Pr, Br, Rad Pharm, TBI, TSET Flashcards
(54 cards)
1
Q
What’s the typical energy range of proton beams?
A
- 70-250 MeV
- Lower energies (60 MeV) exist for tx of ocular melanomas
2
Q
What’s the formula for the range of protons in the tissue?
A
Range (cm) = 0.033E + 0.0005E2
3
Q
When does the skin dose from a proton beam increase?
A
When you increase the SOBP
4
Q
How do you create a SOBP?
A
Using a range modulator wheel, which has thicker and thinner parts and can change the range of the protons as they pass through the different parts.
5
Q
What controls the range of the proton beam?
A
- Beam energy
- Can be controlled using a range shifter
6
Q
What controls the modulation of a proton beam?
A
Range modulator wheel
7
Q
What’s the fx of a compensator for proton beams?
A
- A compensator custom-made, beam-shaping device used to absorb some energy so that it stops just on the distal edges of the target or tumor
- It can also compensate for
– the presence of tissue inhomogeneities (bone, lung, etc)
– Irregularities of patient body contours/surface
8
Q
What devices go into the beam path of a double scattered proton beam?
A
- First, second Scatterers: Widen the beam
- Shifter: Determines where the beam will stop
- Modulator: Creates the SOBP to cover the entire length of the tumor.
- Aperture: Shapes the lateral edges of the beam. Custom-made from thick brass.
- Compensator: Shapes the beam to conform to the distal edge of the tumor. Typically made from low Z materials (wax, plastic, etc) to reduce scatter.
9
Q
Is there an advantage to using protons over photons considering lateral penumbra?
A
No, it is very similar to a photon beam (≥ 6 MV)
10
Q
What’re some of the advantages and disadvantages of a PBS?
A
- Advantages
– Sharper penumbra
– Low neutron dose 2/2 fewer devices in the beam path - Disadvantages
– More susceptible to intrafraction patient motion
11
Q
How are protons accelerated in the cyclotron?
A
- Magnetic field
- Charged particles move in a spiral, gaining energy w/ each revolution
– Continuously bent by the magnetic field
– As they gain energy, the radius of their revolutions increase - Gives a continuous supply of protons
12
Q
How are protons accelerated in a synchrotron?
A
- Beam travels around in a circular path in a vacuum
- Bending magnets bend the beam
- Accelerating cavity accelerates the beam using RF electric fields (akin to a Linac)
- Once the beam reaches desired energy, it’s extracted from the synchrotron
- Outputs protons in bursts
13
Q
How do the PTV margins differ for protons vs. photons?
A
- Proton PTV margins depend on the beam direction
– They are non-isotropic, unlike photon beams
14
Q
What’s an advantage and a disadvantage of a carbon ion beam vs. a proton beam?
A
- Adv: Sharper penumbra
- Dis: ↑ dose past Bragg peak 2/2 nuclear spallation
15
Q
What’re the features of a planar Quimby brachy system?
A
- Uniform source activity
- Non-uniform dose distribution
– Higher dose at the center
16
Q
What’re the features of a planar Manchester (Patterson-Parker) brachy system?
A
- Uniform source spacing (1 cm)
- Non-uniform source activity
– Peripheral sources have higher activity - More uniform dose distribution 0.5 cm around sources (±10%) mainly 2/2 ↑ activity of the peripheral sources
17
Q
What’s the formula for the range of protons in the tissue?
A
Range (cm) = 0.033E + 0.0005E2
18
Q
What’s the typical energy range of proton beams?
A
70-250 MeV
19
Q
What are the units of air kerma strength?
A
- 1 μGy × m2 / h
- Also represented as 1U
20
Q
What are the dose rates for LDR, MDR, and HDR brachytherapy?
A
- LDR - 0.4 - 2 Gy/h
- MDR: 2-12 Gy/h
- HDR: >12 Gy/h
21
Q
How’re unsealed sources given?
A
- Usually given systemically or injected
22
Q
Between LDR and HDR, which technique has more normal (biological) tissue sparing?
A
- LDR: More normal tissue sparing 2/2 ↑ sublethal DNA damage repair
- HRD: Less normal tissue sparing 2/2 high dose rates and fx given over time shorter than that required for DNA repair
– Geometric sparing is used to compensate for ↓ biological tissue sparing
23
Q
What’re the key dosimetric considerations for TBI?
A
- Uniform dose throughout the body
- Limit lung dose
- Limit dose rate (5-15 cGy/min at midplane)
24
Q
What’s the purpose of a lung block, beam spoiler, and compensator in TBI?
A
- Lung block reduced lung dose
- Spoiler: Increase the skin dose by increasing e- contribution to dose
- Compensator: Make the dose more homogenous throughout the body (by reducing the dose to thinner (ankles, neck, etc) parts of the body
– Custom-designed for each patient, and can either be attached to the Linac or to the beam spoiler
25
What's the advantage of using higher beam energies for AP/PA TBI tx? How does normal tissue dose depend on pt thickness?
- ↑ energy → ↓ normal tissue dose
- ↓ thickness → ↓ normal tissue dose
26
What's the formula for calculating TBI dose homogeneity?
Dosepeak / Dosemid
27
How does dose homogeneity vary w/ beam energy, SSD, and patient thickness for a TBI tx?
- ↑ homogeneity w/
-- ↓ thickness
-- ↑ energy
-- ↑ SSD
--- ↓ PDD fall-off w/ ↑ SSD`
28
29
What's the Rx for I-125 prostate implant monotherapy and when coupled w/ EBRT?
- Mono: 145 Gy
- w/ EBRT: 110 Gy
30
What's the Rx for Pd 103 implant monotherapy and when coupled w/ EBRT??
- Mono: 125 Gy
- w/ EBRT: 100 Gy
- Lower Rx doses than I-125 2/2 higher dose rate leading to ↑ BED
31
What imaging modality is normally used for a post-LDR-implant-prostate study?
CT scan
32
How do you verify LDR seed activity after receiving seeds from the manufacturer?
- Well chamber
- Test at least 10 seeds
-- Activity should be within ±3%
33
If you order pre-loaded seeds for LDR, how do you verify their spacing?
Using radiographs
34
How does post-LDR-implant prostate swelling impact the dose?
- Dose higher on day 1 (w/ swelling)
- Dose lower on day 30 (swelling resolved)
35
What're the Rx points for tandem & ovoid applicators?
- Point A: 2 cm above ovoids, 2 cm lateral to the tandem
- Point B: 3 cm lateral to the tandem
- Dose goals:
-- Point B = 30-40% of point A dose
-- Rectum < 4.1 Gy / fx (<70% Rx)
-- Bladder < 4.6 Gy / fx (<75% Rx)
-- Mucosa < 120 Gy (<140% of point A dose)
36
How is I131 made?
Nuclear reactor
37
How is I131 made?
Nuclear reactor
38
What're the indications for I-131 use?
- Indications:
-- Thyroid Ca
-- Hyperthyroidism
- Doses
-- Post Op: 65-150 mCi
-- Nodal: 150-200 mCi
-- Mets: > 200 mCi
- Delivery
-- 30% dose trapped in thyroid
-- rest cleared in urine
39
Why is Y-90 injected intra-arterially?
- 80% of the tumor blood supply comes from the arteries
- 80% of the normal liver blood supply comes from the veins
40
What're the patient release criteria for I-131?
- < 7 mCi and survey <2 mrem/hr at 1m → release w/ no instructions
- <33 mCi and survey <7 mrem/hr at 1m → release no instructions
41
When can you not release a pt who has received radionuclide therapy?
When any member of the public is likely to receive > 5 mSv
42
After HDR source exchange, the activity of the source should be within what % of the manufacturer's certificate?
±3%
43
For castration-resistant prostate cancer, how is the dose for 223Ra calculated?
- 1.49 μCi/kg
- We use mass to calculate the dose
44
What's the max allowable deviation between measured and intended dwell positions and step-size spacing b/w dwell positions?
±1mm
45
Do you need to measure the HDR source output daily?
- No; it's a laborious process involving a well chamber
- Only measure it at the time of source exchange or annually
- On a daily basis, use decay tables and TPS to calculate source activity
46
How does the dose at a point vary w/ distance, r, for a line source?
Dose ∝ 1/r
47
As it relates to the medium, what properties dictate the range of protons traversing it?
- Atomic number (Z)
- Tissue density (ρ)
48
How does the penumbra of the PBS compare to that of double-scattering protons?
- PBS has a less sharp penumbra
- PBS systems lack a physical aperture close to the patient, which worsens their penumbra
49
How does the RBE of protons vary throughout the SOBP?
RBE is highest at the end of the SOBP 2/2 end-range effects
50
What factors affect the penumbra of a proton beam?
- Depth
- Proton energy
- The air gap between the aperture and the patient
51
How often do brachytherapy sources need to be exchanged per regulations?
- There are no regulations
- Seeds are usually exchanged after 1 T1/2 2/2 increasing tx times
52
Why is a vaginal cylinder, as opposed to a naked source, used in brachytherapy?
- Dose fall off within the first few cms would result in a very heterogeneous dose distribution for tissues immediately adjacent to the naked source
- A cylinder pushes tissues away, and the dose fall-off isn't as rapid from its surface into the tissues
-- improved dose heterogeneity
53
What's range straggling?
Small fluctuations in the amount of energy lost by individual protons lead to a sigmoidal Bragg peak
54
What's range uncertainty?
Uncertainties in proton range 2/2 uncertainties in tissue composition and stopping power
