Tx planning - techniques Flashcards
Factors that affect hot spots for parallel opposed beam
- as Pt thickness increases, hot spot increases
- as E decreases, hot spot increases
- as FS increases, hot spot decreases
What energies are optimal for 15cm separation
Co-60 and 4-6 MV
What energies are optimal for 20cm separation
10 MV or higher
Where are the hot spots of a POP beam found?
at the surface of each beam entry point
What is Integral Dose
Measure of total energy absorbed in the treated volume
Define Integral Dose mathematically
ID = mass(kg) x dose(rad)
Integral Dose relationship to energy for parallel opposed beams
As E increases, Integral dose decreases
List from tx techniques from highest to lowest in terms of how much integral dose is produced:
in general - VMAT > IMRT > 3D > Proton
Between a physical and dynamic wedge, which type of gives more dose, why?
Physical; more scatter due to being in path of beam longer
Advantages of IMRT: Forward Planning over Inverse Panning:
- simpler intensity patterns
- Easier to deliver
- Easier to verify
- Easier to calculate MU’s (able to use standard 3D TPS)
Advantages of IMRT: Inverse Planning over Forward Planning:
- Intensity Patterns (better distribution - not necessarily more uniform)
- Less dose to critical structures
For IMRT planning, what is the typical number of fields used?
5, 7, or 9
What energies should be used for IMRT, why?
6 MV or 10 MV - tighter penumbra, better coverage, no neutron shielding, no neutron pt dose
What energies should not be used for IMRT why?
energies over 10 MV - neutron production
IMRT causes more or less dose heterogeneity?
More
Advantages of Compensator based IMRT:
- better resolution than MLC
- Less MU required
- Less pt dose (outside field)
- Larger FS
- Tx planning may be easier
Disadvantages of Compensator based IMRT:
- Slower delivery (RTTs enter room for each field)
- Must be fabricated
- Accounting for transmission
What factors influence dose to contralateral breast?
- distance away from beam edge
- Use of HPP (increased dose if Cerro)
- Use of wedge (Medial get 3x more than lat)
- Alignment of Tangents
- Port film
What factors do not influence dose to contralateral breast?
- Gantry angle
- lung density
- breast volume
Ways to limit dose to contralateral breast
- Don’t use Cerro HBB,
- align tangents,
- decrease wedge angle (or only use lateral wedge;
- Use 2.5cm thick Pb shield
- Limit collimator for port films
Contralateral breast dose limit to women < 45 years old
200 cGy
When is TBI utilizied
as a radioimmuno-suppressant prior to bone marrow transplant
Common TBI Rx
- 5-8 fx
- 1000 - 1200cGy
- Rx to umbilicus
- Lung blocks used post 600-800cGy
- Dose Rate of 5 - 10 cGy/min
- SSD = 350cm and 450cm
Common TBI techniques:
- AP/PA
- Laterals
- Sliding tables
Physics goals of TBI:
- TPR: +-4%
- ISL: +-6%
- OAR: +-5%
For TBI inhomogeneity worsens with:
- Lateral technique
- Decreased E
- Increased Pt thickness
- Decreased distance (SSD)
For TBI, skin dose increases with:
- FS
- Distance (increased SSD)
- Spoilers (**want this for pediatric pts)
When is IORT(e’) typically used?
originally for pancreas, now breasts
Typical Rx for IORT(e’)
- Single fx
- 1000 - 2000 rad
- 6 MeV - 20 MeV
For IORT(e’), use high or low Z material to collimate beam?
low, to not produce bremsstrahlung
A decrease in width will improve/worsen flatness of beam for IORT(e’)
worsen
When is Total Skin Irradiation used (TSCT)?
for Mycosis Fungoides, and t-cell lymphoma
Common Rx for TSCT?
- 3-7 Mev
- SSD = 300 to 400cm
- 6-field tx
- dose prescribed to 1cm
- Do not exceed bone marrow tolerance
For SRS, a large volume gets a smaller/larger dose?
smaller
At what isodose line is Gamma Knife Rx’d?
50%…want a 200% hot spot
At what isodose line is SRS on LINACs Rx’d
90%
For SRS is penumbra better with Gamma Knife or LINAC
Gamma Knife
For SRS, additional field/arcs will effect penumbra how?
worsens
For SRS, what will an overlap of beams cause?
inhomogeneities (hot spots)
How are dose profiles measured for SRS
Radiochromatic film
How is output measured for SRS
small ion chamber or film
For SRS, what is a shortcoming in regards to output measurements with chambers?
volume averaging
What kind and how many sources are present on a gamma knife unit
201 Co-60 Sources - 6000Ci
What is the size of the isocenter for a gamma knife unit
.3mm
What is the gold standard of radiosurgery
Gamma Knife
Gamma Knife is governed by what body
NRC
Daily tests of Gamma Knife include:
- Interlocks
- Intercom
- Radiation Monitors - Geiger Muller
Monthly Gamma Knife tests:
- Timer reproducibility
- Timer linearity
- Timer Accuracy
- Timer Error
- Mechanical Alignment
- Output (for 18mm collimator)
Semi-Annual Gamma Knife tests:
Leak tests
Annual Gamma Knife tests:
- Output for all Helmets
- Mechanical Iso - Rad iso
- Dose profiles
Most accurate SRS delivery by LINAC is ____
cones, especially for lesions < 1mm
Range of SRS cone diameters:
2.5mm to 50mm
Typical number of arcs for LINAC SRS:
4-6
Energies used for LINAC SRS:
< 10MV
Typical Rx isodose line for CyberKnife:
70-80%
SADs used for brain and body with CyberKnife:
80 SAD for brain; 90-100 SAD for body
What is the name used to describe the fixed ranged of heavy charged particles?
Bragg Peak
E range of Proton machines
30-250 MeV
How is the Bragg Peak spread out (SOBP)
using multiple energies, or by a modulator
Whats the purpose of an aperture in Proton therapy
to shape the beam
Features of the Proton Head design:
Double Scattering Head: uniform scan or pencil beam scan
How are protons accelerated
Cyclotrons or Acclerators
What is the RBE of a proton, where is it largest?
1.1; can vary with E (depth) - larger at distal end of Bragg Peak
Largest inhomogeneity corrections are for what kind of energy
Proton Energy
How is Neutron Contamination associated with Proton Energy
- Increases with use of Modulator
- Increases with use of collimator
- Increases with higher E
- Increases if Mod/Coll is closer to pt
Skin dose of Proton Beam is _____ than 15X or 18X
slightly higher
For proton beam lateral dose penumbra is ______ than 15X or 18X
Larger
