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RadPhys > Pr, Br, Rad Pharm, TBI, TSET > Flashcards

Pr, Br, Rad Pharm, TBI, TSET Flashcards

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
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2
Q

What’s the formula for the range of protons in the tissue?

A

Range (cm) = 0.033E + 0.0005E2

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3
Q

When does the skin dose from a proton beam increase?

A

When you increase the SOBP

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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.

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5
Q

What controls the range of the proton beam?

A
  • Beam energy
  • Can be controlled using a range shifter
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6
Q

What controls the modulation of a proton beam?

A

Range modulator wheel

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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
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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.
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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)

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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
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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
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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
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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
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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
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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
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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
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17
Q

What’s the formula for the range of protons in the tissue?

A

Range (cm) = 0.033E + 0.0005E2

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18
Q

What’s the typical energy range of proton beams?

A

70-250 MeV

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19
Q

What are the units of air kerma strength?

A
  • 1 μGy × m2 / h
  • Also represented as 1U
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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
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21
Q

How’re unsealed sources given?

A
  • Usually given systemically or injected
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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
Q

What’s the advantage of using higher beam energies for AP/PA TBI tx? How does normal tissue dose depend on pt thickness?

A
  • ↑ energy → ↓ normal tissue dose
  • ↓ thickness → ↓ normal tissue dose
26
Q

What’s the formula for calculating TBI dose homogeneity?

A

Dosepeak / Dosemid

27
Q

How does dose homogeneity vary w/ beam energy, SSD, and patient thickness for a TBI tx?

A
  • ↑ homogeneity w/
    – ↓ thickness
    – ↑ energy
    – ↑ SSD
    — ↓ PDD fall-off w/ ↑ SSD`
28
Q
A
29
Q

What’s the Rx for I-125 prostate implant monotherapy and when coupled w/ EBRT?

A
  • Mono: 145 Gy
  • w/ EBRT: 110 Gy
30
Q

What’s the Rx for Pd 103 implant monotherapy and when coupled w/ EBRT??

A
  • Mono: 125 Gy
  • w/ EBRT: 100 Gy
  • Lower Rx doses than I-125 2/2 higher dose rate leading to ↑ BED
31
Q

What imaging modality is normally used for a post-LDR-implant-prostate study?

A

CT scan

32
Q

How do you verify LDR seed activity after receiving seeds from the manufacturer?

A
  • Well chamber
  • Test at least 10 seeds
    – Activity should be within ±3%
33
Q

If you order pre-loaded seeds for LDR, how do you verify their spacing?

A

Using radiographs

34
Q

How does post-LDR-implant prostate swelling impact the dose?

A
  • Dose higher on day 1 (w/ swelling)
  • Dose lower on day 30 (swelling resolved)
35
Q

What’re the Rx points for tandem & ovoid applicators?

A
  • 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
Q

How is I131 made?

A

Nuclear reactor

37
Q

How is I131 made?

A

Nuclear reactor

38
Q

What’re the indications for I-131 use?

A
  • 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
Q

Why is Y-90 injected intra-arterially?

A
  • 80% of the tumor blood supply comes from the arteries
  • 80% of the normal liver blood supply comes from the veins
40
Q

What’re the patient release criteria for I-131?

A
  • < 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
Q

When can you not release a pt who has received radionuclide therapy?

A

When any member of the public is likely to receive > 5 mSv

42
Q

After HDR source exchange, the activity of the source should be within what % of the manufacturer’s certificate?

A

±3%

43
Q

For castration-resistant prostate cancer, how is the dose for 223Ra calculated?

A
  • 1.49 μCi/kg
  • We use mass to calculate the dose
44
Q

What’s the max allowable deviation between measured and intended dwell positions and step-size spacing b/w dwell positions?

A

±1mm

45
Q

Do you need to measure the HDR source output daily?

A
  • 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
Q

How does the dose at a point vary w/ distance, r, for a line source?

A

Dose ∝ 1/r

47
Q

As it relates to the medium, what properties dictate the range of protons traversing it?

A
  • Atomic number (Z)
  • Tissue density (ρ)
48
Q

How does the penumbra of the PBS compare to that of double-scattering protons?

A
  • PBS has a less sharp penumbra
  • PBS systems lack a physical aperture close to the patient, which worsens their penumbra
49
Q

How does the RBE of protons vary throughout the SOBP?

A

RBE is highest at the end of the SOBP 2/2 end-range effects

50
Q

What factors affect the penumbra of a proton beam?

A
  • Depth
  • Proton energy
  • The air gap between the aperture and the patient
51
Q

How often do brachytherapy sources need to be exchanged per regulations?

A
  • There are no regulations
  • Seeds are usually exchanged after 1 T1/2 2/2 increasing tx times
52
Q

Why is a vaginal cylinder, as opposed to a naked source, used in brachytherapy?

A
  • 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
Q

What’s range straggling?

A

Small fluctuations in the amount of energy lost by individual protons lead to a sigmoidal Bragg peak

54
Q

What’s range uncertainty?

A

Uncertainties in proton range 2/2 uncertainties in tissue composition and stopping power

RadPhys (10 decks)

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