Advanced Imaging in Spine Radiation Therapy: A Patient's Guide to CT and MRI for Targeted Treatment

Table of Contents

• Introduction: Understanding Spinal Metastases
• Conventional Radiotherapy Versus Stereotactic Body Radiotherapy
• Imaging's Role in Patient Selection for SBRT
• Pre-SBRT Imaging: Preparing for Treatment
• Treatment Planning Imaging: Mapping Precisely
• Source Information

Introduction: Understanding Spinal Metastases

The spine is the third most common site where cancer spreads (metastasizes) after the lungs and liver. Spinal metastases most frequently originate from lung, prostate, and breast cancers, though virtually any cancer can spread to the spine. Approximately 70% of patients with breast and prostate cancer develop spinal metastases according to post-mortem studies.

This high rate of spinal involvement occurs because vertebral bones contain abundant red marrow and have unique blood vessel networks that facilitate cancer spread through the bloodstream. When cancer spreads to the spine, it can cause debilitating back pain and impair quality of life through several mechanisms.

Pain may result from pathological vertebral compression fractures (VCFs) where weakened bones collapse, or from compressed nerves due to epidural tumor spread. In up to 20% of cases, metastatic epidural spinal cord compression (MESCC) can occur, which represents an oncologic emergency requiring immediate treatment when neurological symptoms are present.

Treatment options for spinal metastases include various radiation therapy approaches, surgery (both open and minimally invasive), neurointerventional procedures, and combination treatments—particularly decompressive surgery with stabilization followed by targeted radiotherapy.

Conventional Radiotherapy Versus Stereotactic Body Radiotherapy

Radiation therapy benefits most patients with spinal metastases, with few exceptions. Research shows that a combination of surgery and radiation provides better clinical outcomes than radiation alone in selected patients with metastatic epidural spinal cord compression (MESCC).

Stereotactic body radiotherapy (SBRT) has become an established treatment for spinal metastases, with growing evidence supporting its effectiveness for local symptom relief and disease control. Nearly 50% of radiation oncologists now incorporate SBRT into their daily practice.

SBRT utilizes advanced radiation techniques to deliver ablative (tumor-destroying) radiation doses to the target tissue while minimizing radiation exposure to surrounding normal organs. This approach has demonstrated promising results as:

• Primary treatment for new diagnoses
• Treatment after conventional external-beam radiotherapy (cEBRT) failures
• Treatment for residual tumor after surgery
• Management of benign spinal tumors

SBRT achieves impressive outcomes across different treatment scenarios:

• Pain response rate: 74.3% of patients experience significant pain relief
• Local control rates:
  • Primary treatment: 80-95% tumor control
  • Post-operative treatment: 70-100% tumor control
  • Re-irradiation treatments: 66-93% tumor control

Despite these excellent results, post-SBRT local recurrences still occur, mostly in the epidural space (the area around the spinal cord). The risk of local failure correlates strongly with the pre-SBRT Epidural Spinal Cord Compression (ESCC) score:

• No epidural disease (grade 0): 5% local failure rate
• Low-grade epidural disease (grade 1a–c): 19% local failure rate
• High-grade epidural disease (grade 2/3): 30% local failure rate

These higher failure rates may occur because radiation oncologists must underdose epidural disease to respect spinal cord safety limits, or because metastases with epidural extension may be biologically more aggressive.

Conventional external-beam radiotherapy (cEBRT) typically delivers up to 5 Gy per fraction, while SBRT delivers higher biologically effective doses in 1 to 5 fractions to a more precisely defined target area. The distinctive geometry and steep radiation dose gradients in SBRT contribute significantly to its therapeutic effects compared to conventional radiotherapy.

While these steep dose gradients spare areas outside the treatment field, positional variations as small as one millimeter may result in adverse effects on normal critical tissues. For this reason, cEBRT remains preferred for patients with poor overall health status and prognosis, including those with symptomatic widespread metastatic disease, very low functional scores (Karnofsky performance status ≤40), or life expectancy of less than 2 months.

Imaging's Role in Patient Selection for SBRT

After evaluating a patient's overall prognosis and disease burden, metastatic lesions that are candidates for radiation therapy are evaluated using the MNOP algorithm, which assesses:

• Mechanical Stability
• Neurological Risk
• Oncological Parameters
• Preferred Treatment

To assess spinal stability, clinicians use the Spinal Instability Neoplastic Score (SINS) criteria, which includes six imaging and clinical features scored from 0-18:

• Pain type (none, non-mechanical, or mechanical)
• Location of the spinal segment involved
• Bone density (blastic, mixed, or lytic)
• Vertebral involvement extent
• Posterolateral involvement (unilateral or bilateral)
• Spinal alignment

For SINS scores of 12 or above, spine surgeons should be consulted about the need for stabilizing intervention before SBRT. The specific role of surgery for intermediate SINS scores remains an active area of investigation, but consultation is advised.

Neurological risk is assessed using the Epidural Spinal Cord Compression (ESCC) score, an imaging-based six-category scale that measures epidural metastatic disease resulting in spinal cord encroachment. This scoring system helps determine whether SBRT can be used as primary treatment with limited risk for spinal cord injury.

The ESCC grading system includes:

• Grade 0: No gross epidural disease
• Grade 1a: Tumor spread to epidural space without thecal sac indentation
• Grade 1b: Tumor causing thecal sac indentation without spinal cord abutment
• Grade 1c: Spinal cord abutment without compression
• Grade 2: Spinal cord compression without complete cerebrospinal fluid space obliteration
• Grade 3: Spinal cord compression with complete cerebrospinal fluid space obliteration

When combined with clinical and histopathological features, the ESCC score provides useful guidance for spinal metastasis therapy. ESCC scores of ≤1b represent optimal candidates for primary SBRT. The optimal management of ESCC 1c-3 cases is specific to each situation, while spines with ESCC scores of 2 or 3 may undergo surgical intervention before SBRT to avoid neural complications, depending on tumor radiosensitivity.

For malignant cancers highly sensitive to radiation therapy and/or systemic therapies (such as lymphoma and myeloma), conventional external-beam radiotherapy (cEBRT) is recommended. Conversely, radioresistant tumor types (sarcoma, melanoma, and renal cell carcinoma) may benefit greatly from ablative SBRT doses, especially when tumor burden is limited.

Patients who cannot undergo MRI for pre-SBRT planning or radiation therapy guidance are not good candidates for SBRT. In summary, SBRT for spinal metastases is preferred for patients with good overall prognosis, including:

• Limited systemic disease (oligometastatic disease)
• Small site of spine involvement (limited to 1-3 contiguous spinal/paraspinal levels)
• Limited epidural disease (as graded by the ESCC score)
• Relatively stable spinal column (as defined by SINS criteria)

In clinical practice, radiologists report both SINS and ESCC scores for all spinal metastasis cross-sectional exams to inform treatment urgency and approaches. For SINS, they generally report the level with the highest score when multiple levels are involved, while for ESCC, every level with epidural disease is reported.

Pre-SBRT Imaging: Preparing for Treatment

Treatment planning with pre-SBRT imaging and tumor delineation is crucial to confine the ablative radiation doses precisely to the target volume and prevent inadvertent overdosing of normal tissues. This requires precise and reproducible patient positioning.

For example, spinal cord overdosage can occur with very small patient movements (as small as 1 mm) or if an epidural tumor touches or compresses the spinal cord. Radiation-induced myelitis (spinal cord inflammation), although rare at 0.4%, is one of the most feared and debilitating late post-SBRT complications.

At most institutions, patients undergo pre-SBRT imaging within one week of treatment, preferably as close as possible to the treatment date. The SPINE response assessment in Neuro-Oncology (SPINO) group recommends using both CT and conventional MRI in pre-SBRT imaging.

CT scanning is superior for delineating bony remodeling or erosion by tumor and is needed for SINS scoring, but has limited accuracy in delineating soft tissue and bone marrow tumor infiltration. SPINO consensus guidelines recommend CT slice thickness of at least ≤2 mm, preferably ≤1 mm, for treatment planning.

MRI is the gold standard imaging modality for detecting and characterizing spinal metastases and is recommended for pre-SBRT imaging. Both for cases of single isolated spinal metastases and those with multiple metastases, full spine MRI is recommended, as finding additional spinal lesions is not uncommon.

Both 1.5-Tesla and 3-Tesla MRI scanners are routinely used in clinical practice. The recommended MRI sequences for SBRT planning are volumetric non-contrast T1-weighted and T2-weighted imaging. Sagittal T1-weighted and STIR sequences are usually the most informative unenhanced MR sequences for metastasis detection.

Three-dimensional isotropic volumetric MRI acquisitions facilitate fusion with CT for pre-SBRT planning. With this technology, multiplanar reconstructions can be performed without image quality degradation. Recent 3-D MRI acceleration methods, such as compressed sensing and AI de-noising reconstructions, have shown the image quality advantages of 3-D over 2-D MRI for SBRT planning.

Treatment Planning Imaging: Mapping Precisely

The SPINO group advises incorporating both CT and conventional MRI in treatment planning imaging for SBRT to ensure comprehensive assessment of both bone and soft tissue involvement. To keep radiation to critical tissues at tolerable levels, organs at risk must be accurately delineated.

Treatment planning involves defining several specific volumes:

• Gross Tumor Volume (GTV): Consists of the metastatic tumor and its epidural/paraspinal extensions as they appear on MRI
• Clinical Target Volume (CTV): Includes the entire anatomic compartment containing the GTV plus immediately adjacent anatomic compartments to include potentially viable microscopic invasion
• Planning Target Volume (PTV): Adds an additional margin of normal tissue to the CTV to compensate for motion, including cardiorespiratory and patient bulk motion

Including an entire diseased vertebral body in the CTV has been associated with better post-SBRT local control in spinal metastases. Different cancer centers consider a range of 0 to 3 mm expansion for the PTV.

In post-operative patients with tumor recurrence or residue, the CTV should include the full area of pre-treatment bony and epidural tumor involvement, as well as the surrounding bony structures at risk of microscopic tumor infiltration. The delineation of the CTV in the post-operative setting can be guided according to patterns of epidural failure observed in previous cases.

SBRT requires accurate patient and target volume positioning and localization to ensure complete CTV coverage while decreasing radiation to organs at risk. Patient bulk motion can be addressed by expanding the PTV to 1.5 mm beyond the CTV and using near-rigid body immobilization devices.

Source Information

Original Article Title: The Role of CT and MR Imaging in Stereotactic Body Radiotherapy of the Spine: From Patient Selection and Treatment Planning to Post-Treatment Monitoring

Authors: Javid Azadbakht, Amy Condos, David Haynor, Wende N. Gibbs, Pejman Jabehdar Maralani, Arjun Sahgal, Samuel T. Chao, Matthew C. Foote, John Suh, Eric L. Chang, Matthias Guckenberger, Mahmud Mossa-Basha, Simon S. Lo

Publication: Cancers 2024, 16(21), 3692

Note: This patient-friendly article is based on peer-reviewed research and aims to make complex medical information accessible to educated patients and their families.