Amanda C's notes Flashcards
what does obliquity do?
-increase side scatter at dmax
-shift dmax toward surface
-decrease depth of penetration
virtual electron source point
intersection of backprojections
limit on horns
5% for 40x40 cm2
what cancers spread to the brain?
-small cell lung (prophylactic brain irradiation is standard)
-breast cancer
most common sites of mets
-brain, liver, lungs
whole brain RT
-used for palliation of mets or prophylactically
-30/10
-treat down to foramen magnum or bottom of C1 or C2, typically use POP
-treat brain and brainstem
-can fail at cribiform plate, which is underdosed to spare eyes
-include orbital apex since field since CSF circulates around the optic nerve
time for total dose delivery for 103 Pd vs 125I
56 days vs 200 days
physicist responsisbility in brachy QA
-check daily QA
-perform annual QA
3 things: MP role in brachy per AAPM TG56
- design facility that meets clinical needs
- develop and implement treatment delivery procedures
3.ensure accuracy and safety of each individual treatment
regulatory compliance vs adequate QA
they are not the same thing
goal of QA program
maximize likelihood that each tx is administered consistently, that it realizes RO intent, and is executed wrt safety of patient and others
QA endpoints:
safety
positional accuracy (2 mm)
temporal accuracy (2%)
dose delivery accuracy: 3 % relative to existing air kerma strength, 2 % for dose calcs
what is needed to calculate workload in brachy?
air kerma strength and max source activity
BNC vs TNC connectors
BNC: twist- has inncer conductor to cable core to shield to jacket
TNC: screw - has inner conductor to inner shield to inner jacket to outer shield to outer jacket
male: center conductor is pin
demale: center conductor is socket hole
triax= bettter BW and rejection of interference than coax but more expensive
challenge with measurement for TBI
-large amt of cable in tx field
-chamber leakage and saturation become more problematic at lower TBI dose rates
max dose delivered before MU verification per TG-40
3-4 Gy
4 things to check before using survey meter
-battery
-voltage
-source responde (functional check on all ranges of survey meter)
-calibration date within last 12 months
class II prescribed equipment
-irradiatior that uses > 10^15 Bq of a nuc
-irradiatior that requires shielding and delivers at rates > 1 cGy/min at 1 m
-teletherapy machine
-particle accelerator that produces enegy < 50 MeV for particles with amu </= 4 amu or <15 MeV for particles with amu >/= 4 amu
-remote afterloader
DVH for rectum in IMRT vs brachy prostate tx
-volume of rectum receiving high dose is very small for brachy
-volume receiving low dose may be larger for brachy
-bladder is same effect but less pronounced (ie effect is bigger in rectum)
radiation reaching maze door
-primary scattered off walls
-primary scattered off patient
-leakage scattered off walls
-leakage transmitted
-for > 10 MV, above is negligible
-consider neutron capture gamma rays from maze at door
-consider neutrons at door
-for neutrons, TVL = 4.5 cm (E = 100 kV)
-for photons, TVL = 6.1 cm (E = 3.6 MeV)
what do acceptance tests accomplish?
-specs in purchase order are fulfilled
-environment is free of radiation
-radiotherapy equipment is free of electrical hazards to staff and patients
pratical suggestions for water scanning
-increments for Fs should be < 5 cm but are typically 2 cm
-make measurements to depth of 35 or 40 cm
HDR prostate planning objectives
V100%> 95%
V200%<11%
Urethra D10%< 118%
Rectum V80%<0.5cc
wedge material
lead, brass, steel
who does RSO report to?
RSO reports to licensee and licensee reports to CNSC
replacement RSO
-temporary replacement can only replace RSO for up to 60 days in 365 day period. If absence is longer, replacement must be certified
-regardless of length of absence, notify CNSC in writing of change within 15 days of change
-If RSO gone > 60 days but < 13 months, must review material
-If RSO gone > 13 months, must recertify
-Absence > 60 d requires replacement to be certified
TG 66 CTDI QA tolerance
+/- 20 % of manufacturer spec
TG66 laser tolerance
2 mm
gantry and wall
gantry and imaging plane
TG66 couch motion tolerance
1 mm, 1mm tolerance for couch indexing as well
TG66 gantry tilt accuracy
1 degree
TG 66 scan localization accuracy
< 1 mm
TG66 CT number accuracy
0+/- 5 HU for water (daily)
monthly check more materials
annually check RED curve
TG 66 fiel uniformity spec
within 5 HU
TG66 spatial integrity tolerance
< 1 mm
checked daily and monthly
typical measured dose rate outside a barrier
Gammas:
primary barrier: 20 uSv/h
secondary barrier: 2 uSv/h
door open: 50 uSv/h
door closed: 2 uSv/h
Neutrons
primary barrier: 10 uSv/h
secondary barrier: 10 uSv/h
door open: 600 uSv/h
door closed: 20 uSv/h
typical dose rate allowed for wipe test for HDR source change
15 mRem
positional accuracy for brachy HDR
1mm
source strength 1 %
PDI vs PDD
PDI = ionization released in air
PDD= dose deposited to water
isodose line bulding in electron beams
E > 15 MV- <30% bulges out and >80% constrict
E<15 MV- all isodose lines bulge out, especially < 30%
what to consider for IGRT shifts
PTV margin
site
modality
wedge factor range
0.6 to 0.3 for 15-60 degree wedge
review tolerances in TG142
OAR expected values at commissioning
within 0.02 (think symmetry)
criteria for beam matched beams
dmax position +/- 1 mm
% PDD at 10 cm- +/- 0.5%
dose surrogate measured at or near surface +/- 5 %
equivalent field for a shielded field
square root of (equivalent square field size ^2 * fraction unshielded)
activity of old HDR source when removed
4 Ci
equation to use to calculate surface contamination for wipe test
C= ((cpm(gross)-cpm(background))/efficiency)*100cm2/detector area
when to do leak tests?
> 50 Mq
must be able to detect 200 Bq
-if stored > 12 months, check before use
every 2 years if stored
if damaged
every 12 months if inside class II equipment, every 6 months if not inside device
