Transcript Tomotherapy

‫بسم هللا الرحمن الرحيم‬
‫” قالوا سبحانك ال علم لنا إال ما‬
‫علمتنا إنك أنت العليم الحكيم“‬
OUTLINE
Principle of Tomotherapy.
 CT scan similarity.
 Tomotherapy idea.
 Serial and Spiral.
 Clinical Application and Overview.

Tomotherapy
Tomotherapy
is
intensity-modulated
rotational radiotherapy utilizing a photon
fan beam.
 Radiation therapy device designed on a
CT scanner-based platform.
 Tomotherapy means slice therapy coined
to describe IMRT using fan beam.


Computer Tomography
CT
Prospective
Retrospective
Types of CT Scanner
Serial
Helical (Spiral)
Two Flavor
Serial
Helical (Spiral)
Medco, a company formed by neurosurgeon
Mark Carol began developing the serial
tomotherapy concept.
 Their first presentation was in 1992 (Carol et
al. 1993) and their first patient was treated
with tomotherapy in 1994

Basic Principle
Rotation therapy is relatively insensitive to
the energy of the beam (Johns and
Cunningham 1983).
 Before
megavoltage
beams
were
developed, rotation therapy was used to
deliver x-radiation deep within the body.
 The lower exit dose of orthovoltage photon
beams approximately compensated for the
high entrance dose.

Intensity Modulation
The linac output can be varied.
 The jaw opening can be changed.
 The gantry speed can be altered.
 The couch speed can be modified.
 The leaf states can be changed.

Tomotherapy Information
The primary collimator and jaws are
fabricated from an alloy containing 95%
tungsten.
 The thickness of the clamshell is about 22
cm, which reduces the leakage for IMRT
with high numbers of monitor units to
values typical for conventional delivery
with conventional monitor units.


At the axis of rotation, the leaf thickness is
0.625 cm and the leaf height is 10 cm of
95% tungsten. The leaves have a tongueand-groove design that results in an interleaf leakage less than 0.5% of its open
intensity, which makes it the best-shielded
radiotherapy unit used for IMRT.
can irradiate up to a 160 cm long field.
 The Tomotherapy unit has instead a set of
uniform thickness filters before and after
the monitor chamber to provide buildup to
the monitor chamber, to shield the monitor
chamber from the jaws, and to remove
extremely low-energy photons.



The rate of fall-off at a plane through the
machine axis is about 2.5% to 3% per cm
so that by the edge of the field (20 cm
from the central axis) the output is 40% to
50% of its value at the center of the beam.
The intensity variation leaf-to-leaf is about
2%,but this small discretization cannot be
discerned dosimetrically.

The major advantage of a lack of a fieldflattening filter is the beam quality. The
thin set of filters produces far less head
scatter than a conventional linac equipped
with a field-flattening filter.

This
also
improves
the
imaging
characteristics of the beam. The energy
spectrum at the center of the field is not
significantly harder than the spectrum at
the edge of the field. This means that the
beam is easier to characterize with modelbased dose calculation systems

The
helical
tomotherapy
has
a
conventional xenon ion chamber CT
detec- tor system on board the system that
acquires a transmission signal from the
linac beam. The parameters of the linac
radiofrequency system are adjusted for
imaging from those for treatment so that
the beam has a lower energy (about 3.5
MV),lower pulse repe- tition frequency,and
smaller output.

Absence Of Metal Artifacts Metal artifacts
arise in conventional CT scanners
because the attenuation of the metal is
non-linear due to the photoelectric effect.
In helical CT, the beam is penetrating
enough to eliminate metal artifacts arising
from hip prostheses and dental filings.

This means that a verification CT could be
a more reliable CT system for prostate
patients with hip replacements and head
and neck patients with cancer of the
tongue.
For a typical 30-fraction treat- ment
schedule, daily imaging delivers a total
exposure dose to the imaged volume of
less than 1 Gy.
 For
a typical 30-fraction treatment
schedule, daily imaging delivers a total
exposure dose to the imaged volume of
less than 1 Gy.


Nonetheless, MV CT images are far
superior to planar images (either kV or
MV) for the verification of correct patient
setup. Moreover, helical MV CT imaging is
a slow CT imaging technique that allows
for the encoding of target motion in the
resulting MV CT data set (see fig. 10 , 11
), and therefore the pretreatment
verification of a motion envelope defined
from four-dimensional CT.
Tungsten MLC 2cm in width
CRANE
Rock Mackie with the University of Wisconsin Tomotherapy
Research Unit
Tomotherapy
Hi - ART
Tomos ’Universal’ Signature
Tomotherapy And Adaptive
Radiotherapy

instead of simply delivering the intended
treatment plan, one would take an exit
dose measurement during treatment and
reconstruct the dose distribution that has
been delivered. Using any of a number of
metrics, one could then compare this
actually delivered dose dis- tribution with
the intended one.

the pretreatment dose guidance that is
currently possible with tomotherapy. The
overlaid isodose cloud can be used to
visually verify that the intended targets lie
within the high-dose region and that
organs at risk lie outside the high-dose
region.
Positioning Verification

However, placing a slippery polyethylene
sheet with handholds between the couch
top and mattress can easily effect lateral
movements. The next generation of
couches on helical tomotherapy units has
lateral adjustments. Tomotherapy comes
equipped with a set of moveable CTsimulator lasers so that the modified
position of the patient can be confirmed.

The process is not accurate unless a CT
of the patient at the time of treatment is
used. Dose reconstruc- tion has been
tested using the UW tomotherapy
benchtop unit but not yet implemented on
the UW clinical unit.
Deformable Registration And
Autocontouring

Deformable registration algorithms
typically use common features such as
contours or points that are identified
automatically in both images (Blake and
Isard 1998). The intervening space
between identifiable features is deformed
according to finite element methods using
established elastic property parameters (

Re-optimization is the modification of
subsequent treatment fractions on the
basis of the dose reconstruction and
deformable registration of the regions of
interest and the dose distribution (Wu
2002)

While it is not possible to remove the
excess dose delivered to a sensitive
structure, it may be possible to take this
into account in re-optimization so that the
dose delivered in the whole course of
therapy might be the same or only
marginally higher.

Re-optimization might also be done if it is
decided that the target volume needs to
change part way through the course of
ther- apy. Re-optimization is a research
topic and not yet being tested on the UW
clinical unit
Disadvantage

Ring gantry are minimal since few patients
are treated with non-coplanar radiation
fields and IMRT diminishes the need for
these types of field arrangements.
Clinical Appliaction

Image-Guided Stereotactic Body
Radiotherapy of Non-Small-Cell Lung
Cancer.
Treatment of Multiple
Metastasis
Future
Needs to be able to calculate the dose
actually delivered during therapy.
 The actually delivered dose can be
calculated from collected exit dose
measurements during treatment using
back-projection
 has to have the ability to reliably track
four-dimensional target motion envelopes
since tomotherapy is a fully dynamic
therapy


reliably contour changes in the target and
to quantify organ deformation. Target
variation and organ deformation can be
handled
using
deformable
image
registration and projecting the contours
from the old data set onto the new data set
using nonlinear transformation
MRIgRT
Tomotherapy With MRI
Against
“There are no nonviolent
revolutions…”
Malcolm X
WITH!
Highly Arched Chest Wall
Conventional: 2 tangential fields
More than conventional: 3 fields
IMRT
50%
70%
90%
120%
Spare Time on the treatment Machine.
 Single Plan reduce planning work Load.
 Reduced work on Linac.

Reduces treatment time from 7 to 5 weeks
Cramp?
Better