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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.