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From OncoLog, September 2012, Vol. 57, No. 9

Stereotactic Body Radiation Therapy Achieves High Control Rates in Lung Cancer

By Zach Bohannan

Photo: Dr. Joe Chang and Barbara Pool
Dr. Joe Chang reviews a treatment plan with Barbara Pool, the 1,000th lung cancer patient to receive stereotactic body radiation therapy at MD Anderson.

Stereotactic body radiation therapy (SBRT), a relatively new treatment modality, has recently become a very successful treatment—and may even exceed the effectiveness of surgery—for early-stage lung cancer.

SBRT, sometimes called stereotactic ablative radiotherapy, has many similarities to conventional radiation therapy; both use multiple beams to deliver a therapeutic dose of radiation to the target tissue. However, the difference between SBRT and conventional radiation therapy is that SBRT uses more beams from many more directions, allowing doctors to administer very high radiation doses to very specific targets with less risk to the surrounding tissues.

“The key issue with SBRT is to deliver a high enough dose to ablate the tumor while sparing the surrounding tissues,” said Joe Chang, M.D., Ph.D., an associate professor in the Division of Radiation Oncology, the director of the SBRT program, and clinical service chief of thoracic radiation oncology at The University of Texas MD Anderson Cancer Center.

Planning around critical structures

Dr. Chang explained, “SBRT creates a very sharp dose gradient, which means that within 5 mm, an ablative dose drops to a safe dose. So although multiple beams hit the target, only one beam goes through the surrounding tissue at any given region.”

Because the radiation is delivered from so many angles, SBRT requires precise planning around the target area. This requires a 4-dimensional volumetric imaging modality, such as computed tomography (CT), that accounts for motion. These images are used to create customized treatment plans that direct several radiation beams of different intensities at different angles precisely to the tumor. Volumetric images are also taken immediately before treatment using imaging equipment attached directly to the SBRT machinery. Another option is implanting a metal fiducial marker to track the position of the tumor with x-rays.

These precise imaging methods are more important for SBRT than for conventional radiation therapy because of the higher doses SBRT can deliver to the target. Precise imaging is especially important for treating lung cancer because of the movement associated with respiration and the sensitivity of the lung and surrounding tissue to radiation. Small inaccuracies can result in large doses of radiation to healthy tissue.

Lessons learned from 1,000 patients

The SBRT program in MD Anderson’s Radiation Treatment Center recently treated its 1,000th patient. As more patients have been treated, physicians have been able to adjust doses and treatment plans to ensure a balance of tumor control and patient safety.

Dr. Chang said that conventional radiation therapy doses are limited to 70–80 Gy because patients cannot tolerate higher doses without significant toxicity. However, the primary advantage of SBRT is that it can deliver higher doses to the tumor because of the greater precision and number of beam angles. This difference in dosage is the key difference between conventional radiation therapy, which is limited to a local control rate of about 50%, and SBRT, which has a 98% local control rate.

Dr. Chang said, “For SBRT, we initially used a biologically effective dose of 88 Gy. As we began escalating doses, we got improved control, but we didn’t achieve our 98% control rate until we used biologically effective doses above 100 Gy to the planning target volume.” Despite these high radiation doses, fewer than 5% of patients treated with SBRT for lung cancer experience severe toxicities to surrounding tissues when SBRT plans are well conducted.

Photo: Dr. Joe Chang
Dr. Joe Chang (left) helps lung cancer patient Barbara Pool prepare for stereotactic body radiation therapy.
Image: 4-dimensional computed tomography
Treatment plans for stereotactic body radiation therapy are developed using 4-dimensional volumetric imaging, which accounts for the motion caused by respiration.

The sensitivity of some of these surrounding tissues was unknown when SBRT was first used to treat lung cancer in 2004. For instance, the chest wall can tolerate only a certain radiation dose before patients begin experiencing severe rashes and pain. The same is true of the bronchial tree and brachial plexus, although each tissue’s dose tolerance is unique. However, these toxicities did not develop until very high doses were used, and they could typically be addressed by adjusting treatment plans.

The other major adjustment that had to be made was the assessment method used after treatment. Although up to 20% of post-SBRT CT studies show worsening consolidation—which sometimes indicates local recurrence—positron emission tomography (PET) and biopsies done to confirm these findings are often negative. Dr. Chang and his colleagues have found PET to be much more accurate for post-SBRT assessment than CT, but only beginning 6 months after treatment. By comparison, the effectiveness of conventional radiation therapy can be assessed by PET 3 months after treatment.

Finally, and perhaps most importantly, the physicians in the Radiation Treatment Center have found that SBRT can be used to treat patients with recurrent lung cancer—including some previously treated with conventional radiation therapy—or other cancers that have metastasized to the lung. Many of these cancers were previously believed incurable, but SBRT shows the same local control rate of 98%, and some of these patients are even cured of their cancers. Chang and his colleagues have developed a model to predict the toxicity of SBRT in patients who were previously treated with radiation therapy.

Trends in SBRT

National hospital surveys have shown a sizable leap in SBRT usage in recent years. Similarly, MD Anderson’s SBRT numbers have in creased substantially. In 2004, when the hospital first acquired the equipment, it was used to treat only about 30 patients. Now, the Radiation Treatment Center treats nearly 240 patients a year using SBRT. This number may further increase because of the Lung Cancer Screening Program, which was launched in the summer of 2011.

Because of the success of SBRT for lung cancer, there are proposals that it can be considered a cure for early-stage lung cancer. Previously, the only known cure for such cancer was surgical resection, which can cause significant surgical complications and remove vital tissue from the lung itself.

Because of its success against lung cancer, SBRT also has been studied as a treatment for other cancers, including those of the liver, spine, and pancreas. Many cancer types metastasize to the lung, and there are initial results showing that SBRT can improve local control of these metastases. However, it is not yet known whether this local control can improve survival rates. Dr. Chang and others are currently investigating this possibility.

A Patient’s Perspective

Barbara Pool, the 1,000th patient to receive stereotactic body radiation therapy (SBRT) for lung cancer at MD Anderson’s Radiation Treatment Center, described her experience at her first treatment session. “First, I came in for some preliminary scans, then I came in last week for some treatment planning CT scans, and now I’m here for my first treatment,” she said. These scans were used to design her treatment plan and construct a custom treatment couch.

“Dr. Chang told me that I only need four treatments,” Mrs. Pool said. “The doctors and nurses were very good about explaining everything; they’re really thorough and very nice, and they made sure I understood the schedule and plan before I was treated.” SBRT is a complex procedure that uses advanced equipment, and it is critical that the patient understands his or her own treatment protocol and the possible risks and benefits of it.

Mrs. Pool said that she was confident and calm during her treatment because it was clear that all of the staff treating her understood and could clearly explain what was happening.

Mrs. Pool advises other lung cancer patients to seek treatment at a large cancer center with an experienced staff. She also advises anyone who smokes to quit, no matter how hard it is, because the alternative may be lung cancer. She is proud to have convinced her two granddaughters to stop smoking.

For more information, call the Thoracic Care Center at 713-792-6110 or visit www.mdanderson.org/radiationtreatmentcenter.

FURTHER READING

Chang JY, Liu H, Balter P, et al. Clinical outcome and predictors of survival and pneumonitis after stereotactic ablative radiotherapy for stage I non-small cell lung cancer. Radiat Oncol 2012;7:152.

Zhang X, Liu H, Balter P, et al. Positron emission tomography for assessing local failure after stereotactic body radiotherapy for non-small-cell lung cancer. Int J Radiat Oncol Biol Phys 2012;83:1558-1565.

Liu H, Zhang X, Vinogradskiy YY, et al. Predicting radiation pneumonitis after stereotactic ablative radiation therapy in patients previously treated with conventional thoracic radiation therapy. Int J Radiat Oncol Biol Phys 2012;27. [Epub ahead of print]

Other articles in OncoLog, September 2012 issue:

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