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random from study group Flashcards

1
Q

for door shielding calc, average MV to use for patient and wall scatter and leakage scatter

A

patient and wall scatter- use 0.2 MV
leakage scatter- use 0.3 MV

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2
Q

average energy of neutron capture gamma rays in concrete

A

3.6 MeV
TVL = 6.1 cm of Pb

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3
Q

SFRT

A

Spatially fractionated radiotherapy
Peaks and valleys of dose, rather than a uniform or monotonic distribution
Different spatial grid sizes in grid therapy, minibeam, microbeam
Charactrized by peak dose, valley dose, PVDR, peak width, valley width, pattern, % volume irradiated

Method of killing is multi-faceted
Radiation cytotoxicity
Also bystander effect
Microvascular modulation
Immune system modulation (abscopal effect)
Tumour antigens released on cell death, collected by dendritic cells in valley region to prime immune response
Sort of like radiation induced vaccination

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4
Q

TG63 dose through prosthetic diagram

A

Backscatter upstream of prosthetic
Enhanced attenuation within prosthetic
Loss of backscatter at distal end of prosthetic
Low E – buildup of dose post prosthetic
High E – pp electrons increase dose post prosthetic, build down effect
Narrow beam, loss of dose due to attenuation
Broad beam, in-scatter compensates for attenuation to a degree, increasing effect with depth beyond

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5
Q

transmission through prosthetic

A

0.6 to 0.9

can estimate with TLD/OSLD, EPID

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6
Q

neutron dose with prosthetic

A

extra neutron induced photon dose is less than 0.5% of the photon dose at 1 cm from the prosthesis, and, therefore, can be assumed to be clinically negligible.

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7
Q

what is Reflexion?

A

-Combination 6MV linac, 16 slice FBCT, PET detector
Uses PET signal as a biological fiducial to track tumours (BgRT – biology guided radiotherapy)
Feeback loop of signal to radiation (~400 ms response rate)
-aimed for lung treatments, wants to eliminate or minimize ITV

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8
Q

proton lateral dose

A

distribution laterally is gaussian in nature
-width ~ 2% of proton range

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9
Q

proton vs photon plans

A
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10
Q

PHASER

A

Flash Capable Photon treatments (>50Gy/s)
16 small klystrinos provide RF power (x-band)
16 individual Dragon linacs receive power sequentially
Electron pencil beam scanned over planar target covering the SPHINX collimator
Non-coplanar beamlines at CT imager
Allows real time imaging to facilitate FLASH without damaging CT components

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11
Q

Magnettx Aurora

A

biplanar 0.5T magnet, radiation and magnetic field aligned. Beam path uninterrupted.
Newest clinical machine, 6MV xrays
No electron return/streaming, but electron focusing
Lateral couch movement possible
MR coils can be as close as possible

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12
Q

Elekta Unity

A

1.5T solenoidal magnet, radiation perpendicular to magnetic field. Irradiates through cryostat.
7MV xrays
Strong electron return and streaming
No lateral couch motions, adaptive planning to align patient
Can not have MR coils in contact with patient

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13
Q

Viewray Mridian

A

0.35T split solenoidal magnet. Radiation perpendicular to magnetic field, beam path uninterrupted.
Oldest clinical machine, originally had Cobalt source, now 6MV xrays
Weaker electron return and streaming
No lateral couch movements

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14
Q

effects of misalignment on beam profiles

A
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15
Q

linac dosimetry interlocks

A

-under-over dose rate
-dose 1/2 mismatch
-dose 2over limit
-beam summetry and flatness
-dose per pulse
-other ion chamber charge errors

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16
Q

sources of dosimetry system errors in linacs

A

electronic component failure, drift
MU chamber:
-receives high dose, so subject to radiation damage
-can experience changes in gas (sealed chambers), insulation changes
-results in instability, especially at startup
-may also increase sensitivity to atmospheric conditions, such as high humidity

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17
Q

types of gamma knife

A

Perfexion is 192 sources; 4,8,16 mm collimators, single fraction only
ICON has kV CBCT- can do multiple fractions

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18
Q

calibration of gamma knife

A

Done using ~custom calibration ~TG-21, since GK cannot establish reference 10x10 field
Beam quality is cobalt-60 though, so kQ very nearly equal to 1.
Absolute measurement done in a 16 cm spherical solid water phantom
Dose rate for a new source is ~3 Gy/min, compared to ~6 Gy/min for a linac

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19
Q

dose calculation in gamma knife

A

Dose to water, assumes phantom consists entirely of water
Only “inhomogeneity” correction would be a correction for uneven surface
This is based on a skin surface map determined from CT

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20
Q

shielding/safety for gamma knife

A

There are doors blocking off the entrance of GK
Sources can also go in home position, behind shielding
At safest state (doors closed and sources at home), DR ~ 2-3 uSv/h
Even at worst state (doors open and 16 mm), DR ~10 uSv/h behind GK
Typical safety systems:
Room doors, interlocks, LPO
Radiation warning lights
A/V communication
Source out lights
Emergency procedure for stuck source (source won’t go home)
Press emergency release buttons on the way from console to inside bunker
Retract couch (if doesn’t work, use physical crank)
Close doors/retract sources (if doesn’t work, use physical crank)
Remove patient from couch
Get names of all involved
Report to RSO

21
Q

machine QC for gamma knife

A

Daily
Safety checks
Focus-precision test: coincidence of radiation, imaging, and couch isocentres
Monthly
imgQ and imgG for CBCT
Annual
Dose rate measurement
Field homogeneity (measure profiles with film)
Film-based mech-radio iso coincidence
Radiation survey

22
Q

PTV margin for gamma knife

A

0 mm for frame based
1 mm for mask based

23
Q

comparison of SRS modalities

A

Gamma Knife
Excellent dose conformality due to small collimator sizes, and ~hemispherical distribution of sources around isocentre
Low dose rate, ~3 Gy/min max, leads to long treatment times
Mechanically very simple, requiring less QC than linacs
Requires more real estate, staffing, overhead
Radioactive source leads to different considerations for security and radiation protection
Cone-based linac
Can be achieved using existing linacs
Faster dose rate, ~10 Gy/min, faster treatment times
Efficiency starts to drop off compared to GK at around 4 mets, since for each met a different iso with multiple non-coplanar arcs is required
MLC-based linac
Same advantages as cone-based
Can treat multiple lesions with single iso
Low-dose wash is greater
HD-MLC required (2.5 mm leaf size), with smaller max field size (~25 cm); cannot necessarily be used in all sites, eg ¾-field breast, pelvis with nodes, …
Cyber Knife
Excellent dose conformality due to small collimator sizes, and ~hemispherical distribution of sources around isocentre
Higher dose rate than GK
Can also be used for extra-cranial sites
Requires more real estate, staffing, overhead
Zap
Faster dose rate, ~10 Gy/min, faster treatment times
Linac gimballed to rotate around iso in ~2\pi geometry solid angle
Excellent dose conformality due to small collimator sizes, and ~hemispherical distribution of sources around isocentre
Most complex mechanically, probably most expensive
Can only be used for cranial sites

24
Q

FLASH RT

A

> 40 Gy/s
Conventional EBRT ~6 Gy/min
Hypothesized that FLASH effect caused by oxygen depletion in normal tissues. Some believe fewer lymphocytes being irradiated lead to a greater immune response, and normal tissues have more ability to scavenge peroxide products than tumours.

Researchers don’t fully understand why FLASH RT kills tumors with fewer side effects than conventional radiation and further research is needed to determine the biological mechanisms driving the FLASH effect

25
Q

equation for EQD2

A

EQD2= nd((d+alpha/beta)/(2+alpha/beta))

26
Q

canadian incident learning system

A

canadian institute for health information

others: NRC, SAFRON (IAEA)

27
Q

isodose lines at distance from field edge

A

1% at 10 cm, 10% at 2 cm, 0.1% at 30 cm

28
Q

Ppol should be within what?

A

0.3%

29
Q

error associated with using equation for high E beam vs Pb foil

A

2% (i.e. 0.4% error in kq)

30
Q

As %dd10x increases, kq does what?

A

decreases

31
Q

kecal range of values

A

around 0.9

if possible, obtain using cross calibration for PP chambers (if not possible, use values in TG51)

32
Q

breast tangent gantry angles

A

Gantry angles ranged from 42° to 55° for the medial fields and from 224° to 232° for the lateral fields for patients treated on the right side, and from 305° to 322° for the medial fields and from 133° to 147° for the lateral fields for patients treated on the left side

33
Q

Ir-192 dose rate at 1 m

A

46 mSv/h

34
Q

does gama knife have PTV?

A

no

35
Q

spine SBRT Rx

A

24/2

36
Q

CNS fraction

A

18 Gy in 1.5 Gy fx

37
Q

adrenal/kidney SBRT dose

A

45/5 every other day

38
Q

gyne brachy 28/4- what dose constraint do we aim for for rectum and bladder

A

<620 cGy bladder
<420 cGy rectum

39
Q

esophagus fracionation

A

50/25

40
Q

Rx for CSI

A

36/20

41
Q

what do RTs match to with breast CBCT?

A

CTV contour

42
Q

bladder Rx

A

Phase 1: 46 Gy in 23 fx
Phase 2: 14 Gy in 7 fx

43
Q

annual QA of TPS

A

-check constancy of standard set of plans
-check repeatability of DVH
-check constancy of PDD with TMR data (TPS vs measured)
-E2E test

44
Q

basic photon TPS validation tests

A

-small MLC shaped field (non SRS)
-large MLC shaped field with blocking (ex mantle)
-off-axis MLC shaped field, with max allowed MLC over-travel
-asymmetric field at minimal anticipated SSD
-10x10cm2 field at oblique incidence
-large (>15 cm) field for each non-physical wedge angle
-test tolerance is 2% if one parameter changed from reference, and 5% if > 1 parameter changed
from MPPG5a

45
Q

mean energy of electrons

A

2.33R50

46
Q

electron penumbra

A

At dmax, Generally the 20%–80% width is expected to be 10 mm to 12 mm for electron beams below 10 MeV, and 8 mm to 10 mm for electron beams between 10 MeV and 20 MeV

47
Q

PDD for electron pencil beam

A

PDD for an electron pencil beam is a straight line with negative slope; surface dose is 100%; Rp is the same regardless of field size. There is no in-scatter in this case, hence no buildup.

48
Q

electron output factor

A

Output factor is NOT simply a ratio of ion chamber readings, but is a ratio of dose hence use ratio of readings times stopping power ratio since the stopping power ratios vary with depth and wont’ cancel out in the ratio. This is an important issue associated with the fact that zmax varies with field size.

49
Q

error in ignoring stopping power ratio for electron cutout factors

A

up to 3% per TG25

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