**Abstract**

Advancements in the treatment of systemic cancer have improved life expectancy in cancer patients and consequently the incidence of spinal metastasis. Traditionally, open spinal approaches combined with cEBRT (conventional external beam radiation therapy) allowed for local tumor control as well as stabilization and decompression of the spine and neural elements, but these larger operations can be fraught with one complications and delayed healing as well as additional morbidity. Recently, minimally invasive spine techniques are becoming increasingly popular in the treatment of spinal metastasis for many reasons, including smaller incisions with less perioperative complications and potential for expedited time to radiation therapy. These techniques include kyphoplasty with radiofrequency ablation, percutaneous stabilization, laminectomy, and epidural tumor resection through tubular retractors, as well as minimally invasive corpectomy. These techniques combined with highly conformal stereotactic radiosurgery have led to the advent of separation surgery, which allows for decompression of neural elements while creating space between neural elements and the tumor so adequate radiation may be delivered, improving local tumor control. The versatility of these minimally invasive techniques has significantly improved the modern management of metastatic disease of the spine by protecting and restoring the patient's quality of life while allowing them to quickly resume radiation and systemic treatment.

**Keywords:** minimally invasive spine, spinal tumors, spinal metastasis, separation surgery, spinal stabilization

### **1. Introduction**

The spine is the most common site of bony metastases [1]. Significant improvements in treatment modalities in the form of chemotherapy, immunotherapy, and radiation therapy have led to increased life expectancy for cancer patients [2]. Consequently, the incidence of metastatic cancer has been increasing. Twenty to seventy percent of patients with metastatic cancer are expected to develop spinal metastases during the course of their disease [3, 4]. The increased incidence of spinal metastases has increased the need for surgical treatment of its complications including symptomatic cord compression and mechanical instability. The goal of neurosurgical treatment includes addressing mechanical instability, correcting deformity, halting or reversing neurologic deficit, and improving pain and quality of life [5]. Traditionally, open surgical approaches have been used for surgical management of spinal metastatic disease. Through the advances made in minimally invasive spine surgery (MISS) for trauma and deformity, MISS for tumors

is becoming more common due to decreased perioperative morbidity in cancer patients with similar efficacy to traditional open approaches.

## **2. Evaluation of patients with spinal metastases**

Patients with metastatic spinal cancer often have significant comorbidities and complex clinical scenarios that require multidisciplinary evaluation and treatment. The NOMS (neurologic, oncologic, mechanical, systemic) framework, developed at the Memorial Sloan-Kettering Cancer Center, assesses neurologic, oncologic, mechanical, and systemic factors during the decision-making process for the treatment of spinal tumors across multiple specialties. The oncologic factors include the predicted responses and durability of available treatment modalities including chemotherapy, immunotherapy, and radiotherapy [5, 6]. The systemic consideration predicts both the patient's ability to tolerate multimodal treatments and overall survival based on the grade and stage of disease as well as the overall health of the patient [5]. The neurologic and mechanical criteria are of particular interest to the neurosurgeon involved in the patient's care.

The neurologic component of the NOMS framework assesses the presence of myelopathy or radiculopathy and is related to epidural compression of the spinal cord and nerve roots. It is estimated that symptomatic cord compression occurs in up to 20% of patients with metastatic cancer and may be the initial symptom in 5–10% [7, 8]. Cord compression above the conus may present with myelopathy, weakness, numbness, urinary urgency. Below the level of the conus, compression of the cauda equina may present with lower motor neuron symptoms of unilateral or bilateral motor weakness, radiculopathy, numbness, or urinary retention. Such symptoms require urgent neurosurgical intervention to either stabilize or improve neurologic function.

The mechanical criteria concerns the stability of the vertebral column affected by metastatic tumor and can be further analyzed by the SINS (spinal instability neoplastic score) criteria [9]. The SINS criteria assess the location of the spine affected including junctional areas, the presence of mechanical pain, type of bony lesions, spinal alignment, amount of vertebral body collapse, and posterolateral involvement [9]. Scores of 0–6 are deemed stable, whereas 7–12 are indeterminate but suggest the possibility of instability, 13–18 are unstable. SINS scores of 7–18 warrant neurosurgical consultation. Overall, the sensitivity and specificity of the SINS criteria for potentially unstable and unstable spinal lesions are 95.7% and 79.5%, respectively [9].

### **3. Traditional/open operative intervention**

Historically, symptomatic metastatic spine disease has been treated with open surgical approaches. Traditional open approaches provide adequate visualization of tumor as well as spinal anatomy. The wide exposures allow for sufficient decompression of the spinal cord and nerve roots, stabilization for mechanical instability, and the opportunity for gross total resection, if indicated. However, the larger incisions with open approaches often lead to prolonged wound healing, which may delay postoperative chemotherapy and radiation. Larger surgical incisions also involve greater blood loss, which is concerning given the high rate of bone marrow toxicity and anemia in metastatic cancer patients, essentially all of whom exhibit preoperative anemia. Larger incisions also have higher incidence of wound breakdown and infections. Greater tissue damage contributes

to greater postoperative pain, opioid requirements, and poor quality of life in patients with limited life expectancy.
