Rules of thumb Flashcards by Danilo Maziero

Bremsstrahlung max and mean photon energy

max = designated energy,
mean = 1/3 designated energy

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PDD for 10x10 field @ 10 cm depth
co-60
6MeV 
10MeV
18MeV

co-60 = 55%
6MeV = 65%
10MeV = 75%
18MeV = 80%

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TMR @ 10cm depth
6MeV
10MeV
18MeV

6MeV = 0.78
10MeV = 0.85
18MeV = 0.90

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Approximate Attenuation rates
6MeV
10MeV
18MeV

6MeV = 3%/cm
10MeV = 2.5%/cm
18MeV = 2%/cm

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Depth of Maximum dose (Dmax) for a 10x10 field size
Co-60
6MeV
10MeV
18MeV

Co-60 = 0.5cm
6MeV = 1.5 cm
10MeV = 2.4 cm
18MeV = 3.3 cm

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Surface Dose for photons
Co-60 
6MeV
10MeV
18MeV

Co-60 = 50%
6MeV = 25%
10MeV = 23%
18MeV = 20%

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Neutron Contamination in an 18MeV beam

5% on the central axis

0. 15 outside of the bem

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Neutron head leakage
6MeV
10MeV
18MeV

6MeV = none
10MeV = 0.01%
18MeV = 0.15%

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Percentage depth dose dependencies

Field size - as field size increases, the %DD increases due to increased scatter
SSD - %DD increases with increasing SSD due to the inverse square law

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Scattering at 1m from phantom

1/1000 of the primary beam

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Lateral scatter energy

Maximum energy of about 0.511MeV

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Backscatter energy

maximum energy of 0.255MeV

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Photon Interaction proportionalities per unit mass

Photo Electric = Z^3/E^3
Compton scattering = Z/sqrt(E)
Pair Production = Z^2/log(E)

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Conversion factor for Roentgens to cGY

in air = 0.876cGY/R

in tissue = 0.97 cGY/R

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Conversion factor for Roentgens to cGY in tissue for I-125

0.886 cGY/R

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SRS therapy

LINAC based is prescribed 70% isodose line

Gamma Knife is prescribed 50% isodse line

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Scatter contributions outside the field

Physical wedges - scatter 2.5% the central axis dose
Dynamic wedges - scatter about 1% the central axis dose
** dynamic wedges reduce contralateral bereast dose over physical wedges

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Outside of the beam there is a quantifiable amount of dose, extending perpendicular from the beam in the plane of Dmax

10cm away receives approximately 1% of the central axis dose at Dmax
30cm away receives approximately 0.2% of the central axis dose at Dmax

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Tissue weighting factors for effective dose equivalent
Gonads
Red bone marrow
Bladder
Skin

Gonads = 0.2
Red bone marrow = 0.12
Bladder = 0.05
Skin = 0.01

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Electron beams are nearly monoenergetic with average energy equal to that specified
– Electron Range

the 4, 3, 2, 2.33 rule
Depth max Dose R100 (cm) = E(MeV)/4
Therapeutic depth (80%) = R80(cm) = E(Mev)/3
Electron range in water Rp(cm) = E(MeV)/2
Depth of 50% isodose line R50(cm) = E(MeV)/2.33

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Depth of Maximum dose (Dmax) for 10x10 field size of electrons
6E
9E
12E 
20E

6E = 1.2 cm
9E = 1.9 cm
12E = 2.1 cm
20E = 2.1 cm

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Electron Surface Dose (in %)
6E
12E
20E

6E = 80%
12E = 90%
20E = 95%

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Electron X-ray contamination in %
6E
12E
20E

6E = 1%
12E = 2%
20E = 5%

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Amount of lead shielding needed in mm to stop an electron beam

Eo(MeV)/2

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Occupational limit

50 mSv/year (5mSv if pregnant)

Public limit in Sv

1mSv/year

The average worker in the USA receives X from occupational exposures

2 mSv/year

Neutrons need to be taken into account for energies above?

10MV

Energy of neutrons produced in photonuclear reactions

2MeV

Energy of neutrons at shielded door to the vault

100 keV | * most dangerous with radiation weighting factor = 20

Leakage limit

0.1% of the central axis value

Concrete TVLs for shielding 6 and 18 MV beams

``` 6MeV = 37cm 18MeV = 45 cm ```

Amount of lead in mm to shield an electron beam (to less than 5%)

Rp/10 - in other words | E(MeV)/20

``` Approximate shielding materials and thickness X-ray CT PET Cs-137 HDR Co-60 LINAC/Cyberknife ```

``` X-ray - 1 mm lead CT - 2 mm lead PET - 1 cm lead Cs-137 - 3 cm lead HDR - 50 cm concrete Co-60 - 70 cm concrete LINAC/Cyberknife - 170cm concrete ```

Sources of radiation exposure for the average person

``` Radon - 1mSv/year Ingested isotopes - 0.4mSv/year Isotopes occurring naturally - 0.4 mSv/yr Cosmic rays - 0.4 mSv/yr Medical exposures - 2.0 mSv/yr ```

Lead and Iodine's k-edge

Lead - 88 keV | Iodine - 33 keV

Increase in mortality risk due to acute radiation

8% per Sv

Increase in mortality risk due to chronic radiation exposure

4% per Sv

Radiation quality factors (from 10CFR20)

``` Photons = 1 Electrons = 1 Protons = 10 Neutrons = 5-20 (10 for unknown energy) Alpha = 20 ```

Package receiving wipe test limits for 300cm^2 area

Alpha - 22 dpm/cm^2 | Beta/gamma - 220 dpm/cm^2

ALARA levels

ALARA level 1 - 10% of the allowed limit per quarter | ALARA level 2 - 30% the allowed limit per quarter

Limit for removing sealed source from service due to leaking

185Bq | *sealed sources need to be checked every 6 months

Patient exposure release levels I-125 Pd-103 I-131

I-125: <1mR/hr at 1 meter Pd-103: <3mR/hr at 1 meter I-131: <7mR/hr at 1 meter

Brachytherapy Medical Events

Wrong patient, side or nuclide Total dose differs by 20% Single fraction differs by 50%

Teletherapy Medical Events

Wrong patient or site Total dose differs by 20% (for treatments> 3 fractions) Total dose differs by 10% (for treatments<= 3 fractions)

LDR dose rate at prescription point

0.2 - 2Gy/hr

HDR dose rate at prescription point

>12Gy/hr

Exposure time to achieve occupational limit while 1 meter away from HDR source

1 hour

Ir-192 HDR decayment rate

1% per day

Dose from Cs-137 LDR to Ir-192 HDR

D(Ir-192) = D(Cs-137)*0.65 | * due to radiobiological effects related to the differing dose rates

Spine chords division

7 - cervical 12 - thoracic 5 - lumbar 5 - sacral

Spinal cord ends

L1/L2 and terminates in cauda equina

``` Sample CT numbers - relative to water Air Lung Soft tissue Fat Water CSF Blood and Muscle Spongy Bone Cortical Bone ```

``` Air: -1000 Lung: -700 Soft tissue: -300 to -100 Fat: -80 Water: 0 CSF: 15 Blood and Muscle: 40 Spongy Bone: 400 and up Cortical Bone: up to 3000 ```

Ultrasound - velocities of sounds in different media

Air: 330 m/s Water: 1497 m/s Bone: 3500 m/s Soft Tissue: 1540 m/s

Typical dose associated with CT

50-75 mGy

TG-21 to TG-51 output level

1% higher

MLC leakage

Intraleaf - 2% Interleaf - 3% End leakage - 15%

Pacemaker tolerance

2 Gy

Approximate Z of materials

Air - 7.6 Tissue - 7.4 Bone - 14.0 Prostheses - 20-25

``` density of various materials Air Lung Bone Concrete Steel Lead Tungsten ```

``` Air - 0.0012 g/cm3 Lung - 0.33 g/cm3 Bone - 1.8 g/cm3 Concrete - 2.35 g/cm3 (can vary significantly with composition) Steel - 7.85 g/cm3 Lead - 11.34 g/cm3 Tungsten = 19.25 g/cm3 ```

Predominance and occurrence energies Photoelectric Compton Pair Production

``` Predominance - Occurrence P.E: 10-30 Kev - 10-70Kev Compton 30Kev - 25Mev - Occurs everywhere P.P. >25 Mev - >10Mev ```