GALVESTON, Tex-Last 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.
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. Colman said.
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.