatment times are longer since the beam is off while you wait for the patient to breathe in and hold their breath

Answer 145

The distance per breathing cycle is pitch* S * Tresp/Trot, where Tresp is the respiratory period. The distance is then 9 mm. Note that in theory then you could reconstruct three independent slices in this period (3 × 3 × 3 = 9 mm). To get more independent slices you would need to either decrease the pitch (which increases dose) or make the tube rotation speed smaller

Answer 146

The distance per breathing cycle is pitch* S * Tresp/Trot, where Tresp is the respiratory period. The distance is then 9 mm. Note that in theory then you could reconstruct three independent slices in this period (3 × 3 × 3 = 9 mm). To get more independent slices you would need to either decrease the pitch (which increases dose) or make the tube rotation speed smaller -remember L is length of table travelled per cycle -and breathing is 6 s- ie 6 cycles

Answer 147

-single fraction -> 5 Gy -target diameter > 3.5 cm -brain -accuracy < 1 mm -no PTV margins, CTV in some cases

Answer 148

-4,8,16 mm -collimators are tungsten

Answer 149

volume of prescription isodose line/ volume of PTV

Answer 150

volume of 50% isodose line/ volume of PTV -smaller PTV= smaller R50%

Answer 151

max dose at 2 cm from PTV -smaller PTV = smaller D2cm

Answer 152

CI < 1.2 V20Gy < 10%

Answer 153

-MLC width = 5 mm -detectors for small fields -E2E tests -motion assessment for thoracic and abdominal sites -dose calc grid < 2 mm -use IGRT for alignment

Answer 154

d. Increased to 123% of original If the plan is not changed aside from the prescription, the D2cm will track with the maximum dose. In the original plan the maximum dose was 18x3/0.75 = 72 the rescaled plan the maximum dose is 10x5/0.8 =62.5 ´ which is 87% of the original. The overall dose is decreased (54 Gy to 50 Gy). Also increasing the prescription isodose line will decrease the overall dose.

Answer 155

lung_r + lung_l - iGTV

Answer 156

12-15 Gy in 6-10 fractions BID for myeloablative (ie kill stem cells in bone marrow) -want uniform dose with +/- 10% variation

Answer 157

-big SSD so patient fits in field -spoiler- creates extra superficial dose in patient (for example to get bone marrow in ribs, which is at shallow depth) -lung blocks -compensator- corrects for fact that some parts of patient are thin (neck) and others thicker (abdomen)

Answer 158

-higher energy and bigger SSD = bigger PDD= more homogeneous -smaller patient separation = more homogeneous

Answer 159

AP/PA with patient standing- patient can't tolerate AP/PA with patient lying down - patients move, moving lung blocks. Also, lung on downward side is compressed lateral beams with patient sitting or lying down- cannot apply lung blocks and dose is less uniform in depth direction compared to AP/PA because of larger separation

Answer 160

measure with TLDs, OSLD, MOSFET, diode -make sure detector is calibrated to TBI geometry

Answer 161

total skin electron therapy used to treat cutaneous T-cell lymphoma -36 Gy in 36 fractions over 4 days per week -usually 6 MeV is used so you don't get too much Bremsstrahlung in patient

Answer 162

-2 beams aimed at angle at patient- reduces x-ray component which is highest along CAX of beam -use extended distance so whole patient is in field -at extended distance there is lots of scatter and attenuation in air, therefore, operate linac in high dose rate mode -use multiple fields from different angles around the patient -3 beams from 3 different angles on one day, followed by 3 beams from 3 different angles the following day

Answer 163

-use a scatterer -place near exit window of linac instead of near patient as in TBI -placing scatterer near exit window of patient instead of near patient results in narrower angle for electrons that are scattered which results in a deeper depth-dose curve

Answer 164

-these regions do not see direct electron beam -treat with electron boost field with conventional techniques -skin folds can create regions of low dose

Answer 165

Overall the dose will be higher due to inverse square falloff. Because it is closer the dose is less homogeneous (faster PDD falloff). The head and feet may be cold because the eld size may be too small especially if the patient is tall

Answer 166

-pediatric cancer -CNS tumors -eye cancer -sarcomas -unresectable H and N cancer -liver cancer -re-irridiation cases

Answer 167

90-230 MeV

Answer 168

-spread out bagg peak -modulator (low Z) reduces energy of beam and therefore range -range modulator wheel has different thicknesses of plastic, tuned to provide SOBP when wheel is spun -beams with larger modulation have higher surface dose because each Bragg peak contributes surface dose

Answer 169

in proton therapy, used to create 3D coformity to target- thickness is adjusted to modulate beam at different points -can be very sensitive to patient movement

Answer 170

-if it is bone, actally pulls the edge of the SOBP to a shallower depth -if it is lung, pushes the edge of the SOBP to a deeper depth

Answer 171

reduces scattering of the beam

Answer 172

-first scatterer -second scatterer -range shifter -range modulator -patient aperture -patient compensator

Answer 173

-similar (not much better) to photon beam -increases with depth

Answer 174

-magnetic steering system scans beam back and forth over tumor -one layer is scanned -energy of beam is changed, which moves depth of bragg peak -another layer is scanned -and so on

Answer 175

2000X heavier than electrons linac not strong enough to accelerate it

Answer 176

-apply electric field to 2 half circles -particle moves due to E field -magnetic field is applied perpendicular to proton path, forcing proton to curve -as proton travels, its energy increases due to E field -pick off proton at specific radius to select energy

Answer 177

-particle beam travels in a vaccuum tube -bending magnets bend beam into a circle -each time beam goes around, it travels through an accelerating cavity, which applies E field to accelerate particle -proton is extracted when it reaches desired energy

Answer 178

-average beam current is higher in cyclotron -cyclotron can yield continuous beam whereas synchotron yields beam in pulses -in synchotron, can switch energy of beam quickly whereas cyclotron usually operate at fixed energy -cyclotrons are more compact -synchotron- particles can reach extremely high energies

Answer 179

-not uniform like photons -direction of beam must be accounted for -more margin needed at distal end (where bragg peak falls off suddenly) because uncertainties have bigger impact here due to bragg edge -added margin to account for uncertainty from CT numbers, beam energy from accelerator

Answer 180

-effects of motion can have more impact for proton beams due to the bragg peak and its distal edge location...

Answer 181

-range in tissue for heavy nucleon is less than that of proton, for the same energy -LET for heavy nucleon is higher -for heavy nucleon, dose is deposited beyond bragg peak because of nuclear fragments created through inelastic scattering of carbon ion -penumbra of carbon ion beam is smaller than that of proton beam, due to less multiple coulomb scattering of heavier particle

Answer 182

-actually independent of LET -reason why high LET ions do more damage is because the spacing of DSB is closer in high LET vs low LET, and high LET produces clustered lesions, which is more difficult to repair

Answer 183

multiple Coulomb scattering

Answer 184

Spread in the energy of the protons. Some stop earlier and contribute more at the proximal edge while some are at higher energy and stop deeper at the distal edge. The range in energies can arise from the accelerator and beamline itself or from range straggling in a range modulator

Answer 185

A shorter range beam requires lower energy which is achieved by using a range modulator (plastic slabs in the beam to modulate the energy down). Since there is range straggling in this modulator, the width of the peak increases

Answer 186

uniform loading not uniform dose

Answer 187

uniform dose within 10% uses peripheral loading

Answer 188

-high energy sources only

Answer 189

verify location of brachy seeds

Answer 190

10% of seeds are assayed and must be within 3 % of activity specified by manufacturer

Answer 191

-monotherapy for early cancers -later stage cancers in combination with external beam radiotherapy

Answer 192

-pt A is 2 cm from top of ovoid and 2 cm lateral from tandem -pt B is 3 cm lateral from Pt A (pelvic sidewall) -bladder pt is on posterior of foley balloon -rectal pt is 0.5 cm post to vaginal wall

Answer 193

-7 Gy X 4 -6 Gy X 5

Answer 194

-pt B: 30-40% of Pt A -rectum: < 4.1 Gy/fx -bladder: < 4.6 Gy/fx -mucosa: < 120 Gy

Answer 195

-I131 -90Y- liver -223Ra-chloride- castrate-resistant prostate

Answer 196

For a smaller implant, dose falls off more rapidly with depth so more activity is needed. (i.e. IS is less significant for large cylinder vs small cylinder)

Answer 197

lower Due to swelling on the day of implant, seeds are farther apart than planned. This results in an overall underdose at the periphery

Answer 198

line source, so falls off as 1/r

Answer 199

dose at 0 is 100 dose at 2 cm = 60+ E/10 dost at 5 cm is 30+ E/10 dose at 10 cm is 0 + E/10

Answer 200

d5cm =80+E d10=60+e D20=35+e D30=15+e

Answer 201

2,6,10 in s, p, and d 2n^2 in any shell

Answer 202

197.3 MeV * fm

Answer 203

935 and 938 MeV

Answer 204

e^2/ (4 pi epsilon knot hbar c)

Answer 205

2.82 fm (e^2/(4 pi epsilon knot me c^2)

Answer 206

Shells do not have specific, fixed distances from the nucleus, but an electron in a higher-energy shell will spend more time farther from the nucleus than does an electron in a lower-energy shell. Shells are further divided into subsets of electrons called subshells. The first shell has only one subshell, the second shell has two subshells, the third shell has three subshells, and so on. The subshells of each shell are labeled, in order, with the letters s, p, d, and f. Thus, the first shell has only a single s subshell (called 1s), the second shell has 2s and 2p subshells, the third shell has 3s, 3p, and 3d and so forth s holds 2 electrons, p holds 6 total number of electrons is 2n^2,n is shell number Valence electrons are the electrons in the highest occupied principal energy level of an atom