15 chap 11 treatment planning i

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Chapter 11

Treatment Planning I: Isodose Distributions

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11.1 - Isodose Chart

normalized at dmax, used for SSD setup

normalized at a fixed depth, used for SAD setup

Example: Co-60 beam

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11.1 - Isodose Chart Example: a linac beam

“horn” (usually at shallow depth)

The flattening filter is designed to produce a ‘flat’ dose distribution at a selected depth, typically, at 10 cm. As a result, the dose distributions at shallow depths are over-compensated, exhibiting the ‘horn’ effect.

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‘horn’ diminishes with depth

‘horn’ diminishes with depth, because (1) greater scatter on the central axis with depth, and (2) more penetrating, i.e., harder beam, on the central axis due to the larger thickness of the flattening filter in the center than off-center

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Beam profile

Field width defined at 50% dose level

90-20% penumbra width

For Co-60, the penumbra is due to the ‘geometric penumbra’ (1-2cm) and the scatter in the medium. For linac, the ‘geometric penumbra’ is small (~2mm), the penumbra is mostly due to the scatter in the medium.

Scatter in the medium and

head/collimator leakage

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Isodose curves on a plane perpendicular to the beam central axis. (The field edge is typically 5mm outside the 90% lines. In the penumbra region, the dose gradient is approx. 8-10% / mm.)

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11.2 – Measurement of Isodose Curves

Dose measurement in a water phantom:

A. Moving probe (central axis depth dose, beam profile)

B. Fixed reference probe (position fixed, located in field)

C. Relative dose = A/B

A

B

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11.3 – Parameters of Isodose Curves

200 kVp, bulging penumbra due to greater scatter of low ene

rgy photons.

Co-60, penumbra primarily due to source size (geometric penumbra)

4 MV, small penumbra, due to

small photon scatter and short electron

range

10 MV, penumbra greater than that of 4 MV, due to greater electron range

(Beam quality)

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11.3 – Parameters of Isodose Curves

(source size, source-to-surface distance,source-to-diaphragm distance- the penumbra effect)

geometric penumbra:

SDD

SDDdSSDsPd

)(

To reduce the geometric penumbra, trimmers can be used to increase SDD.

d

s

Pd

SDD

SSD

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11.3 – Parameters of Isodose Curves(field size)

Field size is determined based on dosimetric coverage, not geometric coverage.

For small field (< 6-cm), a large portion of the field is in the penumbra region, hence beam profile tends to be bell-shaped.

For Co-60 beams, no flattening filter is used. Consequently, for large field or elongated field, the beam profile is higher on central axis (due to larger scattered dose), and lower off-axis (due to reduced scattered dose and oblique incidence).

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11.4 – Wedge Filters (wedge angle)

Normalized to dmax with wedgeNormalized to dmax without wedge

(open field)

Physical wedge: use metal, such as steel and Pb.Dynamic wedge: independently move one collimator while the beam is on.

wedge angle

wedge angled

= 1

0 cm

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Measured at some depth beyond the dmax, usually 10 cm.

Old Co-60 isodose curves were normalized to the dmax without the wedge. the isodose curves already included the wedge factor.

With advent of TPS, we normally input the isodose curves normalized to the dmax with the wedge, and introduce a wedge factor to account for the transmission factor of the wedge.

Wedge factor depends on the depth and field size.

11.4 – Wedge Filters (wedge factor)

DwDo

o

w

D

DHWdWF ),(

:factor wedge

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11.4 – Wedge Filters (wedge systems)

WF

Center of wedge fixed at beam axis. Wedge factor varies weakly with field size.

WF

Thin edge of wedge aligns with the field edge. Wedge factor varies strongly with field size, (greater wedge factor with smaller field size), resulting in more efficient use of beam-on-time

(universal wedge) (individualized wedge)

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11.4 – Wedge Filters (effect on beam quality)

wedge

open

depth

dose

For Co-60, photons are nearly mono-energetic, wedge filter does not have much effect on beam quality.

For linac-produced photon beams, the wedge filter preferentially attenuates low-energy photons. As a result, the wedged-beam is ‘harder’ than open field beam, that is, more penetrating.

Thus, the wedge factor, which is the ratio of wedge-field-dose to open-field-dose, increases with the depth.

Backscatter factors are assumed to be not affected by the wedge filter.

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11.4 – Wedge Filters (design of wedge filters)

A B C E G I K M O Q S T UNonwedge isodos

e 40 55 62 65 67 68 68 68 67 65 62 55 40

wedge isodose 35 39 41 47 53 60 68 76 86 95 105 110 115

ratio .875 .710 .660 .720 .790 .880 1.00 1.12 1.28 1.46 1.70 1.20 2.88

transmission - - .387 .425 .462 .515 .590 .660 .750 .860 1.00 - -

mm Pb - - 15.2 13.6 12.2 10.5 8.3 6.5 4.5 2.3 0.0 - -

penumbra region

1. Draw a horizontal line at the reference depth (e.g. 10 cm)

2. Construct fanlines (e.g. 1-cm intervals)

3. Construct parallel wedged isodose lines, use same values as open field

4. At each intersection point, find dose values for non-wedge & wedge fields

5. Find ratio of wedge/open dose values6. Normalize max ratio to 1007. Get thickness by attenuation coeff.

1

2

3

4

5

6

7

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11.5 – Combination of Radiation Fields

Single field treatment can be used, if:

1. Dose distribution inside tumor reasonably uniform (e.g. ±5%) – tumor volume cannot be too big

2. Maximum dose to normal tissue cannot be too large (e.g.110%) – tumor must be relatively shallow

3. Dose to normal tissue within tolerance – no critical organs in the beam

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11.5 – Combination of Radiation Fields (parallel opposed beams)

SSD setupE

ach

beam

del

iver

s 10

0 to

dm

axSAD setup

Eac

h be

am d

eliv

ers

100

to is

ocen

ter

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11.5 – Combination of Radiation Fields (patient thickness vs dose uniformity)

The larger the patient thickness, the greater the energy is needed to produce more uniform dose in the tumor and less dose in normal tissues.

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11.5 – Combination of Radiation Fields

When multiple fields are used, each field should be treated each day! This will minimize damage to the normal tissues outside the tumor region.

(this result is based on cell-survival calculation. For details, see Johns & Cunningham, chapter 17)

Edge effect, lateral tissue damage ?

Integral dose ( = mass × dose )

distance surface-source SSD

dose) 50% of(depth depth value-half d

icknesspatient th darea, field A

axis central along dosepeak Ddose, integral

88.21144.1

21

0

21/693.0210

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SSD

deAdD

dd

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11.5 – Combination of Radiation Fields (multiple fields)

Increase the ratio of tumor dose to normal tissue dose

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11.6 – Isocentric Techniques (stationary beams)

SAD setup

SSD setup

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11.6 – Isocentric Techniques (rotation therapy)

4 MVField Size = 7x12 cm

100° arc 180° arc

360° full rotation

TMRSSDD

TDD

pciso

refiso

0

Past pointing

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11.6 – Isocentric Techniques (example)

Rotation therapy, 4 MV, field size 6 × 10-cm, SAD = 100 cm.

Given: TMR = 0.746, Sc(6x10)=0.98, Sp(6x10)=0.99, dose rate = 200 MU/min

To deliver a prescription dose of 250 cGy to the isocenter, what is the number of MUs ?

MU 345 min 1.73 MU/min 200 MU

min73.1cGy/min144.8

cGy250 timetreatment

min/8.144746.099.098.0200

0

cGyD

TMRSSDD

iso

pciso

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11.7 – Wedge Field Techniques

9080

70

60

50

180

160

140

120

Orthogonal pair – open field Orthogonal pair – wedged field

80

90

70

60

Uniform dose

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11.7 – Wedge Field Techniques (hinge angle and wedge angle)

φ

In clinical reality, this relationship isn’t true in general, due to surface curvature.

Computer treatment plan is needed to find the optimal wedge angle

angle hinge

290

angle wedge

θ

80

90

70

60

80

90

70

60

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11.7 – Wedge Field Techniques (open and wedge fields combinations)

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11.8 – Tumor Dose Specification for External Photon Beams (ICRU Report No.50 and No.62)

Irradiated volume: The volume that receives a significant dose (e.g., 50% of the target dose)≧

Treated volume: The volume enclosed by an isodose surface which adequately covers the PTV.

Planning tumor volume (PTV) CTV + margin (internal+setup);includes CTV with an IM and as a set-up margin (SM) for patient movement and set-up uncertainties. To delineate PTV, IM and SM are not added linearly but are combined subjectively.

Internal tumor volume (ITV) ICRU 62 recommends that an internal margin (IM) be added to CTV to account for internal physiological movements and variation in size, shape, and position of the CTV during therapy.

Clinical tumor volume (CTV) GTV + subclinical (CTV-I)multiple CTV’s possible (CTV-II, CTV-III,…)Additional volumes with presumed subclinical spread (e.g. Regional lymph nodes)

Gross tumor volume (GTV) palpable or imaged; gross demonstrable extent andlocation of the malignant growth; consists of primary tumor (GTV Primary) and possibly metastatic lymphadenopathy (GTV nodal) or other metastases(GTV M).

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Planning Organ at Risk Volume (PRV)The organ at risk (OAR) needs adequate protection just as CTV needs adequate treatments.Once the OR is identified, margins need to be added to account for its movements, internal as well as set-up.

OARPRV

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maximum target dose

minimum target dose

target

1 dosemean i

iDn

most frequently occurred dose

half of the target points with dose < median dose

< half of the target points with dose

Hot spot: an area outside the target that receives a higher dose than the prescribed target dose; clinically meaningful only if the area > 2 cm2

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11.8 – Tumor Dose Specification for External Photon Beams (specification of target dose)

The ICRU Reference Point

• The point should be selected so that the dose at this point is clinically relevant and representative of the dose throughout the PTV• The point should be easy to define in a clear and unambiguous way• The point should be selected where the dose can be accurately calculated• The point should not lie in the penumbra region or where there is a steep dose gradient

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Stationary Photon Beams

• For single beam, the TD should be specified in the central axis of the beam placed within the PTV• For parallel opposed, equally weighted beams, the point of TD specification should be on the central axis midway between the beam entrances• For parallel opposed, unequally weighted beams, the TD should be specified in the central axis placed within the PTV• For any other arrangement of two or more intersecting beams, the point of TD specification should be at the intersection of the central axes of the beams placed within the PTV

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Rotation Therapy

• For full rotation or arcs of at least 270 degrees, the TD should be specified at the center of the rotation in the principal plane.• For smaller arcs, the TD should be stated in the principal plane, first at the center of rotation and, second, at the center of the target volume. This dual point specification is required because in small arc therapy, ‘past pointing’ techniques are used that give maximum absorbed dose close to the center of the target area.

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Additional Information

• The specification of TD is only meaningful if sufficient information is provided regarding the irradiation technique.• Radiation quality, SSD or SAD, field sizes, beam modification devices, beam weighting, correction for inhomogeneities, dose fractionation and patient positioning should be included as well.


15 chap 11 treatment planning i

Technology

wedge filters wedge

dynamic wedge

open field beam

beam central axis

central axis beam

wedge filters effect

ratio openof wedgefield

penumbra penumbra