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| From OncoLog, September 2012, Vol. 57, No. 9 |
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Stereotactic Body Radiation Therapy Achieves High Control Rates in Lung Cancer
By Zach Bohannan
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| 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.
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| Dr. Joe Chang (left) helps lung cancer patient Barbara Pool prepare for stereotactic body radiation therapy. |
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| 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]
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articles in OncoLog, September 2012 issue:
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