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Chapter 13 Treatment Planning III
Field shaping, Skin dose, and Field separation
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Why do we need field shaping?
To minimize the dose to normal tissues outside the target
Methods: Blocks, Independent Jaws, and MLCs
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13.1 Field Blocks - Block thickness
The number of HVLs (n) needed to achieve ‘t’ transmission is:
tn
2
1
For example, to achieve 5% transmission, the number of HVLs needed is:
32.42log/20log
202
20
105.0
2
1
nor
or n
n
For Megavoltage photon beams, typically 7.5 cm of cerrobend is needed to achieve transmission < 5%.
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A primary beam transmission of 5% through the block is considered acceptable.
Block thickness calculation (example: lead, = 11.36 g/cm3):
n: number of half-value layers needed
1/2n = 0.05
n = log 20/ log 2 = 4.32 (between 4.5 to 5 HVL)
Table 13.1: about 6.5 cm for 6MV (HVL ~ 1.4 cm)
Most commonly use: Cerrobend ( = 9.4 g/cm3)
Thickness ~ Thickness of lead (Table 13.1)*1.21
In MV range, ~ 7.5 cm in thickness commonly used.
13.1 Field Blocks - Block thickness
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13.1 Field Blocks - Block divergence
To minimize the penumbra at the block edge, the block is shaped or tapered following the geometric divergence of the beam.
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13.2 Field Shaping – block cutter
Sou
rce-
to-f
ilm d
ista
nce
Sou
rce-
to-b
lock
-tra
y di
stan
ce
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Used for matching fields, beam splitting (without changing the isocenter)
Benefit: no divergence
Caution: MU calculation should be based on points in the open portion of the field, taking into account the beam flatness at the point of reference.
13.2 Field Shaping – independent jaws
Machine central axis
Effective beam central axis
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13.2 Field Shaping – multileaf collimator (MLC)
Used to replace Cerrobend blocks,
(a) Avoids collision between the blocking tray & the couch
(b) Avoids block exceeding the weight limitation of the tray
(c) Delivers intensity modulated beam
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13.2 Field Shaping – multileaf collimator (MLC)
Benefits:
(a) Easy control and modification
(b) Reduce mistakes (record & verify)
(c) Eliminates the need for mold room activity
(d) Avoids the need for block storage
(e) Avoids entry into the room, lifting difficulties
(f) Time saving for a treatment with large number of fields
Limitations:
(a) Limitation of leaf span (Varian: ~14.5 cm)
(b) Mantle field; kidney blocks (island)
(c) Scalloping effect on field edge
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13.2 Field Shaping – multileaf collimator (MLC)
MLC block
Scalloping effect
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13.3 Skin Dose
depth in water
Per
cent
dep
th d
ose
15 MV
6 MVorthovoltage
Skin sparing is an important feature of the Megavoltage beam.
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13.3 Skin Dose – electron contamination
Skin dose is the result of e- contamination of the incident beam + back scattered radiation from the medium.
Contaminated electrons arise from photon interactions in the air column between the machine head and patient surface; in the machine head (flattening filter, collimators); and any objects in the beam (wedge, blocking tray).
Megavoltage beams produce an initial electronic buildup with depth → reduced dose at the surface
Higher energy → the effect of skin sparing more pronounced
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The size of the dosimeter along the beam direction should be as small as possible.
Ion Chambers:
Extrapolation chambers (electrode spacing ~ μm )
Parallel-Plate Chambers with adequate guard ring
TLDs:
Thin layers (<0.5 mm) of TLDs (thermoluminescent dosimeter)
13.3 Skin Dose – Measurement of Dose Distribution in Build-Up region
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13.3 Skin Dose – skin sparing as a function of photon energy
Depth (mm) Co-60 4 MV 10 MV 25 MV
SSD=80 cm 80 cm 100 cm 100 cm
0 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 90.0 63.0 54.5
5 100.0 94.0 72.0 60.5
6 - 96.5 76.0 66.0
8 - 99.5 84.0 73.0
10 - 100.0 91.0 79.0
15 - - 97.0 88.5
20 - - 98.0 95.0
25 - - 100.0 99.0
30 - - - 100.0
Table 13.2 build-up dose distribution in polystyrene for a 10x10 cm field
?
Higher photon energy, more skin sparing
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13.3 Skin Dose – Effect of absorber-skin distance
As the absorber (e.g. blocking tray) moves away from the skin, the electrons generated in the absorber are more likely to scatter laterally out of the beam, thus reducing the dose to the skin, increasing skin sparing.
dFig 13.6 10 MV photons,
15x15 cm field size
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13.3 Skin Dose – Effect of field size
Increased field size Increased contaminated electrons
decreased skin sparing
Small field size
large field size
Depth dose curve
Fig. 13.7
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13.3 Skin Dose – electron filter (?)
With metal filter
Shadow tray onlyDepth dose curve
Electron filter
Open beam
Tray only
Metal filter
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13.3 Skin Dose – electron sparing at oblique incidence
Electron energy deposition electron track length.
In a given slab, increased obliquity
increased electron tracklength
Increased surface dose, decreased dmax
decreased skin sparing
Normal incidence
oblique incidence
Incident electron
dmax dmax
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Skin dose increases when :
(1) Field size increases (increased electron emission from the collimator and air) (Fig. 13.6, Khan)
(2) SSD decreases, especially in larger field sizes
(3) An acrylic tray placed in the beam (beam spoiler to eliminate skin sparing)
(4) Blocked field vs. Open field: skin dose increases
(5) Blocked field vs. MLC field: skin dose increases
(6) Angle of incidence increases
13.3 Skin Dose – summary
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13.4 separation of Adjacent Fields
Adjacent ‘angled’ fields to cover a large treatment area
d
Adjacent fields (with a skin separation) to deliver uniform dose at depth d
sepa
ratio
n
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13.4 separation of Adjacent Fields
Orthogonal split beams to avoid beam divergence at the match line.
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13.4 separation of Adjacent Fields
‘Penumbra spoiler’ to blur the penumbra to prevent spinal cord injury that may arise from setup uncertainty
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13.4 separation of Adjacent Fields – methods of field separation
d
Hot region
cold region
Uniform dose
SS
D1
SS
D2
L1 L2
S1 S2
1
11 1
2 SSD
L
d
S
2
22 1
2 SSD
L
d
S
Field separation on the skin: 2
2
1
121 22 SSD
dL
SSD
dLSSS
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dS
SD
1
SS
D2
Ideal match
Uniform dose, no hot/cold regions
SS
L1
L2
2
2
1
1
SSD
L
SSD
L
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d
If the gap S1+S2 is increased by S, there will be a cold spot at the midline d
midlined’
cord
S’
To avoid hot spot at the spinal cord, one can increase the gap S1+S2 by S’.
d
dd
S
S
''
More conveniently, use the same L and SSD for all 4 fields, (ideal match) but truncate the 2nd pair with independent jaws
S2S1
S=S1-S2
Three-field overlap
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Example: A patient is treated with parallel-opposed mantle and paraortic fields of lengths 30 and 15 cm, respectively. Calculate (a) the gap required on the surface for the beams to intersect at a midline depth of 10 cm and (b) the gap required to just eliminate the three-field overlap on the cord assumed to be at a depth 15 cm from the anterior surface, given SSD = 100 cm for all fields.
(a) Think of two beams incident from the same side:
S1 = L1/2 * d/SSD1 = 30/2 * 10/100 = 1.5 cm
S2 = L2/2 * d/SSD2 = 15/2 * 10/100 = 0.75 cm
Total gap = S1 + S2 = 1.5 +0.75 = 2.3 cm
(b) Think of three-field overlap:
ΔS = S1 - S2 = 0.75 cm
ΔS’ = ΔS * (d’-d)/ d = 0.75 * (15-10) /10 = 0.4 cm
New gap required = S1 + S2 + ΔS’ = 2.3 + 0.4 = 2.7 cm
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S1+S2
Uniform dose Three-field overlap
S1+S2+S
Cold spot
S1+S2+S’
Relatively uniform dose
Spinal cord sparing
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13.4 separation of Adjacent Fields – craniospinal fields
SSD
dLS
2
d
s
L
Technique A:
Divergent fields
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13.4 separation of Adjacent Fields – craniospinal fields
Technique B:
θcoll
SS
DL1
SSD
Lcoll
2tan 11
θcouch
SADL2
SAD
Lcouch
2tan 21
More convenient to use independent jaw to eliminate beam divergence:
1. No couch rotation.
2. Moving junction to smear out hot/cold spot at junction
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13.4 separation of Adjacent Fields – guidelines for field matching
1. The site of matching should be over an area that does not contain tumor or critical organs.
2. If the tumor is superficial at the junction, field separation is not needed provided the hot region below does not exceed the tolerance of normal tissues or critical organs.
3. Beam splitter or beam tilting can be used to eliminate beam divergence.
4. For deep-seated tumors, the fields are separated at the skin so that the junction point lies at the appropriate depth.
5. Move the junction few times during the course of treatment is desirable to smear out the dose distribution at junction.
6. A field-matching technique must be verified by dose distribution. Light field and dose in penumbra region must be accurate.