Dose Calcs Lecture 2

1
Dose Calcs Lecture 2

• Elizabeth Small B.S.R.T.(T)

2
Blocked Field Size

• What is the dilemma?
• Have to get a f.s. that truly represents the
  treated area
• How?
• Subtract the blocked area from the open area
  treatment area
• Divide the treatment area by the dimension of the
  field that changed least (W or L)
• This becomes new dimension
• Now have a rectangular f.s. that truly represents
  what is to be treated
• Do ESFS to get square f.s.

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Example Problem

• Open area f. s. 10 X 12
• Block in each corner
• Triangular and measure approximately 2 X 3
• What is treatment f.s.?
• Open area 120 cm squared
• Blocked f.s. .5 X 2 X 3 four times 12
• Treatment f.s 120-12 108

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Example Problem Continued

• Which dimension is least affected by the blocks?
• Length to 12
• Divide treated area by this dimension
• 108/12 9
• New rectangular f.s. 9 X 12
• What is ESFS?
• 4 X A/P or A/.5 ( W L)
• 108/10.5 10.3

5
PDD and TAR Review

• What are these referring to?
• What factors are they dependent on?
• When are PDD calcs used?
• T or F TAR is independent of SSD.

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Backscatter Factor (BSF)

• BSF TAR at D max
• Dose phantom/dose in air
• Level of D max
• In phantom- scatter
• Air- no scatter
• BSF- often called peak scatter factor (PSF) since
  measured at D max
• Dependent on
• Size/shape of field
• Depth of underlying tissue
• Energy of beam

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Tissue Phantom Ratio (TPR) and TMR

• Tissue Phantom Ratio
• Dose at given depth in phantom
• Divided by
• Dose at the same point at a reference
  depth-usually 5 cm
• If reference depth is D max then called Tissue
  Maximum Ratio (TMR)
• SAD same
• Depth in phantom is different
• SEE book

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TPR and TMR

• Developed because at high energies measurements
  in air (TAR) become difficult to get without
  introducing scatter from the build up cap
• Difference between TPR and TMR
• Reference depth of D max- TMR
• D max changes with
• Energy, D max gt as Energy gt
• F.S., D max lt as f.s. gt due to low energy scatter
  radiation
• TPR avoids f.s. affect since it is not measured
  at D max

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TPR and TMR

• TPR
• Dose at depth / dose at reference
• TMR
• Dose at depth/dose at D max
• TPR
• The numerical value has no upper limit
• Use any value for denominator- reference depth
• TMR
• The numerical value will not exceed 1.0
• Denominator will always be greater than numerator
  since the reference depth is D max

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Transmission Factor

• Tray, wedges, and compensating filters
• Any device placed in the path of the beam
• Attenuate the beam and must be calculated in dose
  calculation
• Transmission factor
• Dose with the device/dose without device

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Tray Transmission

• Trays attenuate beam
• If more than one tray is used want to account for
  it (heavy blocks)
• Tray factor
• Dose with tray/dose without tray
• Dependent on
• Beam energy, as E gt, tray attenuation lt
• Different energies have different tray factors

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Wedge Transmission Factors

• Wedge factor
• Amount of radiation transmitted through the wedge
• Thick end (heel) attenuates more of beam
• Thin end (toe) attenuates less of beam
• Alters beam isodose patterns

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Calculations

• SSD setupwhat is the dose?
• Use PDD
• PDD 100 at D max
• Prescription is written to a depth other than D
  max more dose will be given at D max
• Dose at depth, called tumor dose (TD)

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PDD Calculation

• Given dose (dose at Dmax) TD/PDD X 100
• PDD (d, s, SSD) use table in book
• D depth
• S equivalent square f.s.
• SSD the setup distance
• Example
• PDD (5, 10, 80) 78.3
• Dose and the PDD is a direct relationship
• Dose at point A Dose at point B
• PDD at point A PDD at point B

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PDD Problem

• Given dose TD x 100
• PDD
• TD tumor dose
• Given dose also called Applied dose
• Given dose referring to dose at D max
• TD will be lt given dose (dose at D max) in a
  single field because TD will be at a depth gt D max

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PDD Problem

• Example problem
• PA Spine, where the Dr prescribes to sc
• A patient is treated on the Cobalt 60 at 80 cm
  SSD. Prescription is written to give 30 Gy in 3
  Gy fxs to a depth of 5 cm with a 10 X 10 f.s.
• What is the given or applied dose (dose at D
  max)?
• Given dose 3 Gy X 100
• 78.3
• Given dose 3.83 Gy

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Another PDD Problem

• What is the TD?
• Rx is written to D max
• A patient is to be treated using a 6 MV linear
  accelerator, at 100 cm SSD. The F.S. is 15 X 15.
  
• 3 Gy per fx to be given to D max. What is the
  dose at a depth of 5 cm?
• TD Given dose X PDD (5, 15, 100)
• 100
• 3 Gy X 87.9
• 100
• 2.64 Gy

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Another example PDD

• Two points calculated
• Total dose to D max from both fields
• Cord dose
• APPA
• Exit dose must be considered for both points

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PDD- complex problem

• Patient treated on 6 MV linear accelerator at 100
  cm SSD ( the pt is moved between the fields) The
  f.s. is 15 X 15 , but blocked to an 8 X 8 blocked
  field size.
• 40 Gy delivered to depth of 10 cm in 2 Gy fxs
  using APPA field arrangement
• Each field will give 1 Gy
• Patient separation is 20 cm
• Cord is 3 cm deep
• What is the total dose at D max and cord dose?

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PDD- complex problem

• Draw a picture!!!!!!!!
• Need PDD entrance/exit info from both APPA for D
  max( 1.5 cm)
• AP 1.5 cm and 18.5 cm
• PA 1.5 cm and 18.5 cm
• Need PDD at depth of cord from both fields
• PA 3 cm, AP 17 cm
• Need PDD at depth of TD info, 10 cm (this was
  given in the directions)!

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PDD- complex problem

• PDD (1.5,8,100) 100
• PDD (3,8,100) 95.0
• PDD (10,8,100) 66.7
• PDD (17,8,100) 45.2
• PDD (18.5,8,100) 41.6

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PDD- complex problem

• Ant Field and Post field
• Contributes to dose at D max
• 1.5 cm
• 18.5 cm
• Need to know how much dose is there. Use this
  formula
• Dose at point A Dose at point B
• PDD at point A PDD at point B
• We know what dose is at 10 cm
• 1.0 Gy is prescribed to this depth

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PDD- complex problem

• Insert the numbers
• ? dose at 1.5 cm 1.0 Gy at 10cm
• 100PDD _at_ 1.5 cm 66.7PDD,10cm
• Dose at 1.5 cm 1.0 X 100
• 66.7
• 1.49 Gy

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PDD- complex problem

• What is the dose at 1.5 cm from exit dose?
• ?dose at 18.5 cm 1.0 Gy at 10
• 41.6PDD _at_18.5 cm 66.7PDD,10
• .62 Gy
• Total dose at D max from entranceexit dose
• 1.49 Gy .62 Gy
• 2.11 Gy

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PDD- complex problem

• Cord Dose
• Dose from both fields AP and PA
• From AP depth 17
• From PA depth 3 cm
• Dose at point A 17cm (AP)
• Point B 10 cm- any known dose/PDD
• ? Dose at point A Dose at point B
• PDD at point A PDD at point B
• X 1. 0 Gy
• 45.2 66.7 .68 Gy

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PDD- complex problem

• We have AP cord dose
• Now we need PA cord dose
• From PA depth 3 cm
• Dose at point A 3 cm
• Point B 10 cm - any known dose/PDD
• ? Dose at point A Dose at point B
• PDD at point A PDD at point B
• X 1. 0 Gy
• 95.0 66.7
• 1.42 Gy
• Total cord dose 1.42 .68 Gy 2.10 Gy

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Total doses

• Dmax
• 1.49 Gy .62 Gy
• 2.11 Gy
• Cord
• 1.42 .68 Gy
• 2.10 Gy
• Midplane dose
• 1 Gy 1 Gy
• 2 Gy