6 Total Body Irradiation
6 Total Body Irradiation
6 Total Body Irradiation
. Its purpose is for dose uniformity.
Position the patient's arms to they become natural compensators for the lungs.
Rotate the collimator to 45 degrees so it can subtend the entire body more easily.
Make sure you extend your SSD to around 400 cm. The use of a long SSD serves
two purposes. First, it allows for a single field to cover the entire body. Secondly,
when combined with lowering the pulse repetition rate, it reduces the dose rate to
below 10 cGy/min to prevent higher dose-rate related toxicities.
Use a beam spoiler to spread out the dose.
For ap-pa treatment, the principle difference is that the patient is standing. Therefore:
Dose to the brain, lungs, and kidneys can be reduced using cerrobend blocks.
What are dose inhomogeneities caused by?
Dose inhomogeneities are mainly due to tissue inhomogeneities, the contour of the
human body, and beam energy. Tissue inhomogeneities cause the most pronounced dose
increase for lung tissue. Compensation is sometimes made through the introduction of
partially shielding lung blocks. Energy selection is based on the need to achieve higher
whole-body dose homogeneity and lower-dose deposition to the lung (both require higher
energy) and the need to avoid low doses near the surface in the build-up region (favoring
low energy). A 10-MeV photon beam is often chosen in this situation. Why is all this
important? Because dose inhomogeneity can lead to failure of TBI through either
insufficient dose to the marrow stem cells or excessive dose to critical organs.
What is the typical dose fractionation?
Protocols vary rather widely by institution, but the basics are as follows:
The total dose is typically around 12 14 Gy.
It is delivered via 150 rads x 8 fractions or via 200 rads x 6 fractions.
Most protocols use 2 fractions per day separated by at least 6 hours.
It is advised to use a low dose rate of around 10 cGy/min. This is one of the
reasons we use extended SSDs.
The main goal is to deliver uniform dose to the entire body to within 10% of the
prescription dose.
Discuss some treatment statistics.
Inhomogeneity worsens with:
Decreased energy
Increased patient thickness
Decreased SSD
Lateral fields
Skin dose increases with:
Field Size
SSD
Spoilers
How do you calculate MUs?
You can perform direct output calibration by placing an ion chamber in a water
phantom at the fixed source to body TBI distance, opening the collimator to its
maximum size, and generating a table of output factors as a function of depth so they can
be used to calculate monitor units given a midline depth.
Alternative to direct output factor measurements is the calculation formalism based on
TMRs, Sc, Sp, and the inverse square law:
( ) ( ) ( ) ( )
2
e c c p e block
D
MU =
100
k TMR d,r S r S r OAR d TF
400
Recall that Sc is the collimator scatter factor for the field size projected at the isocenter
(rc), Sp is the phantom scatter factor for the patient equivalent field size (re), and TF is the
transmission factor for the block tray, beam spoiler, or any other absorber between the
machine and the patient.
*The only hitch: The TMR data obtained under standard conditions (100 cm) must be
checked for its validity at TBI distances. A thorough physicist will compare the dose per
MU value to the directly measured output.
How do you QA a treatment plan? How do you even commission TBI in the first
place?
A partial whole body Rando phantom (head through mid-femur) can be used for dose
verification and treatment-planning verification. A prescription dose of 14 Gy (2 Gy per
fraction) can be delivered using two lateral 10 MV fields at 300-cm SSD. With the
collimator rotated 45 and field size set to 40 40 cm, this technique allows coverage of
the entire body. Dose verification can be performed in two ways:
LiF thermoluminescent dosimeters (TLDs) can be placed inside the Rando
phantom at organ sites of interest, such as manubrium, lungs, xyphoid, iliac crest,
and hip, for dose verification during a TBI treatment. The TLDs must be
calibrated and their linearity with respect to dose should be verified for a 50 cGy
to 500 cGy range.
Computed tomography can be performed on the Rando phantom using 5-mm
thick slices on a GE Light Speed CT scanner. During imaging, both arms should
be kept resting at the side, simulating the lateral treatment, in which arms act as a
natural compensator for the lung. A Pinnacle radiation treatment- planning
workstation can be used for dose calculation. Dose distributions can be
normalized to a point located at the level of the hip in the middle of the body.
Dose-volume histograms can then be created for the target and individual
sensitive organs.
After a particular TBI technique is adopted and commissioned for clinical use, it
is recommended that an in-vivo dosimetry check be performed on the first 20
patients. TLDs should be placed on the patients skin at strategic locations and
then compared to predicted doses taking into account the thickness variations, the
compensation, and any off-axis ratios.
What problems can occur?
Again, the problems stem from inhomogeneity. The above tests have the ability to
quantify the variation in doses delivered to the whole body. Specifically, we should
search for which parts of the bone marrow fail to receive the prescribed dose, and which
critical organs, such as the lungs, receive excessive dose.
Consider the results of the following test:
Some of the bone marrow is under-dosed by more than 20%. Under-dose to the bone
marrow can have the immediate effect of rejecting the donor marrow and increasing the
relapse rate in the long term. This is clearly due to the placement of the arms at the side,
which protects the lung but also reduces the dose at this level.
In addition, in the volume of the lungs not covered by the arms, the radiation exposure
resulted in a dose 20% to 30% higher than that prescribed.
For all other regions, we hope that measured doses are within 5% to 10% of what is
prescribed.
The crux of most AAPM reports are quantifying and eliminating any uncertainties in dose
delivery to the target and the normal tissue. The whole point of good physics is to QA the
hell out of anything new until you are super confident.
How would you start a TBI program?
Starting any program involves the scrutiny of numerous protocols specifying many
different regimens. The amateur first need know where to look. TG-29 provides a good
basis for implementing such a program.
First, the medical director would have to choose which type of TBI treatment he or she
wished to perform; single fraction with low dose-rate, single fraction with high dose-rate,
fractionated TBI, hyper-fractionated TBI, AP-PA TBI, Bi-lateral TBI, compensators, no
compensators, etc.
Then we have to ask; do we have the necessary equipment to execute this treatment. Do
we have the space, can we make the blocks, can we use are existing LINAC, etc.
Are we trained? Does anyone have any experience? An important part of this plan is to
put together a team consisting of a radiation oncologist, a medical physicist, a
dosimetrist, and a therapist. A clinic starting from scratch may wish to send staff
members to other clinics that have active TBI programs. All details should be taken note
of; how to make patient measurements, how to set them up, proper dosimetry, QA
procedures, worksheet templates, etc. Besides literature, such a program requires
practical training above all else.
Total Body Electron Therapy:
Treats: T-Cell Lymphoma and Mycosis Fungoides
Energy: 3 7 MeV
Dose/fraction: 200 cGy
Distance: 300 400 cm
Stanford Technique:
6 fields
60 degrees apart
Patient standing up
1 cm Lucite scatter plate
shield eyes and nails
Disadvantages:
Long Tx times
X-ray contamination
4 8% uniformity