Mu Calcs

Front 1

MU Calcs: Dose equation

Back 1

MU Calcs: Dose equation

D = MU * Kref * Sc * Sp * TMR *InvSq

Front 2

MU Calcs: Dose rate equation

Back 2

MU Calcs: Dose rate equation

D/MU = Kref * Sc * Sp * TMR * InvSq

Front 3

MU Calcs: MU equation

Back 3

MU Calcs: MU equation

MU = D/(Kref * Sc * Sp * TMR * InvSq)

Front 4

MU Calcs: Calibration refs
1
2
3
4
5

Back 4

MU Calcs: Calibration refs
1 Ref field size (10x10cm)
2 Ref depth (dmax)
3 Ref SAD or SSD (100cm)
4 Ref medium (Water)
5 Ref dose/MU (1 cGy/MU)

Front 5

MU Calcs: Sc

Back 5

MU Calcs: Sc
Accounts for change in head scatter photon fluence reaching the ref SAD
Changes output (cGy/MU) wrt 10x10 ref

Front 6

MU Calcs: Sp

Back 6

MU Calcs: Sp
Accounts for change in scatter photon fluence reaching the ref depth (dmax) and ref SAD
Changes output (cGy/MU) wrt 10x10 ref

Front 7

MU Calcs: TMR

Back 7

MU Calcs: TMR
As thickness deviates from dmax for constant SAD, accounts for
- Attenuation
- Scatter

Front 8

MU Calcs: PDD

Back 8

MU Calcs: PDD
As thickness changes from dmax and distance from the source changes from SSD+dmax to SSD+d for constant SSD, accounts for
- Attenuation
- Scatter
- Inverse square law

Front 9

MU Calcs: InvSq

Back 9

MU Calcs: InvSq
Accounts for decrease in fluence and hence dose due to speading of photons over a larger area due to divergence (point source geometry)

Front 10

MU Calcs: Rules of Thumb

Changes in TMR per cm

Back 10

MU Calcs: Rules of Thumb

Changes in TMR per cm

6MV photons: 3%/cm change in TMR

18MV photons: 2%/cm change in TMR

Front 11

MU Calcs: Rules of Thumb

Change in InvSq

Back 11

MU Calcs: Rules of Thumb

Change in InvSq: For any geometry, near 100cm SAD 2%/cm change in InvSq

Front 12

MU Calcs: Rules of Thumb

Change in PDD

Back 12

MU Calcs: Rules of Thumb

Change in PDD

6MV photons: 5%/cm change in PDD

18MV photons: 4%/cm change in PDD

Front 13

MU Calcs: InvSq

TMR-based technique

PDD-based technique

Back 13

MU Calcs: InvSq

TMR-based technique

= (SADref/SAD)**2

PDD-based technique

= (SSDref+dref/SSD+dref)**2

= (SSDref/SSD)**2

Front 14

MU Calcs: InvSq

When is it used in
- TMR-based technique
- PDD-based technique

Back 14

MU Calcs: InvSq

When is it used in
- TMR-based technique
-- When SAD for calc point is different from SADref
-- Source to point distances (SAD) considered
- PDD-based technique
-- When SSD for calc geometry is different from SSDref
-- Source to dmax distances used (not source to point) because PDD already has InvSq variation from dmax to d

Front 15

MU Calcs

Dose rate: Linac v Co-60

Back 15

MU Calcs

Dose rate
- Linac: cGy/MU
- Co-60: cGy/min

No monitor chamge in a Co-60 machines since "beam on time" uniquely determined the dose.

Front 16

MU Calcs: Mayenoard F-factors

Back 16

MU Calcs: Mayenoard F-factors
1 PDD at SSD2=F*PDD at SSD1
2 If an "approximate" answer is required, ignore the F-factor
3 If dose at dmax is required, F=1 for any SSD
4 Do not confuse F-factor with SSD factor which caculates changes in absolute dose output (cGy/MU), not relative changes in PDD when SSD changes
5 Do not apply to TMR since it contains no InvSq component to be corrected

Front 17

MU Calcs: Equation for electrons

Back 17

MU Calcs: Equation for electrons

MU=D/(Kref * cone factor * PDD/100 * InvSq)

Front 18

MU Calcs: Are PDDs SSD-dependent?

Back 18

MU Calcs: Are PDDs SSD-dependent?

No because of the InvSq componenet inherent in PDD.
But Mayeonard F-factor must be used to correct PDD from one SSD to another.

Front 19

MU Calcs: Mayeonard F-factor equation

Back 19

MU Calcs: Mayeonard F-factor equation

Front 20

MU Calcs: dmax by energy

6 MeV
9 MeV
12 MeV
15 MeV
18 MeV
22 MeV

Back 20

MU Calcs: dmax by energy

6 MeV: 1.3 cm
9 MeV: 1.9 cm
12 MeV: 2.1 cm
15 MeV: 2.1 cm
18 MeV: 1.5 cm

Front 21

MU Calcs: Radiochromic film
1
2
3
4
5
6

Back 21

MU Calcs: Radiochromic film
1 Tissue equivalence
2 High spatial resolution
3 Large dynamic range (10**-2 to 10**6 Gy)
4 Relatively low spectral sensitivity variation
5 Insensitivity to visible light
6 No need for chemical processing
But are sensitive to UV light and temperature

Front 22

Dose equivalent

Back 22

Dose equivalent

SI unit: J/kg

1 Sievert = 1 J/kg

Front 23

Effective dose equivalent

Back 23

Used to take into account nonuniform irradiation situations

Def: the sum of the weighted dose equivalents for irradiated tissues or organs

H(E)= W(T)H(T)

where W(T) is weighting factor of tissue T and H(T) is mean dose equivalent received by tissue T

Weighting factors represent proportionate risk (stochastic) of tissue when body is irradiated uniformly

Front 24

Shielding: Protection against three types of radiation
1
2
3

Back 24

Shielding: Protection against three types of radiation
1 Primaryn radiation (useful)
2 Scattered radiation
3 Leakage radiation through source housing

Front 25

Shielding: Barriers
1
2

Back 25

Shielding: Barriers
1 Primary - shields useful beam
2 Secondary - shields stray radiation
A Scatter
B Leakage

Front 26

Shielding: Factors
W
U
O
d

Back 26

Shielding: Factors
W: Workload (rad/week at 1m)
< 500 kVp=mAmp min/week
MV=weekly dose at 1m from source
=#pts/week * dose/pt at 1m
U: Use factor-fraction of operating time radiation is directed toward a particular barrier
O: Occupancy factor-Fraction of operating time area is occupied
d: Distance in meters from source to area to be protected

Front 27

Shielding: Primary barrier

Back 27

Shielding: Primary barrier

P=(WUT/d**2)B

P=max dose equivalent to area protected
B=transmission factor for the barrier

Front 28

Shielding: Neutrons

Back 28

Shielding: Neutrons
-The shielding of the maze for photons is adequate for neutrons
-The door must been shielded for neutrons at scatter down the maze
--Longer maze >5m
--A few inches of hydrogenous materail like polyethylene can be added to the door to thermalize neutrons and reduce dose
--Lead or steel sheet may be added to the door to protect against scattered x-rays

Front 29

X-ray spectra: Max photon energy given

Back 29

X-ray spectra: Max photon energy given

I(E)=KZ[E(m)-E]
-I(E) intensity of photons with energy E
-Z atomic number of target
-E(m) max photon energy
K constant

Remember: max possible energy a bremsstrahlung photon can have is the energy of the incident electron

Front 30

MU Calcs: Skin gap for match at depth d

Back 30

MU Calcs: Skin gap for match at depth d

g=(d/SAD) * (C1+C2)/2

g gap on skin
C1, C2 collimator settings
d depth of match

Front 31

MU Calcs: divergence of spinal field

Back 31

MU Calcs: divergence of spinal field

arctan[(spine field length/2)/100]

Front 32

Beam obliquity tends to
1
2
3

Back 32

Beam obliquity tends to
1 increase side scatter at dmax
2 shift dmax toward surface
3 decrease depth of penetration as measured by the depth of the 80% isodose

Front 33

Exposure: Occupational (annual)
1 Effective dose equiv limit
2 Dose equiv limit
A Lens of eye
B All others
3 Cumulative exposure

Back 33

Exposure: Occupational (annual)
1 Effective dose equiv limit: 50 mSv
2 Dose equiv limit
A Lens of eye: 150 mSv
B All others: 500 mSv
3 Cumulative exposure: 10 mSv*age

Front 34

Exposure: Public (annual)
1 Continuous or frequent
2 Infrequent
3 Remedial action if
A Eff dose equiv
B Exposure to radon
4 Dose equiv for lens, skin, exts

Back 34

Exposure: Public (annual)
1 Continuous or frequent: 1 mSv
2 Infrequent: 5 mSv
3 Remedial action if
A Eff dose equiv >5 mSv
B Exposure to radon: >0.007 Jhm**-3
4 Dose equiv for lens, skin, exts: 50 mSv

Front 35

Exposure: Ed and training (annual)
1 Eff dose equiv
2 Dose equiv for lens, skin, exts

Back 35

Exposure: Ed and training (annual)
1 Eff dose equiv: 1 mSv
2 Dose equiv for lens, skin, exts: 50 mSv

Front 36

Exposure: Embryo-fetus
1 Total dose equiv
2 Dose equiv / month

Back 36

Exposure: Embryo-fetus
1 Total dose equiv: 5 mSv
2 Dose equiv / month: 0.5 mSv

Front 37

Exposure: Negligible individual risk level (annual)

Back 37

Exposure: Negligible individual risk level (annual): 0.01 mSv

Front 38

X-ray contamination at the end of the electron range

Back 38

X-ray contamination at the end of the electron range
-Determined from tail of depth-dose curve where tail becomes straight
-Dose in patient contributed by brem interactions of electrons
--In collimation system
--In patient

Front 39

MU Calcs: Electron energy at depth

Back 39

MU Calcs: Electron energy at depth

E(z)=E(0)(1-z/Rp)

E(z)-mean energy at depth
z-depth

Front 40

MU Calcs: Electron's most probable energy

Back 40

MU Calcs: Electron's most probable energy

Ep(0)=C1+C2Rp+C3Rp**2

Front 41

MU Calcs: Electron's mean energy

Back 41

MU Calcs: Electron's mean energy

E(0)=C4*R50

C4=2.33 MeV/cm
R50=depth at which dose is 50% max dose

Front 42

Exposure

Back 42

Measure of ionization per unit mass of air