Stereotactic radiosurgery (SRS) is a form of external beam radiation therapy that has demonstrated efficacy in managing metastatic tumors of the spine (with or without surgery). The high radiation doses delivered per treatment fraction, along with the precision of delivery, have improved tumor control while preventing excessive toxicity to normal tissues.
The potential for injury to normal tissue with SRS is minimized because the dose is delivered over a relatively large area, with multiple radiation beams intersecting on the tumor. Careful patient immobilization and image guidance reduce uncertainty regarding where the beams are directed.
Unfortunately, a small number of patients experience recurrence following SRS. And while re-irradiation is possible, there is a limit to the amount of radiation that normal tissues can safely receive from the externally directed beams (the bowel, kidneys, esophagus, and spinal cord are examples of structures commonly in the radiation field).
Once these limits are reached, external beam radiation therapy is no longer a safe and effective treatment modality, as the likelihood of injury outweighs the potential benefits of treatment.
Brachytherapy is a technique in which the radiation source is placed within the target, delivering treatment without the need to travel through healthy tissue. With this technique, tumoricidal doses of radiation can be delivered directly to lesions in the vertebral bodies and paraspinal tissues using the radioactive isotope Iridium-192 (192Ir)—a radionuclide often used in high-dose-rate (HDR) treatments for gynecologic and prostate cancers.
By placing multiple catheters in the lesion, a conformal dose can be generated, enabling definitive treatment of the lesion while sparing critical structures within and near the spine such as the spinal cord, cauda equina, bowel, kidney, and esophagus.
A Pilot Study
In 2013 we reported(1) on a small pilot series of patients treated with HDR interstitial brachytherapy using catheters implanted into vertebral or paraspinal lesions in the intraoperative setting under direct visualization, or percutaneously using image guidance. The five patients had all previously received at least two and up to four courses of radiation therapy in the area and were unable to receive additional external beam radiation therapy due to the dose limits of nearby structures, including the esophagus, bowel, kidney, spinal cord, and cauda equina.
In the study, the patients received a median dose of 14 Gy to the lesion using interstitial HDR brachytherapy techniques. In all cases, the treatment was delivered with no brachytherapy-related complications.
At a median followup of 7.2 months, there has been no local progression of the disease on the basis of clinical neurological exam and serial MRI imaging (every two to three months). Three of the five patients reported significant pain relief within one to four weeks (either more than a five-point change on the numeric pain rating scale or complete cessation of opiate pain medications); the other two patients reported a mild decrease in pain symptoms.
Considerations with HDR Interstitial Brachytherapy
While this pilot study has shown promise, we are working to optimize the process.
The current image-guided catheter placement process is very time consuming, as exact placement is critical for providing optimal coverage of the target volume. To address this issue, we are developing techniques using image-guided surgical navigation that might improve catheter placement and reduce overall treatment time.
A prospective feasibility study has been approved by our Institutional Review Board (MSKCC IRB Protocol 12-260) and is currently open for accrual. Eligible patients have recurrent disease in the spine after prior radiation therapy and are unable to receive therapeutic doses of radiation via external-beam techniques.
As the prognosis for patients with metastatic cancer improves and survival times increase, the demands for effective salvage therapies that maintain quality of life will likely increase.
While access and use of SRS has improved local control for spinal lesions, a portion of these patients will recur. For these patients, HDR interstitial brachytherapy is a potentially powerful tool to address the need for an alternative. Treatment is completed in a single session, allowing patients to quickly return to systemic treatment options.
Our hope is that incorporation of image-guided surgical navigation will make this technique accessible to many institutions and further improve our patients’ outcomes and quality of life.