random from study group Flashcards
for door shielding calc, average MV to use for patient and wall scatter and leakage scatter
patient and wall scatter- use 0.2 MV
leakage scatter- use 0.3 MV
average energy of neutron capture gamma rays in concrete
3.6 MeV
TVL = 6.1 cm of Pb
SFRT
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
TG63 dose through prosthetic diagram
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
transmission through prosthetic
0.6 to 0.9
can estimate with TLD/OSLD, EPID
neutron dose with prosthetic
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.
what is Reflexion?
-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
proton lateral dose
distribution laterally is gaussian in nature
-width ~ 2% of proton range
proton vs photon plans
PHASER
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
Magnettx Aurora
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
Elekta Unity
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
Viewray Mridian
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
effects of misalignment on beam profiles
linac dosimetry interlocks
-under-over dose rate
-dose 1/2 mismatch
-dose 2over limit
-beam summetry and flatness
-dose per pulse
-other ion chamber charge errors
sources of dosimetry system errors in linacs
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
types of gamma knife
Perfexion is 192 sources; 4,8,16 mm collimators, single fraction only
ICON has kV CBCT- can do multiple fractions
calibration of gamma knife
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
dose calculation in gamma knife
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
shielding/safety for gamma knife
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
machine QC for gamma knife
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
PTV margin for gamma knife
0 mm for frame based
1 mm for mask based
comparison of SRS modalities
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
FLASH RT
> 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
