# PDD

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```					RANZCR Radiation Oncology Physics

Dose Distribution in
Tissue Part I            12
Helen Gustafsson

Medical Physics and MTS Sections
Acknowledgements

   This lecture is based on Khan (chapters 9-10)
and tutorials by Graham Stuart and Leonard
Wee (Royal Perth Hospital)
Overview (Part I)
   Primary and Scatter dose
   Dose distribution on central axis
   PDD
   TAR
   TPR
   Dose distribution off axis
   Beam profiles
   Penumbra
Overview (Part II, next week)
   Manual dose calculations using PDD and
TPR
   Effects on dose distribution from
inhomogeneities
Primary and Scatter Dose
   Total dose will be made up of:
   Primary dose – photons originating from the
source that have not been scattered elsewhere
   Head scatter dose – photons that have been
scattered anywhere in the gantry, i.e. flattening
filter, collimators etc.
   Phantom scatter dose – photons that have been
scattered in the phantom/patient.
Percentage Depth Dose (PDD)

Describes the ratio of dose    Fixed SSD

at an arbitrary depth to the
dose at a reference depth
(usually Dmax):
Chamber
PDD(%)=100x(d/dref)         moves
deeper
into
phantom
PDD
Variation with beam quality

(Figure from Khan, Chapter 9)
PDD
Variation with field size

(Figure from tutorial by Leonard Wee )
PDD
Variation with SSD
   Source can be considered as a point at large SSD –
dose decreases with inverse square of the distance*
   Change in dose is faster closer to the source, i.e.
PDD at depth increases with SSD
   Standard SSD (100 cm) is a compromise between
dose at the tumour relative to surface dose and
dose rate

*Not completely true because of scatter contribution
PDD
Effect of beam modifiers

(Figure from tutorial by Leonard Wee )
Tissue Air Ratio (TAR)
   Defined as the dose at a point inside a
phantom relative to the dose at the same
point in space
   Originally intended to simplify calculations for
rotational arc therapy ~independent of SSD
   Varies with energy, depth in phantom and
field size
   TAR at Dmax=backscatter factor
Tissue Phantom Ratio (TPR)
   Defined as the dose at a point
(usually the isocenter) inside a
phantom with tissue “on top”           fixed SCD
relative to the dose at the same
point in space, but with a reference
amount of tissue “on top                            ‘thickness’
of water or
   TPR20,10 is commonly used as a                      tissue above
the chamber
measure of beam quality for photon        Chamber   increments
beams.                                    remains
fixed
   TPR is ~independent of SSD
Dose distribution off axis
   The beam profile shows the
variation in dose across the beam
(inplane or crossplane)
   Field size is defined at the 50%
dose level
   The Off axis Ratio (OAR) is defined
as the dose at a point away from
the central axis relative to the dose
directly under the central axis

(Figure from Khan )
Off axis – flatness and
symmetry
   The field flatness is defined as the
absolute variation in dose over the
central 80% of the beam profile.
   Field symmetry may be defined as
the largest ratio of dose between
any pair of symmetrical points in
the flat region of the beam
   Usually within ±3%, tweaked at one
depth (10 cm)

(Figure from tutorial by Leonard Wee )
Profiles – variation with depth
   At shallow depths regions of high
the penumbra region, due to the
shape of the flattening filter
   The mean energy is lower off axis
(off axis softening, OAS) because
of the shape of the flattening filter
   As a consequence, at depths
greater than 10cm, the beam
profiles   have      with      rounded
“shoulders”

(Figure from Podgorsak )
Penumbra
   The distance between the 20%-
80% dose levels
   The width of the penumbra will
determine how sharp the the dose
fall of outside the tumour can be
   Three contributions:
 Geometrical (source size)

 Scatter (photons scattered
sideways)
 Transmission (through
collimators)
References
   Podgorsak, Radiation Oncology Physics: A handbook for
teachers and students
   Kahn, The physics of Radiation Therapy (second edition)
   Van Dyk, The Modern Technology of Radiation Oncology

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