GALVESTON, TexLast October, radiation oncologists at the
University of Texas Medical Branch in Galveston treated their first
patient with intensity modulated radiation therapy (IMRT). The
patient received a computer-planned prostate boost involving seven
beam angles and 25 beam segments (see Figure).
This complex plan allowed for maximum targeting of the tumor while
minimizing radiation to the urethra, rectum, and bladder.
I think IMRT is going to offer a terrific advantage for
treatment of tumors that are close to a vital organ like the spinal
cord or kidney, said Martin Colman, MD, professor and chairman
of the Department of Radiation Oncology. He said that eventually IMRT
may be used to treat a variety of cancers, but at the moment,
research is directed primarily at prostate cancer, head and neck
cancers, and brain tumors.
Dr. Colmans department is using an integrated IMRT system from
Siemens Medical Systems that provides automated delivery of IMRT
plans using a multileaf collimator, with considerable time savings.
With autosequencing, the prostate cancer patients intricate
treatment procedure was finished in 12 minutes. Using a manual rather
than automated multileaf collimator, it would have taken two to three
times longer, Dr. Colman said. With conventional radiation technology
involving blocks, the treatment would have taken five to 10 times longer.
If you treat 30 to 35 patients on a single unit in an 8-hour
day, which is fairly average, you might find that you could treat 60
or 70 patients using this autosequencing technology, he said.
In an interview with Oncology News International, Dr. Colman said
that IMRT takes conformal radiation therapy to a new level of
precision. Conventional radiation therapy, he noted, is delivered by
a rectangular beam defined by four collimators. Secondary shielding,
in which a shaped lead shield is placed in front of the rectangular
beam, allows some conformation of the beam to the target volume.
The next advance in conformal therapy was three-dimensional (3D)
treatment planning in which a 3D target volume is defined using CT
scans or MR images. Then you can aim your beam from more beam
angles (maybe five or seven) instead of the conventional two or four
directions, he said. Using whats called a
beams eye view, you could go to another degree of conformation
of the dose distribution to the target volume.
But, with this technique, he said, clinicians were still, in effect,
treating the whole target volume within each beam and relying on a
cross firing effect to give a higher dose to the target.
Intensity modulation takes this conformal technique one step further.
It defines the target volume not only as an outline but also in terms
of subvolumes within the target volume. You can define the
dosage you want much more specifically in much smaller volumes,
Dr. Colman said.
To explain the IMRT technique, Dr. Colman used the analogy of an
inverse CT scan. A CT scan looks at pixels of tissue, very
small volumes within a larger volume, he said. It defines
mathematically how much radiation each of those pixels is absorbing
and then displays this radiographically.
With intensity modulation, he said, you can do the inverse of
that. You can take those same pixels and define how much radiation
you want to deliver to each pixel.
With the Siemens technology, a beam angle is set and the leaves
of the multileaf collimator automatically change position to deliver
a radiation dose to each pixel as prescribed by the radiation
oncologist. Each of these dose deliveries is called a segment.
So for each field, which is a beam angle, you might have
anywhere from one or two to a large number of segments, Dr.
The Siemens integrated IMRT system in use at Galveston was introduced
at the 40th annual meeting of the American Society for Therapeutic
Radiology and Oncology (ASTRO) in
Phoenix and is being marketed as the IMART solution. It
includes a package of components starting with the inverse treatment
planning system (CORVUS, developed by the Nomus Corporation).
CORVUS allows the radiation oncolo-gist to stack the patients
CT images, define the target volume, and derive an optimal treatment
plan. The computer then uses that information to develop a
prescription to deliver the radiation in a way that best
fits the clinicians desired doses.
This prescription is then transferred through Siemens
sequencing intensity modulated technology (SIMTEC) software to the
accelerator and collimator controller. The software provides
instructions to the linear accelerator (PRIMUS) and the multileaf
collimator for each beam angle and all the beam segments within each angle.
You position the patient appropriately, and the sequencing
software guides the equipment through the entire treatment plan,
which may include many different beam angles and many different
segments, in an automated fashion, Dr. Colman said.
The IMRT system also includes a software program known as PRIMEVIEW
that allows the intensity modulated treatment to be visualized,
verified, and recorded step by step as it progresses.
At Galveston, the PRIMUS high-energy linear accelerator with
multileaf collimator is being used to treat several different
cancers, primarily head and neck, while intensity modulation has been
used so far only for treating prostate cancers.
Finding the Cancer
Dr. Colman noted that cancer in solid organs is not necessarily
homogeneous. For example, with prostate cancer, the prostate
may be diffusely involved or involved only in certain parts.
The limitation right now is not in how we deliver the radiation
dosage, but rather in not knowing the exact location of the
cancer, he pointed out. Often, we treat a larger volume
than may be necessary to ensure that weve treated the whole tumor.
He said that researchers at the University of California, San
Francisco, are using new imaging technologyMRS (magnetic
resonance spectroscopy)to define volumes within the prostate
that are more or less likely to be cancerous. Then, using IMRT, they
can deliver higher radiation doses to volumes within the prostate
that have a higher probability of cancer.
Using this technique, Dr. Colman said, you can still give the
same dose to the whole prostate gland that you know it will tolerate,
and you can then give a higher dose to the area that is more likely
to have tumor. We think that principle will probably apply to other
tumors in other areas of the body as we become more expert in
defining the location of the cancer.