light vs. radiation field
- Daily:2 mm
- TG-142(IMRT, SRS_SBRT)
- IMRT-Daily:1.5 mm
- SRS(SBRT)-Daily: 1 mm
- Monthly IMRT +/- 1mm SRS(SBRT):<1mm
- Weekly-picket fence
- Travel speed, loss of leaf speed 0.5cm/sec
- Leaf position accuracy vs. gantry angle-1 mm
gantry, collimator, couch isocenter
2 mm or 1 degree
monthly QA tests for the CT sim
- Alignment of lasers to midplane center
- –CT number
- –Image noise
- –Spatial integrity (+ 1mm)
- Laser positioning, spacing, etc. + 2 mmTable
- motions, vert, long, lat,indexing 1mm
- Spatial integrity + 1mm
- Field uniformity + 5 HUCT
- number accuracy (4-5 different materials)
“rule of thumb” for electron energy loss in tissue
electron range (Rp)
The practical electron range. Seen on a depth-dose curve where the intersection of the depth dose slope and the bremsstrahlung tail.
ie-a 10 MeV beam has a range of 5 cm
electron beam rule of thumb 80%
electron beam rule of thumb 90%
electron beam rule of thumb 50%
Describe the characteristics of electron isodose curves.
The low -level isodose curves (<50%) bulge out beyond the field borders -The higher level isodose curves (>80%) tend to show lateral constriction, which becomes worse with increasing energy and depth
PDD(increases/decreases) with E?
PDD(increases/decreases) with fs?
PDD(increases/decreases) with depth(beyond depth of max dose?
What are the parameters for calculating primary shielding?
- 1)P is the permissible limit to an individual outside the barrier
- 2)dpri is the distance from the target to the protected point
- 3)W is workload or Gy / week produced by machine
- 4)U is use factor or fraction of time the beam falls on the barrier
- 5)T is the occupancy factor or fraction of time person will be in area adjacent to barrier
___ is the permissible limit to an individual outside the barrier
___ is the distance from the target to the protected point
__ is Gy / week produced by machine
____ is the fraction of time the beam falls on the barrier
U (use factor)
__ is the fraction of time person will be in area adjacent to barrier
T is the occupancy factor
What are the parameters for calculating secondary shielding?
- PATIENT SCATTER
- Bp = P / aWT * (dsec)2 * (dsca) 2 * 400/F
- 1)a: the fraction of primary workload that will de scattered
- 2)dsec: distance from patient to protected point
- 3)dsca: distance from xray target to patient
- 4)F: field size area
- HEAD LEAKAGE
- BL = 1000 P (dsec)2 / WT
Secondary Shielding includes calculations for ____ and _____.
- patient scatter
- head leakage
BL = ?
head leakage(secondary shielding)
a: the fraction of primary workload that will de scattered
distance from patient to protected point
distance from xray target to patient
What is a maze & why is it used?
A wall that is placed at the entrance of a vault & results in a 90 degree turn from the entrance into the vault to reveal the linacTypically used to reduce radiation levels at the room entrance so that a massive door is not neededMostly decreases head leakage that reaches the door so the door only shields for neutrons
What type of shielding is used in the door?
- First layer of lead (closer to source) reduces the energy of the incoming neutrons
- Neutrons are absorbed in the second layer (BPE; borated polyethylene equivalent) Second layer of lead (away from source) attenuates captured gamma rays produced in the BPE (478keV)