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Dose Tolerances > Calculations > Flashcards

Calculations Flashcards

1
Q

Equation for Equivalent Square of Rectangle:

A 
ES = (4 x A) / Perimeter
A = area
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2
Q

Equation for Equivalent Sq of a Circle:

A

ES = (4 x A) / Pie x P; OR, ES = .89 x diameter

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3
Q

Output factors for a circle are ______ those for a rectangle?

A

greater than

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4
Q

Step and Shoot method with really low MU is not used due to what?

A

timer error

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5
Q

What is the cause of Timer Error for Co-60?

A

transit time of the source

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6
Q

What is the cause of timer error for LINAC?

A

time needed to get to full dose rate

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7
Q

What is the Timer Error for Co-60?

A

0.02 min

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8
Q

What is the Timer Error for a LINAC?

A

0.10 MU

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9
Q

Whats another name for LINAC Timer Error?

A

End Effect

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10
Q

PDD equals:

A

Dd / Ddmax (SSD conditions)

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11
Q

What is the dmax for E < 400kVp?

A

dmax = surface

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12
Q

Properties of PDD:

A
  1. PDD increases with FS (less independent with higher E)
  2. PDD increases with E
  3. PDD increases with SSD (due to ISL)
  4. PDD decreases with depth (exponentially) after the build-up region
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13
Q

What equation is used to determine a PDD with a new SSD?

A

PDD_2 = PDD_1 x MF (mayneord factor)

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14
Q

What factor accounts for change in PDD

A

Mayneord Factor

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15
Q

What are the disadvantages of the MF?

A
  1. it overestimates increases in PDD with increases in SSD

2. it overestimates for small fields

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16
Q

What is the MF equation?

A

MF = ((SSD_2 + dmax) / (SSD_1 + dmax))^2 x ((SSD_1 + d) / (SSD_2 + d))^2

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17
Q

For photons, as E increases, surface dose

A

decreases

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18
Q

What is the surface dose, dmax, and d10 for Superficial (1mm Al) Unit?

A

100%, 0.0cm, 5%

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19
Q

What is the surface dose, dmax, and d10 for Orthovoltage (2mm Cu) Unit?

A

100%, 0.0 cm, 35%

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20
Q

What is the surface dose, dmax, and d10 for Co-60?

A

30-50%, 0.5 cm, 56%

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21
Q

What is the surface dose, dmax, and Dd10 for 6MV?

A

20-30%, 1.5 cm, 67%

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22
Q

What is the surface dose, dmax, and Dd10 for 10MV?

A

18-27%, 2.0 cm, 72%

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23
Q

What is the surface dose, dmax, and Dd10 for 15MV?

A

15-22%, 2.9cm, 77%

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24
Q

What is the surface dose, dmax, and Dd10 for 18MV?

A

13-20%, 3.5cm, 80%

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25
Q

PDD takes measurements at ______ to make a graph?

A

individual depths

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26
Q

TAR equals:

A

TAR = Dd / Dfs (free space in air - SAD conditions)

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27
Q

Properties of TAR:

A
  1. TAR increases with E
  2. TAR increases with FS
  3. TAR Decreases with depth (approx exponentially) beyond build-up
  4. TAR is independent of SSD
  5. TAR = TMR x BSF
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28
Q

SAR equals:

A 
SAR = dose scatter / dose free space;
SAR = TAR - TAR(0) --- (TAR(0) = total TAR - TAR of primary beam)
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29
Q

SAR is _____ of SSD

A

Independent

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30
Q

BSF equals:

A

BSF = Ddmax / Dfs

31
Q

Properties of BSF:

A
  1. BSF = TAR at dmax
  2. BSF is independent of SSD
  3. highest for .7mm HVL Cu
  4. 1.5 for Orthovoltage; 1.036 for Co-60
32
Q

Explain the ISL, BSF, TAR, PDD box:

A

2 routes to get output factor: if given PDD or BSF use route 1 (BSF to PDD); if given ISL or TAR use route 2 (ISL to TAR).
*both routes begin in Air, Route 1 goes through Water (between BSF and PDD)

33
Q

TPR equals:

A

TPR = dose at given point in phantom / dose at same point at fixed reference depth (in SAD conditions)

34
Q

Properties of TPR:

A
  1. may be normalized at any depth
  2. dref > dmax
  3. TPR increases with FS and E
  4. TPR is independent of SSD
35
Q

TMR equals:

A

TMR = dose at given point in phantom / dose at same point at dmax (in SAD conditions)

36
Q

TMR Properties:

A
  1. Assumes scatter is independent of divergence
  2. TMR increases with FS and E
  3. TMR is independent of SSD
  4. TMR = 1 at dmax
37
Q

What is Sc, Sp, and Scp normalized to ?

A

10

38
Q

What is Sc measured in?

A

air

39
Q

What is Scp measured in?

A

water

40
Q

What is Sp measured in?

A

cannot be measured, must be calculated

41
Q

Sc (collimator scatter factor) equals:

A

Sc = Output in air of a given FS / Output in air of a ref field size

42
Q

Sc Properties:

A
  1. Sc increases with FS due to increased exposure of flattening filter and surface area of collimator exposed
  2. Measurement requires entire buildup is covered
43
Q

Scp (total scatter factor) equals:

A 
Scp = Sc x Sp;
Scp = dose of given FS at ref d / dose of ref FS at ref d
44
Q

At what depth is Scp measured?

A

at damx

45
Q

Sp (phantom scatter factor) equals:

A

Sp = Scp / Sc;

46
Q

Properties of Sp:

A
  1. Sp increases with FS

2. Sp is not directly measurable

47
Q

Sc, Sp, and Scp all ________ with FS?

A

increase

48
Q

BSF _____ with E, then ______

A

increases; decreases

49
Q

Does BSF increase or decrease with FS?

A

increase

50
Q

What therapeutic E range has the highest BSF?

A

2mm Cu (Orthovoltage) - 1.360

51
Q

What therapeutic E range has the lowest BSF?

A

18 MV - 1.01

52
Q

Off axis ratio (OAR) equals:

A

OAR = output at point of off-axis (d) / output at CAX (d)

53
Q

What creates OAR?

A

created from design of flattening filter and photon scatter

54
Q

Properties of OAR:

A
  1. OAR increases with distance off CAX point
  2. OAR are greater for 6x than 18x at shallow depths
  3. OAR decreases with depths beyond 10cm
55
Q

At what point is the wedge factor (WF) measured?

A

at dmax or d10 on CAX

56
Q

WF equals:

A

WF = output with wedge / Output of open field (without wedge)

57
Q

Properties of WF:

A
  1. WF are higher for smaller wedges

2. WF increase with FS and depth

58
Q

What is Cerrobend blocks made of?

A

Bismuth, lead, tin, cadmium (BLTC)

59
Q

Melting point of cerrobend compared to lead is:

A

70 degrees C for Cerro, 327 degrees C for lead

60
Q

At room temp Cerro is harder or softer than Lead?

A

Harder

61
Q

Therapeutic thickness for Cerrobend is, and what thickness of lead does that equal?

A

7.5cm; 6cm of lead

62
Q

How many cm of Cerrobend equals 1cm of lead?

A

1.2cm

63
Q

What is the required lead thickness for Co-60, 6MV, and 18MV?

A

5.0cm, 6.5cm, 7.0cm

64
Q

What creates less penumbra, Cerrobend or MLCs, why?

A

Cerrobend; blocks are cut with divergence

65
Q

How many HVLs of Cerrobend does it take to allow only 5% transmission?

A

5 HVLs

66
Q

What is the Gap calculation

A

G = d/SAD (L1 / 2 + L2 / 2)

d=depth; L=length of field size

67
Q

ISL formula

A

I1 / I2 = (D2 / D1)^2

68
Q

Temperature and Pressure formula

A

[((273 + T) / 295)] x (760 / P)

69
Q

Temperature Coversion

A

C = 5/9 (F -32)

70
Q

Pressure Conversion: 760 mmHg = ?kPa

A

760 mmHg = 101.3 kPa

71
Q

Wedge Angle formula

A 
WA = 90 - (HA/2)
HA = hinge angle
72
Q

ISL with VSD

A

((VSD + Dmax) / (VSD + Dmax + gap))^2

gap = distance from 100 SSD to tx’d SSD, ie. tx’d at SSD = 11 would result in gap of 11

73
Q

Formula to determine # of TVLs

A 
#TVL = log(exposure @ point / limit)
*Exposure at point = W x U x T (workload, use factor, output factor)
74
Q

Biological Effective Dose =

A

BED = TD x RBE; or BED = n x d (1 + d / (alpha/beta))

Dose Tolerances (23 decks)

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