general Flashcards
what is FLASH?
> “FLASH radiotherapy (FLASH-RT) is a new technique, involving treatment of tumours at ultra-high dose rates which actually reduces the trauma to normal tissue around the tumour, whilst equalling the anti-tumour effect of conventional dose rate radiotherapy (CONV-RT). However, very little is known about the mechanisms behind the FLASH effect.”
> 40 Gy/s
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.
FLASH effect may be tissue specific.
Dose constraints (and alpha beta) could also be dose rate dependent.
gamma knife calibration
Calibration
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
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
gamma knife shielding/safety
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
why do we prescribe to 50% in gamma knife?
his is done because for a single shot, the 50% line is also the point of max gradient; so we are maximizing dose falloff at that point.
target definition in gamma knife
Contrast-enhanced T1MRI, or contrast-enhanced CT if MRI not possible
prescription for mets on gamma knife
15-21 Gy / 1 @ 50% line
Very small mets ~1 cm get 15 Gy; above this 21 Gy, then Rx decreases with increasing size (since the volume of irradiated healthy brain increases with met size)
At larger met sizes, can also fractionate: 21, 24, 27 Gy / 3
OAR constraints on gamma knife
Healthy brain: V12Gy < 10 cc
Brainstem 15 Gy max
Optic nerves, chiasm, motor strip
trigeminal neuralgia on gamma knife
80 Gy @ 100%, 50 Gy on retreat
acoustic schwannoma prescription on gamma knife
12 Gy at 50%
PTV in gamma knife for frame-based vs mask-based
-0 mm PTV for frame based
-1 mm PTV for mask based
cone-based linac for SRS
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 for SRS
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,
cuber knife dose rate is higher or lower than gamma knife?
higher dose rate than GK
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
gyne total BRT + brachy EQD2gy constraints
CTVs use a/b = 10 Gy, OARs use a/b = 3 Gy
HR CTV
D90% > 90 Gy, < 95 Gy
D98% > 75 Gy
IR CTV
D98% > 60 Gy
Bladder
D2cc < 90 Gy (80 Gy optimally)
Small bowel, sigmoid, rectum
D2cc < 75 Gy (small bowel and sigmoid 70 Gy optimally, rectum 65 Gy optimally)
disadvantages of protons
Cost – proton facilities can cost upwards of >$100 million
Cost continues to drop, some single unit designs can cost less!
Difficult to secure funding for such facilities
Inhomogeneities (bowel gas, metal implants, weight loss) more of a concern due to precision required in water equivalent depth for dose deposition
Immobilization devices must not interfere with the beam, avoid excess scatter or energy degradation
Motion management becomes more critical, particularly for scanned beams
RBE increase at distal track
Could include depth dependent RBE in planning
Concern for OARs behind target volume
what is Reflexion X1?
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 at targeting metastatic and primary lesions, particularly in lung, using biology based tumour tracking to minimize or eliminate ITV.
Dual 90 degree PET detectors (64 counters each) mounted orthogonal to CT/Linac
Entire system rotates at 60 RPM
2cm thick lead septa – reduce scattering
PET signals ignored after MV pulses to mitigate MV scatter false signals
64 leaf Binary MLC, 85CM SAD (like tomo)
MV detector
Unclear if suitable for MVCBCT
Marketed as BgRT SBRT/SRS
Will require modified calibration (as with tomo)
Limited by tracer activity and annihilation resolution
what is SFRT?
Spatially fractionated radiotherapy
Peaks and valleys of dose, rather than a uniform or monotonic distribution
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
transmission factors through prosthetics
0.65 to 0.89
neutron dose from prosthetics
nowing the dose rate of the linear accelerator (at least 200 cGy/min) and the dose delivered by the beam passing through the prosthesis, one can estimate that the 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.
from TG63
PDD through prosthetic
TG63
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
2 new parameters in TG1 addendum
Prp and Pleak
what is embrace II?
image guided BT (volume based planning)
f factors for bone
4 at 80 kV to 1.2 at 300 kV
Thought process that goes into choosing an ortho fractionation
-longer fractinations may be difficult for older patients
-F-factor- if treating a lesion directly over bone, the bone dose may be very high and fractionation can help with this
-more fractions= better cosmetic outcome
