10. THERAPEUTIC NUCLEAR MEDICINE

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10. THERAPEUTIC NUCLEAR MEDICINE
10.2 DOSE AND ISODOSE IN RADIATION TREATMENT
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At the position dmax of maximum energy loss of
radiation, the number of secondary ionizations
products peaks which in turn maximizes the dose
at that location.
The dose is denned (see section on dosimetry) as
total energy loss of radiation per mass. It can
be formulated in terms of the activity A(t)
(number of incident particles/second in cases of
external beam treatment N(t)) and energy loss or
stopping power dE/dx. The total absorbed dose
D(t) after a period t of irradiation is expressed
in terms of number of particles N(t), total
amount of energy lost ER, and irradiated area A
with m, V, and ? as mass, volume, and density of
the exposed organs. This results in a absorbed
dose D(t,d) after an irradiation period t at a
certain depth d
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The absorbed dose at a certain depth d is
directly proportional to the stopping power 1/? ?
dE/dx !
Within the area A each point at a certain depth
d receives the same dose ? ISODOSE.
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The average dose due to energy loss of ?
radiation within a depth d over a period t is
The dose is directly proportional to the
transfer and absorption coefficients which change
with depth.
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The dose distribution is less well defined
compared to particle beams.
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Within the area A each point at a certain depth
d receives the same dose ? ISODOSE. Isodose
profiles are plotted in terms of the percentage
depth dose DD because absolute dose
measurements are difficult. The percentage depth
dose is the absorbed dose at a given depth d
expressed as a percentage of the absorbed dose at
a reference depth dmax along the central axis of
the beam.
In figure above the percentage depth dose at
point A is 75 .
Isodose charts are usually plotted in increments
of 10 . They depend on the beam geometry and the
various absorption effects within the body tissue.
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Examples of isodose profiles
For electron beam the percentage depth
dose increases with depth, the maximum range
depends on the initial energy of the electron
beam.
The isodose profile widens rapidly due to wide
angle scattering.
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For heavy ion beam the profile remains well
defined but the percentage depth dose increases
rapidly at well localized position due to Bragg
curve behavior plus decay radiation from on-line
produced activities.
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For ?-radiation the percentage depth dose peaks
at small depths but ranges deeply into the tissue
proportional to the absorption coefficient.
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The angle scattering is small, the beam profile
and therefore the isodose profile remains well
defined.
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A carefully designed treatment plan is necessary
to maximize the dose at the tumor location while
minimizing the dose in the surrounding body
tissue! Notice, while tumor might get maximum
dose, the surrounding tissue may be exposed to at
least 50 of it which may cause problems.
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Dose calculation should consider the following
aspects

1. geometry of treatment

1. energy loss effects

1. straggling and widening of beam

1. backscatter

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Treatment plan needs to be carefully designed,
should rely on careful localization of tumor with
modern imaging techniques (CT, MRI). Dose and
dose losses should be simulated (three
dimensional simulation).
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Typically, the prescribed dose depends on the
size of the tumor and the specific organ which
has been effected. The prescribed total doses
range between 40 Gy to 70 Gy.
For external beam therapy the dose will be
administered over a period of five to six weeks
with a daily dose ranging between 1.9 and 2.2
Gy/day (five days a week).
The treatment time depends on the intensity of
the radiation source!
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For brachytherapy a radioactive source is
implanted in a location near the tumor. Therefore
the radiation is constant until the desired dose
has been reached.
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Example
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For calculating the dose to be delivered
geometrical aspects and backscattering have to be
taken into account. Critical is the
source-surface distance SSD which determines
intensity losses between source and body.
d is the depth of the tumor location!
A dose rate of DR1 1.17 Gy/min delivered over
a distance of SSD1 d 80.5 cm reduces over a
distance of SSD2 d 100 cm to
Substantial losses can occur by back scattering,
the backscattered radiation will increase the
dosage in the surrounding body tissue. Therefore
a further modification has to be introduced by
subtracting the amount of backscattered radiation
BS in the body tissue.
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The backscatter is defined as the ratio of
scattered dose at depth d of body tissue to the
scattered dose in air at the same length d.
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To optimize treatment often multiple beam
treatment is applied.
This approach maximizes the dose at the location
of the tumor and minimizes the dose in the
surrounding body tissue.
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Alternative options are the introduction of
wedges which allow beam attenuation and
absorption to shape the radiation field for
optimal treatment.
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