New hope for winning the war against cancer
For years, oncologists have faced a frustrating challenge. They've known that if they can deliver a high dose of radiation to a localized tumor, they have a very good chance of eradicating the tumor and possibly curing the patient. On the other hand, such a dose may pose a substantial risk to a patient's surrounding tissue and organs. This fact has required some patients to be treated with less radiation than ideal for local tumor control and has made it difficult, if not impossible, to optimally treat some tumors.
Clinicians at leading institutions such as Memorial Sloan-Kettering Cancer Center, Stanford University, Emory University, the Medical College of Virginia and Providence Cancer Center are using a new technique that delivers high doses of radiation to tumors while substantially reducing risk to normal tissues. This therapy, called Intensity Modulated Radiation Therapy, or IMRT, is a type of radiation therapy that links treatment planning and driver software to the actual treatment delivery devices. As a result, clinicians can determine and deliver an optimum plan of individualized radiation for each patient.
"IMRT offers tremendous potential for improving the efficiency and efficacy of radiotherapy," says Kenneth Haugen, M.D., Medical Director of Providence Cancer Center Radiation Oncology Department. "In terms of advances in radiation technology, this is the final frontier."
In addition to IMRT, Providence Cancer Center now offers BAT (B-mode Acquisition and Targeting), an ultrasound-based targeting system used with external beam radiation to precisely localize targets that may move from one treatment day to the next. BAT allows for the delivery of high intensity radiation directly to the tumor, while decreasing the risk of complications.
Optimized Treatment Delivery
With IMRT, clinicians are no longer confined to using a single, large, uniform beam of radiation that may cause complications in normal tissue surrounding the tumor. Now they can vary dose intensity and "paint" the tumor with radiation, while sculpting or shaping the beam to limit radiation exposure to surrounding tissue, and then delivering higher dose to special areas of the tumor.
"IMRT allows us to treat difficult cases optimally because we can minimize radiation to adjacent normal critical structures," explains Dr. Haugen. "Because we are able to precisely target tumors and spare the normal tissues, we are able to enhance the dose concentration to the tumor itself."
For example, at Providence Cancer Center, using IMRT, Dr. Haugen is able to treat challenging cases such as patients with tumors of the prostate, breast, head and neck.
Enhancing the dose concentrations to the target gives clinicians a greater chance of completely eradicating the tumor. Because cancer cells are fast replicating by nature, damage to the genetic structure of these cells by radiation is incredibly powerful. In addition, increased optimization enables clinicians to use radiation to treat areas that would have been considered too risky just a few years ago.
In order to achieve such precision safely, medical physicists work with the dosimetrists to determine the best way to deliver the radiation for each patient. Much of this is done with computers using mathematical equations based on the digital imaging scans of the patient.
"We simulate the radiation treatment before the patient even gets on the table," explains Don Jacobs, Dosimetrist at Providence Cancer Center. "Using mathematical models and visualization software, we mimic the radiation beams and emulate the patient. Then, we enable the computer to determine how the radiation dose will accumulate in different parts of the body, depending on how the radiation fields are organized."
"It's like going to the tailor and having a suit made just for you," adds Jacobs. "We can tailor the radiation beams to conform to the tumor from almost any direction -- like a perfectly fitting suit."
"With IMRT, the clinician can use a computer to optimize thousands of treatment options," says Suzanne Schultz, Physicist at Providence Cancer Center. "This makes it easy for the clinician to look at the risk/benefit scenario of various doses of radiation to find out how best to deliver the maximum dose of radiation to the tumor and the minimum dose to healthy tissue and organs."
Previously, a dosimetrist would manually look at as many options as he or she could. However, even the best dosimetrist cannot practically process more the than nine or ten options per patient. Now, IMRT systems can evaluate such an extensive array of options that they can determine an optimal arrangement for the beams. "All you need is a linear accelerator that can deliver this optimal dose and you basically have the best possible scenario for treating your patient," said Schultz.
More than a Century in Development
For more than 100 years, physicians have used radiation to treat cancer. In fact, it was less than a month after Nobel laureate Wilhelm Conrad Roentgen reported his discovery of the X-ray that the first cancer patient was treated with radiation in January 1896.
The discovery of radioactivity and X-rays brought the understanding that radiation could cause damage to cells by interfering with the cells' ability to grow and reproduce. Cells that are growing and multiplying are especially sensitive to the effects of radiation.
Because cancer cells reproduce faster than normal cells, they are more susceptible to damage from radiation. While normal cells are affected as well, they have the ability to regenerate, unlike cancer cells, and tend to recover from radiation damage.
The medical linear accelerator, first used in the 1950s, revolutionized the way radiation was delivered to treat cancer. Using electromagnetic waves, the device accelerates electrons through a specially designed tube to hit a target and generate X-rays.
However, until the advent of digital diagnostic imaging capability, powerful computers and specialized software tools, it was virtually impossible to conform the radiation beam to the shape of the tumor. Initial beam-shaping events were manual, arduous, time-consuming and expensive. Heavy lead blocks had to be cut and repeatedly repositioned for each exposure. In addition, the treatment therapist had to go in and out of the room to change the machine angle, field size, and to insert the new blocks and other field modifiers.
"The introduction of the multi-leaf collimator, or MLC, has significantly contributed to recent advances in radiation therapy," explains Dr. Haugen.
The MLC is a specialized, computer-controlled device with as many as 120 tungsten fingers, or leaves, inside the linear accelerator. This enables the clinician to sculpt the radiation beams automatically to conform to the shape of the tumor. And by automatically adjusting the beam shape and moving the linear accelerator to treat the tumor from several different angles, it is possible to deliver a prescribed dose across all three dimensions of the tumor.
IMRT is an advanced form of this conformal approach, which not only uses 3-D imaging and treatment delivery, but allows for varying intensities of radiation to produce dose distributions that are far more conformal than those possible with standard 3-D CRT. For difficult tumors, such as those very close to or wrapped around critical organs, modulating the intensity of the beam provides a much more viable treatment approach.
Implications of this technology
"There is little doubt that because of the greatly increased control possible over dose distributions, intensity modulation is a significant technological advancement in the state of the art," says Dr. Haugen. "Consequently, we expect to see higher tumor control for the same, or lower, normal tissue toxicities."
Doctors at the Cleveland Clinic have been using IMRT for several years, and experimenting with significantly higher than normal radiation doses. "Originally, we thought we might see an increase in acute side effects as a result of the higher doses of radiation used, but that hasn't been the case," says Roger Macklis, M.D., Chairman, Department of Radiation Oncology, Cleveland Clinic. "So far, our treatments have been extremely successful."
"The benefit of IMRT is that we can escalate our doses to the tumor at the same time keeping treatments practical," says Dr. Haugen. "Ultimately this should allow us to cure more patients."
IMRT has certainly caught the attention of the professional community. In fact, beliefs about the potential of using this treatment modality to increase radiation doses to more effective levels are so strong that the American Society for Therapeutic Radiation and Oncology (ASTRO) recently released this statement pertaining to the treatment of prostate cancer:
"Strong data show unequivocally that cure rates in [prostate] cancer patients can be improved by 15-40 percent by increasing the dose from 72 Gy to 78 Gy. However, the majority of radiation oncology facilities in the U.S. have neither the tools nor the know-how to allow delivery of this higher dose. As a result, many potentially curable cases may fail." ASTROgram #133.
In our region, only Providence Cancer Center has this capability.
Links:
www.varian.com
www.psa-rising.com/medicalpike/ebr_intensitymod99.htm
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