CT/X-ray-Guided Techniques in Vertebral Tumors: Radio-ablation

(1)

Department of Diagnostic and Molecular Imaging, Interventional Radiology and Radiation Therapy, University “Tor Vergata”, Viale Oxford, 81, Rome, 00133, Italy

7.1 Introduction

Primary spinal tumors are relatively rare, estimated around 10% of all cancers interesting spine [1]. The spine is instead the most common site of metastasis in patients with cancer: up to 70% of cancer patients develop secondary spinal disease [2]. New cases of spinal tumors are detected in North America with a rate of incidence near to 18000 patients/year [3].

Spinal metastasis usually arise from tumors of the breast, lung, prostate gland, and hemopoietic tissues (e.g., lymphoma or multiple myeloma). In addition, the spinal lesion represents the first symptomatic manifestation of cancer in 12 to 20% of patients [4, 5].

The dorsal and lumbar segments are the most frequently involved, respectively 70% and 20%, compared to those cervical and sacral; in 30% of cases the disease is multi-segmental [4, 6]. Spinal metastasis, like all spinal tumors, are classified according to their origin, clinical and histological behavior (malignant or benign), and anatomical distribution: secondary lesions are extradural in 94–99% of cases, intramedullary metastases are extremely rare with a rate of 0.5%, while intradural extramedullary tumors account for the remaining percentage [7].

The majority of systemic neoplasms metastasize to the spinal column through hematogenic spread. Vertebral bodies have in fact an intense vascularization especially in their posterior third [6]. Hematogenic spread is possible via arterial emboli to the abundant bone marrow of the vertebral bodies or via retrograde spread through the extradural Batson’s venous plexus [1, 8]. The second mechanism involves mainly the prostatic gland tumors, which have a high incidence of metastasis to the spinal column [8]. Hemopoietic neoplasms as multiple myeloma or lymphoma have a direct contiguous extension of the tumor into the epidural space [1, 8].

Finally tumor spread may be also by seeding through cerebrospinal fluid [1, 8].

Pain without trauma is the initial symptom in the majority of patients. Symptomatic lesions are more frequently in the thoracic region (70%) than in other spinal segments [9]. Therefore, in a patient with history of cancer, the appearance of a sudden and progressive spinal pain is suspicious for spinal metastases. The pain is referred as local and non-radicular; it worsens with rest and at night. This pain is usually elicited by palpation over the spinous process at the level of involvement.

The appearance of radicular symptoms and neurologic deficits, motor sensory or visceral, usually occurs secondarily to the extension of the lesion [6, 9].

Imaging techniques play an important role in the detection of spinal disease. Instrumental diagnosis includes techniques as plain radiography, computed tomography (CT), magnetic resonance (MR), and radionuclide imaging [6].

CT scanning provides a detailed study of the osseous architecture of the spinal axis. Physician detects also data regarding the extent of neoplastic destruction (Fig. 7.1) [6]. MR is the first imaging technique in the evaluation of the soft tissues, therefore is the method of choice for the study of the spinal cord. MR provides essential information for surgical planning, as the epidural and bone marrow tumor infiltration (Fig. 7.2) [10].

Fig. 7.1

An osteolytic lesion located within the D12 vertebral body in a 72-year-old patient with multiple myeloma: CT multiplanar reformation on axial (a), coronal (b), and sagittal (c) plans

Fig. 7.2

MRI of the same patient: sagittal T2 weighted (a), sagittal T1 weighted (b), short-tau inversion recovery (STIR) (c), and axial T2-weighted (d) images show the osteolytic lesion, which caused vertebral collapse

The treatment of symptomatic spinal metastasis has mainly a palliative function. We should always consider the goal of preserving or restoring, even partially, the neurological function [11, 12]. The therapeutic choice has to consider the clinical status of the patients, the presence of a single or multiple painful lesions, and the involvement of the neural structures [12]. While conservative therapies with analgesic and chemotherapy drugs are not always successful, radiotherapy (RT) and surgery are the most used therapeutic options for a radical treatment [11, 12].

RT is particularly effective in the treatment of various radiosensitive histological subtypes of metastasis, such as hemopoietic tumors, small-cell lung carcinoma, and prostate carcinoma [11, 13]. The standard radiation dose schedule consists of 20 to 30 Gy administered in five to ten treatment sessions to the interested area, usually beaming an area of two vertebral bodies; many variations are related to the general status of the patient and the extent of spinal tumor [14, 15].

While in conventional RT the dose is fractioned over the time, stereotactic RT has the rational to focus multiple doses at the same time on the volume of interest, with an important reduction of the radiation exposition for the surrounding soft tissues. The typical dose is 8–20 Gy in one to two sessions. Stereotactic RT has indication in patients when conventional RT has failed or surgery is not applicable [16, 17]. The disadvantage is the need of several re-treatments.

The aim of surgery is the decompression of the spinal cord and spinal roots and the stabilization of the spinal column [12]. Good results with the use of surgery need a careful patient selection. Surgical decompression and stabilization of the spinal axis is necessary in patients with pathological fracture and dislocation of the fragments in the vertebral canal [12]. In patients with no history of primary cancer, surgery should be performed with a diagnostic and therapeutic purpose. Finally, in patients in whom RT has failed to control the progression of symptoms or with a known radioresistant tumor, surgery should be considered [11]. Anyway some studies suggest that infections drastically increased when surgery is performed after radiotherapy [18, 11].

Less invasive therapeutic options include augmentation techniques as vertebroplasty and kyphoplasty (the reader is directed to relative chapters) [19].

In recent years the technology development has provided techniques, as thermal ablation with radio frequency (RF) or cryoablation, in association with vertebral augmentation methods [20]. The goal is both the pain relief and a better quality of life in patients, whose poor clinical status does not allow the use of more invasive techniques.

In this chapter we will focus the attention on radiofrequency ablation (RFA).

7.2 Radiofrequency Heat Ablation

RFA was introduced in clinical practice to treat painful cancerous lesions, not approachable with traditional therapeutic methods [20]. RFA has the purpose to destroy or to stop the tumor progression, and it tries also to fire endosteal nerve endings, which are involved in the origin of pain as a result of their stimulation by chemicals such as prostaglandins and bradykinin, substance P, or histamine, released by the destroyed bone [20]. RFA of bone tumors has specific advantages, related to the electrical and thermal properties of the bone structure [21]. Trabecular bone conducts heat less well than the muscle. In addition, the cortical bone has heat insulating activity, which protects neighboring structures [21].

Osteoid osteomas were the first tumors treated by RFA more than 20 years ago, and nowadays percutaneous RFA has become the primary well-established approach as treatment of these benign lesions [22].

In the last 15 years, chondroblastoma was the second benign tumor considered for ablation. This rare tumor of children/young adults is usually located in the epiphyses/apophyses, and it is often painful [23]. When a small chondroblastoma is detected, RFA offered a shorter postoperative hospitalization and a lower rate of recurrence in comparison to surgery, especially if the proximity to cartilage and growth plates is taken into account [24, 25].

The role of RFA in treatment of painful metastasis is well established, particularly in the axial-loading locations of the spinal column or peri-acetabular region, mainly to avoid potential complications from tumor progression, primarily fracture [26].

7.2.1 Physics of Radiofrequency Ablation

The aim of RFA is the destruction of cancer cells with the use of high temperature. When an ablation electrode is applied to the target tissue, a high-frequency (200–1200 KHz) alternating current moves from the tip of the electrode into the surrounding tissue [27]. The indirect current causes local ionic agitation and subsequent frictional heat, with a significant increase in temperature. The frictional heat emitted from ionic agitation then spreads by convection, with the final results of enlarging the area of ablated tissue [28].

Some authors documented that injury to cells begins at 42 °C and the tissue composition influences the time of heat exposure required to achieve cell death [29, 30]. As the temperature increases above 42 °C, the necessary exposure time decreases exponentially. In vitro experiments with white eggs documented the size of the coagulum did not increase, if a time exposure between 60 and 90 seconds and a target temperature of 80 °C are adopted [31]. Other studies supported the hypothesis in vivo coagulation necrosis of cancer cells could be achieved in only 2 min at 51 °C [29]. Anyway some authors proved the cells die, resulting in tissue necrosis, when the temperature exceeds 60 °C [32]. The tissue temperature decreases rapidly with increasing distance away from the electrode [33]. Authors then supposed the use of a “higher-than-ideal temperatures”, typically over 90–100 °C, for ablation of tissues with a greater distance from the RF source: hence, it should be used to obtain a complete coverage of the tumor by the ablation zone with an adequate margin, typically 1 cm otherwise the boundary of the treated cancer lesion [34].

The physician may preset some parameters to maximize the energy deposition inside the target tissue, as temperature, voltage, impedance, and pulse duration, and then to improve the efficacy of treatment [35]. Also the choice of the RF electrode may influence the results of treatment.

There are RFA systems with single or multiple clustered electrodes. A conventional RF electrode is a metal cylinder entirely covered, except its tip [36]. The coating has an insulating function. In the inner distal tip, a thermocouple is positioned with the aim of monitoring the tissue temperature. There are many types of electrodes, which differ substantially for the length and the thickness of the tip, and the choice depends on the characteristics of the lesion to be treated. The area of tissue coagulation necrosis is a sphere proportional to the square of the RF current and increases linearly in function of the tip length [37]. The surface area of the electrode tip is also many orders of magnitude smaller than the entire coated surface RF probe [28]. Hence, the density of the field lines that are forced through the electrode tip is huge. There is a major ionic agitation in the molecules of the neighbor tissues to the electrode tip, therefore a higher temperature increase [28].

In single-electrode RFA systems, a closed-loop circuit is created with the RF generator, a large dispersive electrode (ground pad), the patient, and a needle electrode in series [33]. Some RFA systems have hollow electrodes, whose cavity is flushed with saline solution to outside the tip with the aim of controlling impedance and temperature; other companies produce water-cooled electrodes, with the saline flush in a closed circuit and without an external flush. Water-cooled RF electrode was performed in order to prevent drying or charring of tissue at the interface with the electrode tip, because the impedance may increase with an insulator function and frictional heat cannot spread out [38]. The internal cooling then supports an increased power deposition in the target tissue and drives RF heating from the electrode-tissue interface deeper into the tissue to create more clinically relevant ablations. Some studies described the efficacy of monopolar RFA since 2000 [39, 40]; e.g., in a multicenter trial on 43 patients with painful osteolytic metastases treated by monopolar RFA, a significant reduction of pain in 95% of the patients was documented [41].

Some companies produce bipolar electrodes: two serially non-insulated metallic surfaces at the tip electrode act as double poles. Xavier Buy in 2005 described the bipolar technique as a therapeutical method for vertebral tumors on three patients [42]. In 2014 Angelos Ng et al. reported good results in a cohort of 36 patients, and this was the first prospective study on the treatment of spinal tumor with bipolar RFA [43].

For larger tumors, a single electrode in different positions can be used to produce multiple overlapping lesions. RFA systems with multiple electrodes are more eligible because of their faster and wider coagulation than single-electrode systems [44, 45]. Multi-tine conventional or cooled electrode systems range from 3 to 12 active tips of variable size and cluster configurations, with a final ablation zone of 3–4 cm [44, 45]. Although the treatment time is reduced with the use of multi-tined electrodes, such systems are not indicated for the treatment of lesions neighboring neurovascular structures (spinal cord and spinal roots must be spared) [44, 45].

7.2.2 Patient Selection, Role of Imaging, and Technique

History of metastatic cancer is the first clue for physician. Clinical examination reveals acute pain, usually refractory to conservative therapies, and tenderness over the spine at or near the involved vertebral level [46].

Imaging techniques play an important role in detection of painful metastasis and their therapeutic planning, as in the follow-up period. Vertebral fractures are generated when the combination of the axial and rotational charges on the spine exceed the resistance offered by the vertebral body [47]. At X-ray examination the vertebral compressive fracture is defined as a reduction in height, which must be at least 20% beyond its initial dimensions [48]. CT examination is useful for the evaluation of the bone erosion. MR scans allow an evaluation of the soft tissues as neural and vascular structures. The presence of intra-spongious edema, particularly in fat suppression sequences, indicates a recent fracture [6, 10].

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