1. The physics of Radiation Therapy, pp. 315 - 345
Ch13 Treatment Planning III: Field Shaping, Skin Dose, and Field Separation

2. 13.1 Field Blocks A. Block Thickness B. Block Divergence
13.2 Field Shaping
A. Custom Blocks
B. Independent Jaws
C. Multileaf Collimators
13.3 Skin Dose
A. Electron Contamination of Photon Beams
B. Measurement of Dose Distribution in the Build-up region
C. Skin Sparing as a Function of Photon Energy
D. Effect of Absorber – Skin Distance
E. Effect of Field Size
F. Electron Filters
G. Skin Sparing at Oblique Incidence
13.4 Separation of Adjacent Fields
A. Methods of Fields Separation
B. Orthogonal Field Junctions
C. Guildlines for Field Matching

3. A. block Thickness Material of block Required thickness of block
lead (commonly)
Required thickness of block
Beam quality
The allowed transmission (5% transmission is acceptable)

4. A. block Thickness
Calculation of the of the number (n) of HVL to achieve 5% transmission
Recommended of block thickness = 4.5 – 5 HVL
Further increase of block thickness is not significant
The predominance of scattered radiation from the adjoining open areas of the field

5. Recommended Minimum Thickness of Lead for Shielding Beam Quality
Required Lead Thickness
1.0 mm Al HVL
0.2 mm
2.0 mm Al HVL
0.3 mm
3.0 mm Al HVL
0.4 mm
1.0 mm Cu HVL
1.0 mm
2.0 mm Cu HVL
2.0 mm
3.0 mm Cu HVL
2.5 mm
137 Cs
3.0 cm
60Co
5.0 cm
4 MV
6.0 cm
6 MV
6.5 cm
10 MV
7.0 cm
25 MV
* Approximate values to give ≦5% primary transmission

6. B. Block Divergence
The blocks is shaped with the geometric divergence of the beam
Minimized the block transmission penumbra
Little advantage for beams with large geometric penumbra (e.g. 60Co)
Most suited from beams having small focal spots

7. C. Multileaf Collimators
13.1 Field Blocks
A. Block Thickness
B. Block Divergence
13.2 Field Shaping
A. Custom Blocks
B. Independent Jaws
C. Multileaf Collimators
13.3 Skin Dose
A. Electron Contamination of Photon Beams
B. Measurement of Dose Distribution in the Build-up region
C. Skin Sparing as a Function of Photon Energy
D. Effect of Absorber – Skin Distance
E. Effect of Field Size
F. Electron Filters
G. Skin Sparing at Oblique Incidence
13.4 Separation of Adjacent Fields
A. Methods of Fields Separation
B. Orthogonal Field Junctions
C. Guildlines for Field Matching

8. A. Custom Blocking Lipowitz metal (Cerrobend®) Main advantage
A low melting point alloy
Density: 9.4g / cm3 at 20℃ (83% of lead)
Consisted of :  bismuth (50%),  lead (26.7%),  tin (13.3%),  cadmium (10%)
Main advantage
Melting point: 70℃ (c.f. 327℃of lead)
Harder than lead in room temperature
Common thickness of block (Cerrobend®) using in MV range of photon
7.5 cm
Density ratio relative to lead (～lead thickness × 1.21)
Equivalent to about 6 cm of pure lead

9. A. Custom Blocking Procedure for constructing Cerrobend block
Outline of the treatment on simulator radiography (port film)
Construct divergent cavities in a Styrofoam block by cutting device
Cutted stylofoam was placed on Lucite plate and carefully aligned relative to the central axis
Pouring Cerrobend into the cavity
Mounting the block on the Lucite plate

14. B. Independent Jaws
Blocking off a part of the field without changing the position of the isocenter
In the past: using beam splitters
In present, simply moving the collimators (or jaws) independently
Application: matching field (blocked off at the central axis to remove divergence)
Effects of the asymmetric collimation
Change in physical penumbra
The tilt of the isodose curves toward the blocked edge
Eliminating photon and electron scatter from the blocked portion of the field
Reducing the dose near the edge

15. Fig.13.3 Comparison of isodose distribution with half the beam blocked by an independent jaw versus a block on a tray
Notice close agreement as well as the tile of the isodose curves toward the blocked edge

16. C. Multileaf Collimators
Large number of collimating blocks (or leaves) that can be driven automatically and independently, to generate a field of any shape
Thickness of the leaf (along the beam direction): ≦1% transmission
Width of the leaf: 1 cm as projected at the isocenter (usually)

17. C. Multileaf Collimators
Double focused MLC
Form a cone of irregular cross-section diverging from the source position
Move on a spherical shell centered at the source
Provide a sharp beam cutoff at the edge
Difficult to manufacture
In high-energy beams, the dose falloff at the edge is largely determined by scattered photons and electrons
Rounded leaf edges MLC
Directions of travel perpendicular to the central ray
Provide constant transmission through a leaf edge, regardless of its position in the field

18. Conformity between the planned field boundary and MLC boundary
Continuous vs. jagged
Depending on:  projected leaf width,  shape of the target volume,  angle of the rotation of the collimator
Optimization of the MLC rotation
Direction of motion of the leaves is parallel with the direction in which the target volume has the smallest cross-section
Potential applications of the MLC systems
Replacement of custom Cerrobend blocking
Automatic beam shaping for multiple fields
Dynamic conformal RT (beam shaping during rotation of the gantry)
Modifying dose distributions within the field (computer-controlled dwell time of the individual leaves)

19. A. Electron Contamination of Photon Beams
13.1 Field Blocks
A. Block Thickness
B. Block Divergence
13.2 Field Shaping
A. Custom Blocks
B. Independent Jaws
C. Multileaf Collimators
13.3 Skin Dose
A. Electron Contamination of Photon Beams
B. Measurement of Dose Distribution in the
Build-up region
C. Skin Sparing as a Function of Photon Energy
D. Effect of Absorber – Skin Distance
E. Effect of Field Size
F. Electron Filters
G. Skin Sparing at Oblique Incidence
13.4 Separation of Adjacent Fields
A. Methods of Fields Separation
B. Orthogonal Field Junctions
C. Guildlines for Field Matching

20. 13.3 Skin Dose
Skin sparing is one of the most desirable features of high-energy photon beams
This effect may be reduced or even lost if the beam is excessively contaminated with secondary electrons

21. A. Electron Contamination of Photons Beams
Contribution to the skin dose
Electron contamination
Backscattered radiation from the medium
Source of the contaminating electrons
Photons interactions  in the air,  in the collimator,  other scattering material in the path of the beam (e.g. shadow tray, thick enough to absorb most of the electrons)

22. A. Electron Contamination of Photons Beams
Controversy as to the relative contribution of secondary electrons vs. low energy scattered photons to the dose of the build-up region
Field size , PDD in the build-up region  (dmax shift to a shallower depths)
Current evidence favors that the secondary electrons is the predominant cause of the effect

23. B. Measurement of Dose Distribution in the build-up region
The size of the dosimeter should be as small as possible
Steep dose gradient
Type of the dosimeter used
Extrapolation chamber (Fig.6.15, p.113)
Fixed-separation plane-parallel ionization chambers (most common used)
Variable electrode spacing (section 6.7B)
Thin layers (<0.5cm) of thermoluminescent dosimeter (TLD)

24. C. Skin Sparing as the Function of Photon Energy
Factors affecting the build-up region (shown by the studies)
 beam energy,  SSD,  field size,  configuration of secondary blocking tray
The effect becomes more pronounced as photon energy increases

25. Build-Up Dose Distribution in Polystyrene for a10 × 10 cm Field
Depth
(mm)
60Co
80 cm
4 MV
10 MV
100 cm
25 MV
18.0
14.0
12.0
17.0 1
70.5
57.0
30.0
28.0 2
90.0
74.0
46.0
39.5 3
98.0
84.0
55.0
47.0 4
100.0
63.0
54.5 5
94.0
72.0
60.5 6
96.5
76.0
66.0 8
99.5
73.0 10
91.0
79.0 15
97.0
88.5 20
95.0 25
99.0 30
* Data from Velkely DE, Manson DS, Purdy JA, Oliver GD. Buildup region of megavoltage photon radiation sources. Med Phys 1975; 2: 14.
Phenomenon:
The dose increase rapidly within the first few mm, and gradually achieves its max value at the depth of peak dose

26. C. Skin Sparing as the Function of Photon Energy
A practical application
Build-up bolus to maximized the dose on the skin
e.g. covering a scar with a strip of bolus
Thickness of the bolus required < the depth of maximum dose

27. D. Effect of Absorber – Skin Distance
Absorber is introduced in the beam to eliminate the “collimator electrons” (main source of electron contamination)
e.g. Lucite shadow tray
Thickness > the range of secondary electrons
Absorber becomes the principal source of electron contamination
Solution: increase the distance between the tray and the surface
Reasons:  Divergence,  absorption and scattering of electrons in the air
Preserve the skin sparing
e.g. 60Co γ rays, 15 – 20 cm air gap, skin dose <50% of the Dmax

28. Figure 13.5 Effect of Lucite shadow tray on dose buildup for 10 MV x-rays
% Depth Dose
Beam: 10 MV
Tray thickness: 1.5 g/cm2
Field size: 15 x 15 cm
SSD: 100 cm
Phenomenon
The relative surface dose 
The Dmax move closer to the surface

29. D. Effect of Absorber – Skin Distance
Beam spoiler
A low atomic number absorber (e.g. Lucite shadow tray), placed at an appropriate distance from the surface, can be used to modify the build up curve
e.g. H&N cancer using 10 MV x-rays, using a beam spoiler to increase the dose to the surface neck nodes

30. E. Effect of Field Size
The relative skin dose depends strongly on field size
Field size, the dose in build up region
Reasons: increase electron emission from the collimator and air
Fig.13.6 Percent surface dose as a function of field size

31. Radius of an equvalent circular field
E. Effect of Field Size
Relationship of the field size and the tray-to-skin distance (studied by Salylor and Quillin)
The ratio of optimum skin sparing is
For large treatment field, tray-to-skin distance is not possible for isocentric treatment
Maintaining the skin-sparing effect using electron filter
Tray to skin distance
Radius of an equvalent circular field
Field size
Optimum tray-to-skin distance
5 × 5 cm
12 cm
30 × 30 cm
67 cm

32. F. Electron Filter
Using absorber of medium atomic number can reduce the skin dose
e.g. Z in the range of 30 – 80
Reduces the secondary electron scatter in the forward direction (showed by Hine)
Proved to reduce the surface dose, and improve the build-up region of large field in 60Co teletherapy (studied by Khan)
Thickness of electron filter
Equal to the maximum range of secondary electron
e.g. 60Co, 0.5 g/cm2 or 0.9 mm of tin

33. Fig.13.7 Variation of percent surface dose with atomic number of absorber
Field size: 15 × 15 cm
Phenomenon
The minimum occurs at about Z = 50 (Tin)

34. Fig.13.8 demonstration of the effectiveness of tin in reducing skin dose

35. G. Skin Sparing at Oblique Incidence
Surface
Phantom
Phenomenon: Angle of incidence , skin dose 
The concept of electron range surface (suggested by Jackson)
The ERS is a 3D representation of secondary electron range and distribution, produced by a pencil beam of photons interacting with the medium
Electron generated insides the ERS volume will reach P and contribute dose to there
ERS for 60Co γ rays is an ellipsoid with axial dimensions of 5 × 2.4 mm P

36. G. Skin Sparing at Oblique Incidence
For tangential beam incidence
1/2 of ERS is below the phantom surface
upper estimate dose to the skin P

37. G. Skin Sparing at Oblique Incidence
Oblique factor (defined by Gerbi et al.)
The dose at a point in phantom on central axis of a beam incident at angle θ°, with respect to the perpendicular to the surface
Representing dose enhancement due to beam obliquity for the same depth along central axis

38. Fig.13.10 Oblique factor at the surface plotted as a function of beam angle for various energy beams
Phenomenon
The angle of incidence ,
Obliquity factor 
First gradually and then
dramatically beyond 45°
45°

39. 13.4 Separation of Adjacent Fields A. Methods of Fields Separation
13.1 Field Blocks
A. Block Thickness
B. Block Divergence
13.2 Field Shaping
A. Custom Blocks
B. Independent Jaws
C. Multileaf Collimators
13.3 Skin Dose
A. Electron Contamination of Photon Beams
B. Measurement of Dose Distribution in the Build-up region
C. Skin Sparing as a Function of Photon Energy
D. Effect of Absorber – Skin Distance
E. Effect of Field Size
F. Electron Filters
G. Skin Sparing at Oblique Incidence
13.4 Separation of Adjacent Fields
A. Methods of Fields Separation
B. Orthogonal Field Junctions
C. Guildlines for Field Matching

40. 13.4 Separation of Adjacent Fields
Examples of adjacent fields
“Mantle” or inverted-Y fields for Hodgkin’s disease
Craniospinal fields for medulloblastoma (orthogonal)
Placing lateral neck fields adjacent to the ant. supraclavicular field

41. A number of techniques have been devised to achieve dose uniformity in the field junction
A. Angling the beams
aways from each other
B. Field separation
C. Isocetric split beam
technique
D. Penumbra generator
Fig Schematic representation of various techniques used for field matching

42. Geometry of two adjacent beams
The dose above the junction will be lower
The dose below the junction will be higher
The total separation S on the surface is given by:

43. An ideal geometry
No overlap between a field and its adjacent opposing neighbor

44. Creates regions of “Three-field overlap”
Bigger fields divergence into the opposing smaller fields
The maximum length of three-field overlap (ΔS)
ΔS = S1 – S2
Field #1 30 × 30 cm
Field #1 15 × 15 cm
Fig.13.14a
Field separate at surface = 2.3 cm

45. Eliminated “Three-field overlap” by adjusting SSD
ΔS = 0, if
A cold spot
occurs at the midline
Field #1 30 × 30 cm
Field #1 15 × 15 cm
Fig.13.14b
Field separate at surface = 3 cm

46. Eliminating the “Three-field overlap” in a specific region
e.g. spinal cord
Depth of the cord
Midline depth
Ant. surface
Field #1 30 × 30 cm
Field #1 15 × 15 cm
15 cm
Depth of spinal cord
Fig.13.14c: Field separate at surface = 2.7 cm
elimination of the 3-field overlap at the cord

47. B. Orthogonal Field Junction
The central axes of the adjacent fields perpendicular to each other
e.g.1. The craniospinal irradiation of medullobastoma
Lateral parallel opposed brain field + posterior spine field
e.g.2. Head & Neck
Lateral parallel opposed head field + anterior supraclavicular field

48. Methods for not possible separation of the field
Beam splitters (abut the field along or close to their central axes)
Drawing match line each time before treatment
Additional block for sensitive structure (e.g. spinal cord)
A geometrical method of orthogonal field separation (Werner et al.)
d  depth at which the orthogonal
fields are allowed to join

49. B.1. Craniospinal Fields Techinque A
Bilateral cranial fields adjacent to a spinal field
The inferior border of cranial field meet at a point midway on the posterior neck surface

50. Technique B (1) Rotation the couch and collimator
Calculation of the two angles θcoll and θcouch

51. Technique B (2)
Rotation of the collimator only with using hemiblock of the cranial fields
Calculation of the collimator angles θcoll only

52. C. Guidelines for Field Matching
The sites of field matching should be chosen over an area that dose not contain tumor or a critically sensitive organs
If the tumor is superficial at the junction site, the fields should not be separated since a cold spot on the tumor will risk recurrence
For deep-seated tumors, the fields may be separated on the skin surface so that the junction point lies at the midline
The line of field matching must be drawn at each treatment session on the basis of the first field treated
A field-matching technique must be verified by actual isodose distribution before it is adopted for general clinical use