Palliation Radiation Therapy of the Spinal Cord

Anatomy and Physiology

Most tumors in adults that compress the spinal cord involve the vertebral column and thus compress the spinal cord at the anterior aspect of the cord. Children, in contrast, usually experience compression from a paravertebral mass, which results in lateral or posterior compression.[9]

Pain relief is not immediate in palliative radiation therapy, as cells do not die until the next cell division. Thus, symptom relief may take several days after initiation of treatment.[5]

The principle underlying radiation therapy is to target malignant cells by hindering the cellular response to DNA damage and or replication. Radiation therapy is applied over the course of multiple treatments to reduce toxicity to the normal cells and target rapidly proliferating cells. Radiation acts in two ways:

• 1: To directly effectuate both single and double-stranded breaks to the DNA structure
• 2: To indirectly produce free radicals derived from the ionization of the water component of cells.[10]

Radiation therapy also benefits from the activation of the NFκB transcription pathway, which increases the radiosensitivity of the cancer cells.[11]

Indications

There is a wide range of indications for palliative radiation therapy. Alleviation of pain is the most obvious indication as metastases, such as those to the vertebrae or proximal femur, may result in significant and debilitating pain. Additionally, it also may be a preventative measure of incurring a pathologic fracture in the spine or extremities. Radiation therapy may be used to control a fungating or ulcerative mass. In highly vascular tumors, radiation therapy can be used to help control bleeding. Finally, although less utilized, radiation therapy can be used in oncologic emergencies such as spinal cord or brain compression.[5]

Preparation

In many tumors and particularly those that occur in the spine, magnetic resonance imaging (MRI) is imperative to delineate the spinal cord and to identify the volume of the target mass. A computed tomography (CT) myelogram, while less ideal, may be used in patients with previous spine hardware placement due to image distortion and artifact, which occur in a metal obstructed MRI.[12] 

Patients must be immobilized with a rigid body immobilization device prior to therapy to minimize intrafraction motion or small patient movements and to increase the precision of radiation delivery.[13] A plan must be made to provide the appropriate radiation dose for therapeutic effect while sparing surrounding organs at risk, such as the spinal cord and esophagus. Finally, it is important to use the appropriate Gantry angles to optimize treatment.[5]

Technique or Treatment

Radiation therapy is delivered via high-energy x-rays targeted at the disease site using linear accelerators. In contrast to curative radiotherapy delivered in multiple small doses, palliative treatments require lower total doses, focusing on relieving symptoms derived from the mass effect, particularly in tumors on the spinal cord. By utilizing largely daily doses, practitioners can decrease the overall treatment burden and shorten the course of therapy.

New techniques such as targeted stereotactic radiotherapy will deliver more focused therapy to a smaller area and thus minimize damage to surrounding healthy tissue.[14]

Complications

The underlying premise of radiation therapy is to expose the body to treatments that destroy or impede malignant cells more efficiently than healthy cells. Therefore, there will inevitably be damage to nearby cells. In radiation therapy of the spine, there is an obvious concern for damage to the spinal cord itself. This risk should be minimized by targeted stereotactic treatments and ample pre-procedural planning.

Research into radiation therapy reveals that damage is not limited to cells in close proximity to the target cells. Gap-junction or cytokine-mediated cellular toxicity can lead to damage in cells far from the targeted radiation track. This phenomenon is coined the "bystander effect." The bystander effect results in genomic instabilities that lead to cell death, the formation of micronuclei, sister chromatid exchanges, and a delay in the cell cycle of non-irradiated cells. The outcome of this bystander damage can lead to unintended consequences in peripheral healthy tissues.[10][15][16]

There is debate on whether radiation therapy should be given prior to palliative surgery or after in cases where resection is indicated. Post-operative radiation therapy is given in a higher dose due to a larger treatment field, resulting in a higher incidence of local fibrosis and lymphedema. Preoperative radiation therapy, in contrast, has a higher risk of wound complications than post-operative radiation.[17]

Clinical Significance

Spinal cord compression that results in neurological deficits is a medical emergency that necessitates intervention with radiation therapy and steroids to preserve neurological function.[5]

When considering management for metastatic lesions to the spine, consultation with an orthopedic or neurological surgeon who specializes in spinal stabilization is indicated to determine the mechanical stability of the spine. The "Spinal Instability Neoplastic Score" or "SINS" score is a scoring system that is the most widely accepted instrument for assessing the stability of the spine with metastatic lesions. The SINS scoring system utilizes six criteria to determine a score that predicts the need for surgical stabilization of the spine. The SINS scoring system is based on:

1. The location of the affected vertebral segment in the spine
2. The presence of mechanical pain
3. The bone lesion quality
4. The presence of deformity or malalignment at the affected segment
5. The extent of vertebral body involvement
6. The presence or absence of posterior element involvement

A total score is calculated by the sum of the individual scores in each of the above categories and results in being able to classify all lesions as either "stable," "potentially unstable, possibly requiring surgical stabilization," and "unstable requiring surgical stabilization."[18]

Enhancing Healthcare Team Outcomes

The management of malignancy is complex, as cancer can develop from innumerable sources, and each type having unique patterns of growth and or metastatic progression. Different malignancies have a propensity to metastasize to different tissues, and thus each may respond differently to various treatment strategies. Nearly half of the patients diagnosed with malignancies will undergo radiation treatment, and almost half of those receive some form of palliative radiotherapy. Therefore, practitioners need to have a basic understanding of the principles of palliative radiation therapy to appropriately counsel and guide patients through what is undoubtedly a difficult time in their life.

Patients who require palliative radiation therapy to the spinal cord must be treated by an interprofessional healthcare team with a familiarity with the complexities of palliative cancer treatment. Radiation oncologists should lead the way in the particulars of palliative spine radiotherapy in conjunction with medical clinicians, oncology nurses,  and all associated healthcare staff. An orthopedic or neurosurgical spine specialist should be involved to assess the stability of the spine using methods discussed in order to determine if surgical stabilization is indicated for an individual patient. Finally, surgical and medical oncologists can serve in an advisory role for all adjunct treatments to maximize patient outcomes. This interprofessional healthcare model will optimize patient outcomes when using radiation palliation therapy on spinal tumors while minimizing potential adverse events. [Level 5]