Jon Strasser, MD
Dayee Jacob, MS
Breast conservation therapy with whole breast radiation may pose a significant problem for breast cancer patients with cardiac implants, particularly if the implant sits within the radiation field. If additional surgery is not performed to relocate the implant, radiation may not be able to be safely delivered without causing significant damage to the device.
To preserve the option of BCT for these patients and eliminate the need for multiple surgeries, physicians at Christiana Care Health System in Newark, Delaware, are pioneering the use of accelerated partial breast irradiation with the SAVI® applicator for eligible patients with cardiac implants. Radiation oncologist Jon Strasser, MD, and senior medical physicist Dayee Jacob, MS discuss their experience with this novel application of APBI with the SAVI device.
What are the radiation therapy treatment options for breast cancer patients with implantable cardiac devices, i.e. pacemakers, defibrillators, etc.?
Jon Strasser, MD:
Patients with implantable cardiac devices certainly have every option open to them with respect to radiation. The problem is when the device is within the radiation field. Often these devices are in the chest, just below the collarbone. Standard tangential fields will often be passing through at least a portion of the pacemaker, which has the potential to be fairly damaging to the device.
With traditional whole breast radiation, our best option is to consider having the cardiologist move the device to another location in the contralateral chest or to the abdomen. If the patient doesn’t want to move the pacemaker, we would then have to contemplate a mastectomy, as the risk to the device may be too significant to safely deliver radiation as part of breast conservation therapy.
However, if the patient is a candidate for accelerated partial breast irradiation, we believe use of the SAVI applicator can limit the radiation dose to the pacemaker below manufacturer specifications.
What advantages does APBI offer over external beam for these patients?
For patients who are potentially eligible for partial breast irradiation, the idea of leaving the pacemaker in place and using APBI to treat that breast is a great advantage. It’s a big time savings for the patient, as it spares them from an additional surgery.
We first looked at this question about six months ago when we had a patient who was sent to us by a referring breast surgeon who knew the pacemaker would have to be moved. It was a small, very favorable looking cancer. We pre-scanned her to see how far the pacemaker would be from the lumpectomy bed. We felt it was far enough away, so we created a mock plan and felt comfortable that we would be able to deliver sufficient dose to the tumor bed without exceeding the recommended manufacturer dose to the pacemaker.
We think this is quite a novel use for APBI. Pacemakers are more prevalent in older patients, which is also the target population for APBI. This is a way of providing an effective cancer treatment without having to do multiple procedures or worry about moving the pacemaker. It’s a new use for partial breast radiation that many people have probably never considered, but for appropriate patients it may be an inherently better option than traditional radiation.
What specific advantages does the SAVI applicator provide in treating these patients with APBI?
With SAVI, we have the option for differential source dwell time loading in the device’s multiple catheters to reduce dose to the pacemaker. The SAVI struts can be expanded to encompass the cavity, and we have the ability to deactivate the catheter that is contributing the most dose to the pacemaker and compensate for that in the other channels. With seven or more catheters to work with, you can very easily turn off one or two complete channels and still come up with a very good plan.
Balloon catheters aren’t as flexible and don’t have the same dose modulation capabilities of SAVI. Balloons have only three or four channels that are more centrally located, so removing just one can have a more significant impact on the quality of the plan.
The typical tolerance of a pacemaker is about 200 cGy. In the case of APBI, this is approximately six percent of the total prescribed dose, and it can be easily accomplished with a multi-channel APBI device like SAVI.
The dose customization is one of the reasons we adopted SAVI in the first place, as it enables us to do a better job of shaping the dose in other areas where we try to minimize exposure, such as near the skin, nipple or chest well. That’s the same principle we apply to treating a pacemaker – we just consider it another avoidance structure.
How do you go about creating a treatment plan for a pacemaker patient receiving radiation with SAVI?
When contemplating APBI for a patient with a nearby pacemaker, we actually bring them in for what we call a pre-plan prior to the surgeon placing the SAVI device. The patient undergoes a CT scan and we contour the pacemaker and the cavity. We simulate a multichannel interstitial treatment for virtual use in the patient so we can determine whether or not we can achieve a reasonable dose to the cavity without delivering high doses to the pacemaker. If they pass that step, the patient is sent back to the surgeon to have the device implanted and then returns for the actual treatment planning. We also advise the surgeon regarding the optimal placement of the SAVI device, based on the pre-insertion CT study.
Do you have any strategies in place to minimize the risk of pacemaker malfunction when delivering radiation therapy to these patients?
We have a very strict departmental policy in place to identify patients with pacemakers. One of the very first questions as part of any patient’s intake is if they have a pacemaker. If so, our policy requires that the patient has an EKG and the device is interrogated by the cardiologist prior to beginning treatment to be sure it is functioning properly. Once we begin treatment, we typically take a TLD measurement to monitor the surface dose on the pacemaker. We also put additional lead shielding over the pacemaker to prevent external scatter. We have the cardiologist interrogate the device once or twice during the course of treatment and once again at the end of treatment just to make sure that it’s functioning according to the appropriate specifications.
We’re pretty aggressive about making sure that we haven’t caused any damage. To us, it’s an important enough issue that we make decisions and treatment plans based around the device and determine what risks we need to look out for. None of us want to be held responsible for affecting a device which may be critically controlling someone’s heart rhythm.
Do the pacemaker manufacturers offer any published guidelines for dose limits to their devices? What type of support should a treating facility expect from the pacemaker manufacturer?
There are no solid guidelines as to how much the devices can tolerate. All the pacemaker manufacturers list the dose that they think the devices are capable of receiving without a problem. Most of them recommend staying below 200 cGy, which is what we typically aim for. However, Medtronic has verified that one of their devices has a higher threshold and can actually go up to 500 cGy.
Overall, the manufacturers have been supportive in working with us. We recently used SAVI to treat a patient with a Medtronic pacemaker. The local Medtronic representative was here on the first day to check the patient’s pacemaker before the treatment, and then he returned on the last day after the final treatment fraction to do a repeat performance evaluation of the device. The pacemaker was found to be functioning as expected without any change in the baseline reference values of the parameters.
Dr. Strasser is board certified in radiation oncology and is a Diplomate of the American Board of Radiology. His primary clinical interests include breast, gastrointestinal, gynecologic, thoracic, and pediatric malignancies. He has specialty training in Intensity Modulated Radiation Therapy (IMRT) and brachytherapy.
Mr. Jacob is a Senior Medical Physicist with the Christiana Care Health System and is board certified in Therapeutic Radiologic Physics by the American Board of Radiology. He directs a busy HDR brachytherapy program at the Christiana Hospital. His interests include HDR brachytherapy treatment optimization techniques and have extensive experience in gyn, breast and prostate HDR applications.