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RPM Respiratory Gating to Protect Women's Hearts During Treatment for Left-Sided Breast Cancer

RPM Respiratory Gating to Protect Women's Hearts During Treatment for Left-Sided Breast Cancer

According to Dr. Francine Halberg, radiation oncologist at the Marin Cancer Institute in Greenbrae, California, standard techniques for treating left-sided breast cancer can place too much heart tissue at risk. "For some women, when we use standard breast tangent techniques, a fair bit of cardiac volume gets encompassed in the treatment field," she says. "At Marin Cancer Institute, we have made a commitment to our breast cancer patients to no longer treat their hearts."

Recently, Dr. Halberg and her colleagues began using Real-Time Position Management (RPM)™ Respiratory Gating technology from Varian Medical Systems to minimize dose to the heart when treating some of their left-sided breast cancer patients with external-beam radiation therapy.

Dr. Halberg begins by evaluating patients at the time of treatment planning, to see if they might benefit from respiratory gating. Using fluoroscopy, she checks to see if taking a deep breath moves the patient's breast up and away from the heart without putting extra lung tissue at risk.

"It turns out to be a function of breast geometry," she reports. "With smaller-breasted women, the normal tangential beam usually encompasses the breast, passing just below the ribs without affecting deeper normal tissues. However, for some large-breasted women whose breasts flatten and wrap around the ribcage, treating the entire breast can involve treating a significant portion of the heart and lungs. While not all patients will realize a benefit from respiratory gating, there are some for whom it makes an enormous difference."

Halberg's normal protocol for breast cancer has been to deliver standard tangential beam irradiation with or without blocking to shield the heart. If the cardiac silhouette is in the radiation field, then some of the medial breast is treated with an electron beam that is matched to the breast tangents. "That effectively wraps the dose around the chest, avoiding the heart," she said. "However, there are drawbacks to this approach. Some women get a significant skin reaction in the electron field, and it's a cumbersome technique that strains departmental resources."

RPM respiratory gating enables clinicians to compensate for normal respiratory motion during radiation treatments. The system uses an infrared camera to track a passive marker block placed on the patient's chest or abdomen. In the case of Halberg's breast cancer patients, this block is taped to the bottom of the patient's xiphoid process. The system then converts the tracked motion into a respiratory waveform that characterizes the patient's normal breathing.

Halberg then selects the optimal phase of the breathing cycle for "gating" the radiation beam on and off. "Respiratory gating protocols for lung cancer usually seek to deliver treatment at the point of the patient's maximum exhalation," Halberg explains. "That's a very stable point; for most patients it is very consistent relative to other parts of the breathing cycle. However, for treating breast cancer, we're looking for the farthest inhalation, because that's when the breast moves furthest from the heart," she says.

Case study

One of Halberg's respiratory gating patients was a 41-year old woman who presented with a mass that she could feel in her left breast, but that had not shown up on mammograms. A needle aspiration revealed cancer, and so a lumpectomy was performed to remove a 1.5 cm tumor with wide clear margins around it. No lymph nodes were involved.

This patient received four cycles of an adriamycin-based chemotherapy, which is cardiotoxic. "That gave us another incentive to do whatever we could to preserve her cardiac tissue during radiation therapy," Halberg says. "We could immediately see that we were going to catch a large volume of heart in the treatment fields using standard breast tangent techniques. Instead, we decided to treat with respiratory gating. Her deep breath moved the heart out of the radiation field. In addition, we felt that not using an electron field would produce a better cosmetic result."

Halberg explained to the patient how respiratory gating would work, taking care to let her know that it is not standard treatment. "Although respiratory gating is not a standard procedure for treating breast cancer, when we explain it to patients they invariably want to be evaluated for it," she said. "And those who are eligible want to take advantage of it, even though there is a learning curve involved. They often like the idea of being positively involved in their own care." Halberg and her staff teach these patients to take consistent deep breaths and to hold their breath for about 30 seconds at a specific point in the breathing cycle.

Halberg treated the patient to 4680 centigray (cGy) using normal breast tangents, "gated" to deep inspiration, and then brought the tumor bed up to 6,000 cGy, using an enface electron field boost.

This patient had a treatment planning CT scan with normal shallow breathing and using the RPM respiratory gating system at the same phase of deep inhalation. "This allowed us to compare the amount of heart in the treatment field with and without respiratory gating. With normal techniques, 28 cubic centimeters of the left ventricle of the heart would have received 3000 rads. With respiratory gating, we could bring that down to 2 cubic centimeters. Treating with normal techniques, the 4,000 rad isodose curve would have affected 14 cubic centimeters of heart; respiratory gating could bring that down to 0 and we could avoid treating her heart altogether," she said.

According to Halberg, an added benefit of RPM respiratory gating is that it has increased her department's efficiency. "When compared to the time it takes to plan for breast tangent fields matched to an electron field, this approach involves less work on the part of the treatment planning personnel and less time for daily treatment," she said. "Our radiation therapists had an easy time learning to use the system. In terms of daily treatment delivery, it adds only a few minutes to the time it takes to deliver the standard breast tangent treatment."

 

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