Transcript File - Patty Sponseller, CMD

Hodgkin’s Lymphoma
T R E AT M E N T P L A N N I N G
ABOVE AND BELOW THE DIAPHRAGM
WHY IS IT SO COMPLEX?
PAT T Y S P O N S E L L E R , C M D
It’s just AP/PA right?
Mantle
Para-aortics
Although AP/PA a lot going on here!
Radiation Fields
 Mantle covers chin and just
below diaphragm
 Laterally “flash” skin to
include both axilla
 Involved field radiation
 Inverted Y covers paraaortic,
pelvic and inguinal lymph
nodes
Mantle RT
 R A D I AT I O N T O A L A R G E A R E A O F T H E N E C K ,
CHEST AND AXILLA
 C O V E R A L L T H E M A I N LY M P H N O D E A R E A S
ABOVE THE DIAPHR AGM
 S H I E L D I N G T O PA R T O F T H E L U N G S , H E A R T
AND SHOULDERS
 MANTLE IS DERIVED FROM THE NAME OF A
G A R M E N T M U C H L I K E A C L O A K I N M E D I E VA L
TIMES
Variation with the DD
throughout the fields
•
PAT I E N T T H I C K N E S S C H A N G E S
• PAT I E N T C O N T O U R I R R E G U L A R I T I E S
• DIFFERENT HETEROGENITIES
• MISSING TISSUE
• BEAM CHAR ACTERISTICS
• G E O M E T R I C M AT C H
 Basic dose distribution data is
Why is this?
obtained under standard
conditions
 Homogenous density in a water
phantom
 Perpendicular beam incidence
 Flat surface
During Treatment
• The beam may
be obliquely
incident with
respect to the
surface
• Surface may be
curved or
irregular in
shape
• Under these
conditions,
standard DD are
not applicable
without
corrections
Corrected in Pinnacle
 Beam data includes profile scans across the beam,
depth doses and output factors (ratio of the dose
measured for a specific F S standard to a 10 cm2)
measured for every machine energy and many square
field sizes. This measured data is used to determine
the many, many free parameters in the Pinnacle
model for EACH linear accelerator.
 In Pinnacle we are then modeling the energy that
comes from the head of the Tx unit including all the
scatter within the head that affects the energy
spectrum.
Modeling – Convolution Algorithm
 An integral that calculates at every point within the
dose grid. That dose calculation includes the
attenuation of the beam as it travels through tissue
times the energy fluence of energy spectrum times
the energy kernel.
Convolution Algorithm
Want to know more?
Imaging
 Conventional X rays
 Ultrasound
 CT scan
 PET (FDG)
Ability to screen for distant Dz
Uptake in nodal areas that
look normal on CT
False positives
Not good at detecting
marrow involvelment
Simulation for Adjacent Fields
 Maintain patient position when all fields are treated
 Proper immobilization for entire torso
 Clam shell used if pelvis RT for males
 Consider oophoropexy for females
 Supine position with arms akimbo
 Immobilization indexed to treatment couch
Matching new fields with previously TX
 Tattoo documentation cannot be overemphasized
 Compare prior DRR’s and portal images
 Examine the chart carefully for possible changes
during prior RT
 Reproduce prior RT with new scan to ensure which
segment of spinal cord was irradiated
Goal is a
Homogenous
Dose
Distribution
Adjacent fields
have the same
divergence
Adjacent
Fields
Adjacent fields do not
match divergence
Dose discrepancies over
spinal cord
Inverted Y
 May need to split up
inverted Y into 2 fields for
adult or tall adolescent
 Otherwise-
Paraortics/Spleen
You have this problem
 The larger inverted Y does
not match the divergence of
the mantle field
 Hot and cold spots
Blocking
Spleen/Paraortic Fields
or Inverted Y usually
can accommodate MLC
Mantle Fields typically
are a combination of
MLC and Cerrobend
blocking
Lung Block shapes are
usually too complex for
MLC
May consider a larynx
block on AP field
Combination of MLC’s and custom blocks
Portal Imaging
 See the
combination
here
Field Blocks
 Blocks are shaped or tapered to match the
divergence of the beam
 This minimizes block transmission pneumbra
(partial transmission of the beam at the edges of the
block)
 Do we do this for all our photon blocking at UWMC?
Custom Blocking
 Lipowitz Metal
 Cerrobend (brand name)
 Approx 83% of Pb density
 Low melting point
 Materials making up Cerrobend are
 Bismuth
 Lead
 Tim
 Cadmium
How thick are the blocks?
 Half Value layer
 The thickness of the material required to attenuate
the intensity of the beam to half its original value
 How many HVL’s in MV photon blocking?
Original Value
 100%
 50%
 25%
 12.5%
 6.25%
 3.12%
 1.56%
 5 HVL’s
How thick are the blocks?
 5 HVL’s of Cerrobend needed
 What is the HVL thickness of Cerrobend?
 1 HVL of Cerrobend is 1.5 cm thick
 In the megavoltage range of photons the most
common used thickness is 7.5 cm
 Which is equivalent to about 6 cm of pure lead
Mantle Fields
Dose Distribution is
calculated with
cerrobend blocking
Pinnacle will not
allow Step N Shoot
with custom blocking
Here we use a “Poor
Man” Technique
Treat more than one
field, close jaws and
adjust weighting
Other Techniques
 Use wedge on
AP field if FS
allows
Compensators
Decimal
Para/Spleen or
Inverted Y
Usually MLC’s can be
used for blocking on all
fields
May require Step N
shoot for hot spots
Point under a block
17 cm
4.5 cm
6.25 cm
17 cm
Fields
 17 X 17 cm = 17 cm 2
 4.5 x 17 cm= 7 cm 2
 6.25 x 17 cm= 9 cm 2
 Calculate the dose under a block at 10 cm depth
Which blocks are bettter?