1. CHAPTER 3 DOSE DETERMINATION FOR EXTERNAL BEAMS
TREATMENT PLANNING
CHAPTER 3
DOSE DETERMINATION FOR EXTERNAL BEAMS

2. Key Terms Maximum Electronic Buildup Dose Depth Half-Value Thickness
Dose Rates
Percentage Depth Dose
Tissue-Air Ratio
Tissue-Phantom Ratio
Tissue-Maximum Ration
Isodose Chart
Dose Profile
Dose Rate
Dose
Depth
Separation
SSD
SAD
Isocenter
Scatter-Air Ratio
Scatter-Maximum Ratio
Backscatter Factor
Collimator Scatter Factor

3. WHAT IS DOSE? Refers to the energy deposited at that point
Measured at a specific point in a medium
Measured in Gray (Gy)
1 Gy = 100 rads
1 Gy = 1 J/kg
1 rad = 100 erg/g
*ergs are a very small amount of energy, considerably less than one-millionth of calorie

4. Methods of Dose Determination
IONIZATION CHAMBERS
THERMOLUMINESCENT DOSIMETERS
PHOTOGRAPHIC FILM

5. Factors Affecting Dose Calculations
Essential step in dosimetry
Establish measured data tables for each treatment machine
Tables prepared as a result of measurements in phantoms (tissue equivalent material)
Constancy of the machine must be routinely checked

6. Depth
Distance beneath the skin surface where the prescribed dose is to be delivered
Specified depth
Midplane
Isocenter

7. Dmax Depth of maximum dose Depth of maximum equilibrium of electrons
Buildup region: region before Dmax
Skin-sparing effect
High-energy photon beams strike a medium, and secondary electrons are formed. Electrons penetrate the medium to a depth at which electron equilibrium is reached.
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

8. Approximate Depths of Dmax
Dmax Table
Approximate Depths of Dmax
Beam Energy
Depth of Dmax (cm)
≤ 200 kV
0.0
1.25 MV
0.5
4 MV
1.0
6 MV
1.5
10 MV
2.5
18 MV
3.5
24 MV
4.0

9. Output
The first major step in determining radiation dose is machine output
Output tells us dose delivered at a specified point in a medium, at a specified distance from the target, and in a specified medium
The second step is absorbed dose in a medium
It is necessary to know the composition of the irradiated material, geometric relationship of the material and radiation beam, and field size

10. Dose Rate
Also referred to as machine output
Measured in the absence of a scattering phantom and in tissue equivalent material
Amount of radiation exposure produced by a treatment machine or source as specified at a reference field size at a specified reference distance
Measured with equipment calibrated according to a national standard
1 MU/cGy in a 10 x 10 at Dmax (standard dose rate for LINACS when not specified in a chart 100cm from the target)

11. Dose Rates are affected by:
Field Size- Increases with increased field size
This occurs due to increased scatter Sc
Collimator scatter factor Sp
Phantom or patient scatter factor
Sc,p (Sc * Sp)
Combined collimator and phantom scatter
Distance- Decreases with increased SSD
Change in beam intensity caused by the divergence of the beam
Describes change in beam intensity caused by divergence
Can be used to find the change in dose rate

13. ISF = (SSD1 + Depth of Dmax/SSD2+ Depth of Dmax)2
Inverse Square Factor
Used to express ratio of change in output vs. distance
1 MU/cGy in a 10 x 10 at Dmax
ISF = (SSD1 + Depth of Dmax/SSD2+ Depth of Dmax)2

14. Tissue Absorption Factors
Nonisocentric or SSD Technique
Percentage depth dose (PDD)
Isocentric or SAD Technique
Tissue-air ratio (TAR)
Tissue-maximum ratio (TMR)
Tissue-phantom ratio (TPR)

15. Source-Skin Distance (SSD) vs. Source-Axis Distance (SAD)
Distance from source or target of machine to surface of patient
Measure with ODI, front pointer
The dose is measured at a specific depth
Distance from source of photons to the isocenter
The dose is normalized to isocenter
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

16. SSD/SAD Approach
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

17. Percentage Depth Dose
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18. Percentage Depth Dose  PDD  Energy  Field size  SSD  PDD  Depth
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

19. Mayneord’s F Factor
Used to convert the PPD at the reference distance to the PPD at a nonreference distance. This would occur at extended distance setups.
where
d is depth

20. Tissue-Air Ratio
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

21. Tissue-Air Ratio
 TAR is used to perform calculations for SAD treatments involving low-energy treatment units (e.g., cobalt-60, 4-MV)
 TAR
 Energy
 Field size
 TAR
 Depth 
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

22. Tissue-Phantom Ratio
Absorbed dose at a given depth in phantom to absorbed dose at the same point at a reference depth in a phantom
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

23. Tissue-Maximum Ratio/Tissue-Phantom Ratio
 TMR/TPR
 Energy
 Field size
 TMR/TPR
 Depth 

24. Tissue-Phantom Ratio
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

25. Scatter-Air Ratio
Primary radiation interacting with matter may result in scatter
Photons or electrons
Change in direction
Ratio of the scattered dose at a given point to the dose in free space

26. Backscatter Factor
Ratio of dose rate with scattering medium to dose rate in air at Dmax
Backscatter: TAR at Dmax
Dependent on energy and field size, not on SSD
BSF is high for low energies
BSF is lower in high energies (8MV and higher)
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

27. Backscatter Factor
Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.