Calculations Flashcards
Equation for Equivalent Square of Rectangle:
ES = (4 x A) / Perimeter A = area
Equation for Equivalent Sq of a Circle:
ES = (4 x A) / Pie x P; OR, ES = .89 x diameter
Output factors for a circle are ______ those for a rectangle?
greater than
Step and Shoot method with really low MU is not used due to what?
timer error
What is the cause of Timer Error for Co-60?
transit time of the source
What is the cause of timer error for LINAC?
time needed to get to full dose rate
What is the Timer Error for Co-60?
0.02 min
What is the Timer Error for a LINAC?
0.10 MU
Whats another name for LINAC Timer Error?
End Effect
PDD equals:
Dd / Ddmax (SSD conditions)
What is the dmax for E < 400kVp?
dmax = surface
Properties of PDD:
- PDD increases with FS (less independent with higher E)
- PDD increases with E
- PDD increases with SSD (due to ISL)
- PDD decreases with depth (exponentially) after the build-up region
What equation is used to determine a PDD with a new SSD?
PDD_2 = PDD_1 x MF (mayneord factor)
What factor accounts for change in PDD
Mayneord Factor
What are the disadvantages of the MF?
- it overestimates increases in PDD with increases in SSD
2. it overestimates for small fields
What is the MF equation?
MF = ((SSD_2 + dmax) / (SSD_1 + dmax))^2 x ((SSD_1 + d) / (SSD_2 + d))^2
For photons, as E increases, surface dose
decreases
What is the surface dose, dmax, and d10 for Superficial (1mm Al) Unit?
100%, 0.0cm, 5%
What is the surface dose, dmax, and d10 for Orthovoltage (2mm Cu) Unit?
100%, 0.0 cm, 35%
What is the surface dose, dmax, and d10 for Co-60?
30-50%, 0.5 cm, 56%
What is the surface dose, dmax, and Dd10 for 6MV?
20-30%, 1.5 cm, 67%
What is the surface dose, dmax, and Dd10 for 10MV?
18-27%, 2.0 cm, 72%
What is the surface dose, dmax, and Dd10 for 15MV?
15-22%, 2.9cm, 77%
What is the surface dose, dmax, and Dd10 for 18MV?
13-20%, 3.5cm, 80%
PDD takes measurements at ______ to make a graph?
individual depths
TAR equals:
TAR = Dd / Dfs (free space in air - SAD conditions)
Properties of TAR:
- TAR increases with E
- TAR increases with FS
- TAR Decreases with depth (approx exponentially) beyond build-up
- TAR is independent of SSD
- TAR = TMR x BSF
SAR equals:
SAR = dose scatter / dose free space; SAR = TAR - TAR(0) --- (TAR(0) = total TAR - TAR of primary beam)
SAR is _____ of SSD
Independent
BSF equals:
BSF = Ddmax / Dfs
Properties of BSF:
- BSF = TAR at dmax
- BSF is independent of SSD
- highest for .7mm HVL Cu
- 1.5 for Orthovoltage; 1.036 for Co-60
Explain the ISL, BSF, TAR, PDD box:
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)
TPR equals:
TPR = dose at given point in phantom / dose at same point at fixed reference depth (in SAD conditions)
Properties of TPR:
- may be normalized at any depth
- dref > dmax
- TPR increases with FS and E
- TPR is independent of SSD
TMR equals:
TMR = dose at given point in phantom / dose at same point at dmax (in SAD conditions)
TMR Properties:
- Assumes scatter is independent of divergence
- TMR increases with FS and E
- TMR is independent of SSD
- TMR = 1 at dmax
What is Sc, Sp, and Scp normalized to ?
10
What is Sc measured in?
air
What is Scp measured in?
water
What is Sp measured in?
cannot be measured, must be calculated
Sc (collimator scatter factor) equals:
Sc = Output in air of a given FS / Output in air of a ref field size
Sc Properties:
- Sc increases with FS due to increased exposure of flattening filter and surface area of collimator exposed
- Measurement requires entire buildup is covered
Scp (total scatter factor) equals:
Scp = Sc x Sp; Scp = dose of given FS at ref d / dose of ref FS at ref d
At what depth is Scp measured?
at damx
Sp (phantom scatter factor) equals:
Sp = Scp / Sc;
Properties of Sp:
- Sp increases with FS
2. Sp is not directly measurable
Sc, Sp, and Scp all ________ with FS?
increase
BSF _____ with E, then ______
increases; decreases
Does BSF increase or decrease with FS?
increase
What therapeutic E range has the highest BSF?
2mm Cu (Orthovoltage) - 1.360
What therapeutic E range has the lowest BSF?
18 MV - 1.01
Off axis ratio (OAR) equals:
OAR = output at point of off-axis (d) / output at CAX (d)
What creates OAR?
created from design of flattening filter and photon scatter
Properties of OAR:
- OAR increases with distance off CAX point
- OAR are greater for 6x than 18x at shallow depths
- OAR decreases with depths beyond 10cm
At what point is the wedge factor (WF) measured?
at dmax or d10 on CAX
WF equals:
WF = output with wedge / Output of open field (without wedge)
Properties of WF:
- WF are higher for smaller wedges
2. WF increase with FS and depth
What is Cerrobend blocks made of?
Bismuth, lead, tin, cadmium (BLTC)
Melting point of cerrobend compared to lead is:
70 degrees C for Cerro, 327 degrees C for lead
At room temp Cerro is harder or softer than Lead?
Harder
Therapeutic thickness for Cerrobend is, and what thickness of lead does that equal?
7.5cm; 6cm of lead
How many cm of Cerrobend equals 1cm of lead?
1.2cm
What is the required lead thickness for Co-60, 6MV, and 18MV?
5.0cm, 6.5cm, 7.0cm
What creates less penumbra, Cerrobend or MLCs, why?
Cerrobend; blocks are cut with divergence
How many HVLs of Cerrobend does it take to allow only 5% transmission?
5 HVLs
What is the Gap calculation
G = d/SAD (L1 / 2 + L2 / 2)
d=depth; L=length of field size
ISL formula
I1 / I2 = (D2 / D1)^2
Temperature and Pressure formula
[((273 + T) / 295)] x (760 / P)
Temperature Coversion
C = 5/9 (F -32)
Pressure Conversion: 760 mmHg = ?kPa
760 mmHg = 101.3 kPa
Wedge Angle formula
WA = 90 - (HA/2) HA = hinge angle
ISL with VSD
((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
Formula to determine # of TVLs
#TVL = log(exposure @ point / limit) *Exposure at point = W x U x T (workload, use factor, output factor)
Biological Effective Dose =
BED = TD x RBE; or BED = n x d (1 + d / (alpha/beta))
