3.1.1 Patient selection

Image-guided interventional procedures in patients with MSK tumours have been found to be minimally invasive, effective, generally well tolerated and safe.³ However, appropriate patient selection is essential for optimal results. A thorough clinical and imaging assessment of the patient is a vital prerequisite to confirm the abnormality, rule out systemic causes, select the appropriate treatment option, plan the approach and reduce the complications. Up-to-date imaging is necessary before the procedure. Prior discussion in a soft tissue sarcoma or bone tumour multidisciplinary team meeting is essential to decide the necessity of the interventional procedure and finalise the desired approach considering possible future surgical approaches to prevent contamination of tissue planes.^3,4 (3,4).

3.1.2 Image guidance

Percutaneous minimally invasive interventions are done under image guidance which has improved the safety and accuracy of the procedures. The choice of imaging modality depends on the type of tissue to be traversed, the depth of the target site and the operator.

Ultrasound (US) is a commonly used imaging modality in musculoskeletal interventions. US is widely available, provides high spatial resolution for superficial structures, is easy to use and allows real-time needle tracking, making it the ideal imaging modality for superficial soft tissue biopsies. A high-frequency (12–18 MHz) linear transducer is generally used for superficial soft tissues. For the deeper soft tissue lesions, a transducer with a frequency range of 7–12 MHz may be required. A high-frequency probe with a smaller footprint (hockey stick probe) is used for smaller structures such as fingers and toes (Fig. 1).

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Fig. 1 US guided biopsy – T1 axial (a) and T2 coronal (b) images of the foot demonstrating a heterogenous lesion in the 1st web space. US image visualizing the position of the biopsy needle in the lesion (c).

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Fluoroscopy is an imaging modality for some bone biopsies and augmentation procedures. It is cost-effective, provides real-time images of the flow of contrast to confirm the needle position, and with a C- arm, images can be obtained in a non-axial plain (Fig. 2).

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Fig. 2 Fluoroscopic guided biopsy and vertebroplasty of the L1 lesion – Sagittal T1 and Proton density fat saturated images of the spine demonstrating multiple vertebral lesions with pathological fracture of L1 (a and b). Biopsy and vertebroplasty of the L1 was done under fluoroscopic guidance, Image c demonstrating position of the needle and d showing post vertebroplasty cement in L1.

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CT (Computed tomography) is routinely used imaging modality to guide interventional procedures targeting bony lesions, spine and joints. It provides excellent direct visualisation of soft tissues and osseous structures and lesions. CT is preferred for deeper soft tissue lesions (Fig. 3), bone lesions and spinal procedures. The disadvantage of CT is the relatively high radiation dose and inability to provide real-time visualisation of the needle, which requires CT fluoroscopy.

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Fig. 3 CT guidance for deep seated intraarticular synovial lesion – Sagittal T1 (a) and axial PDFS images (b) demonstrating a soft tissue lesion in the posterior aspect of the knee joint (arrow). CT guided biopsy of the lesion using a coaxial biopsy system (c) which confirmed the diagnosis of pigmented villonodular synovitis.

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Magnetic Resonance Imaging (MRI): Although MRI is the modality of choice in the imaging of musculoskeletal lesions due to its ability to characterise soft tissue/bony lesions, it is not routinely used to guide the interventions due to the limited availability of compatible hardware and expertise. MRI findings can be used as a road map in CT-guided procedures for the lesions that are not well visualised on CT by identifying a bony or soft tissue landmark on MRI which can be used to target the lesion while performing CT guided biopsy.^4,5

3.1.3 Preprocedural medications

As these interventional procedures are minimally invasive and done under strict aseptic technique, routine prophylactic antibiotics is not recommended but may be used in selected cases if required.

Bleeding is one of the important complications associated with any interventional procedure. Soft tissues/bone biopsies and spinal procedures such as vertebral biopsies carry relatively more bleeding risk, especially in patients on anticoagulants, haematological malignancy, bleeding diathesis and chemotherapy. Hence a careful review of anticoagulant medications and bleeding parameters is required before the procedure. For most procedures, an INR value of <1.5 and a platelet count of >50000/mcL is desired.

There may be a need to alter or stop the anticoagulant medications, or additional medications may be required before the procedures. These decisions are taken after discussion in multidisciplinary meetings, evaluating the bleeding risk, underlying abnormality and comorbidities.^4,5

3.2.1 Musculoskeletal biopsies

Despite advances in imaging, histological diagnosis remains the definitive way of diagnosing a tumour or tumour-like lesion, its histological subtype and grading. Open biopsy was the gold standard for obtaining the issue for histopathological diagnosis; however, percutaneous image-guided biopsies have largely replaced them as preferred first-line procedures for tissue sampling. They are minimally invasive, safe and have a comparable diagnostic yield to open biopsies.⁶ Another advantage of image guidance is the ability to target the desired area within a large lesion for tissue sampling to get a good diagnostic yield and appropriate grading of the lesion. A lesion may contain necrosis and haemorrhage, which lack viable tissue. Similarly, some lesions may have a significant cystic component or fluid-fluid levels, which have low diagnostic yield. Post-contrast imaging, Diffusion MRI and PET scan are often helpful in identifying viable areas within the lesion for biopsy. Under the image guidance, a particular region in the tumour that can provide a sample with a high diagnostic yield can be targeted for tissue harvesting.⁵

As a rule, biopsy should be reserved for those lesions which cannot be confidently characterised as non-aggressive based on imaging and clinical parameters, and a histopathological result will alter the patient management.⁷

Commonly soft tissue lesions are biopsied using US guidance under local anaesthesia. A CT guidance with sedation or general anaesthesia may occasionally be required for deep-seated lesions and lesions close to vital structures such as vessels or nerves. Bone biopsies are generally performed under CT guidance with general anaesthesia or sedation (Fig. 4).

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Fig. 4 CT guided bone biopsy – Axial PDFS MRI images demonstrating a hyperintense lesion in the right pubic bone (a). CT guided biopsy (b) confirmed the diagnosis of chondrosarcoma.

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Tissue samples are preserved in 10% formalin solution after the biopsy. Patients are observed for an hour after the procedure to identify post-procedural complications.

Various automatic and semi-automatic Tru-cut core biopsy needles are available for soft tissue lesions. A semi-automatic 14 G biopsy needle is favoured as it provides control and precision. They come with adjustable penetration depths of 10 and 20 mm. A coaxial system can be used to obtain multiple samples with a single puncture.

A Jamshidi needle or its modifications are commonly used to obtain the bone biopsy specimen. These needles have an outer cannula and inner trocar with a cutting tip. The needle is introduced into the bone, and just proximal to the margin of the lesion, the trocar is removed, and the outer cannula is advanced through the lesion to obtain a core biopsy sample. Usually, a needle of 8 or 11G is used. However, getting a sample from these types of biopsy needles may be challenging in densely sclerotic lesions, and a coaxial biopsy system with an eccentric drill tip, such as Bonopty bone biopsy system (Apriomed), may be required. This system consists of a penetration set and an eccentric drill. The penetration set is advanced to reach the bone surface, and then the stylet is exchanged with an eccentric drill and rotated a few turns, which drills a channel through with outer cannula that is advanced through the sclerotic bone cortex. Once the position of the cannula is secured, the drill is removed, and a biopsy set is introduced through the cannula to obtain the tissue sample. The outer cannula can be used as a coaxial system to obtain multiple biopsies or pass an electrode to perform radiofrequency ablation.⁸ However, the size of bone biopsy specimen using Bonopty bone biopsy system is much smaller as its 15G.

Recently battery powered coaxial drill biopsy sets have been available, which require significantly less physical effort from the operator. They are efficient, requiring less time to complete the biopsy hence reducing the radiation dose to the patient. They have also been found to increase the diagnostic yield in sclerotic lesions.⁶

In CT-guided bone biopsies, the patient's position depends on the site of the lesion and the planned approach to biopsy the lesion. Following an optimal position, the bone biopsy needle is passed towards the lesion, and multiple CT slices are obtained to visualise the direction and angulation of the needle. One critical step in a successful biopsy is initial cortical puncture/purchase. It can be challenging to achieve an optimal purchase/cortical puncture at a desired site in the superficial bones due to the lack of soft tissues to support the needle and hold it in correct direction and angulation when the operator's hand is removed during the CT image acquisition. Recently a few new techniques have been described to aid this. Surface adhesive and hand-aided needle-assisted bone biopsy technique (SAHNA technique) use plastic surgical forceps and an adhesive dressing applied to the forceps to support the needle.⁹ Dual steristrip technique uses steristrips, one end of which is fixed to the skin, and then this steristrip is wrapped around the biopsy needle and the other end is affixed to the skin on the opposite side.¹⁰ Birmingham Intervention Tent Technique (BITT) again describes using the plastic surgical forceps clamped to the biopsy needle at an angle creating a shape of a tent, and the fingers of the forceps are stabilised on a table. BITT is helpful in the body parts with uneven skin surface or angulated body parts such as the cervical spine.¹¹ The aspiration biopsy technique is used when a bone lesion is cystic or contains fluid-fluid levels with no significant solid component, where biopsy can be challenging. In this technique, the needle is introduced into the lesion with suction, targeting any solid components or the wall to obtain scrapings from the wall by making a circular or Figure of 8 motion.

Image-guided percutaneous biopsies have a high success rate, with reported accuracy ranging from 68% to 98%. A 10-year retrospective study done in the senior author's institute showed a high diagnostic yield of 98%. Despite this, a negative biopsy is not uncommon. Various factors affect the biopsy outcome.¹²

In general, a large lesion, a high-grade lesion, osteolytic lesions, lesions with cortical destruction and soft tissue component are likely to give a positive biopsy result. Percutaneous biopsy of an osteolytic lesion is more likely to be diagnostic than an osteoblastic lesion. Diagnostic yield increases with increasing number of samples obtained, sample length and larger needle gauge.¹³

Some lesions, such as Langerhans cell histiocytosis, lymphoma and similar round cell neoplasms, are likely to give a false negative biopsy result. A point to be noted here is that not all negative biopsies need repeat biopsy. These cases are discussed in the multidisciplinary meeting, and in the senior author's experience following discussion in MDT, 60% of cases can be managed without requiring a repeat biopsy.¹²

With the emergence of new technologies, some interesting new concepts are being tried in image-guided biopsies. Park B.J et all; used 3D Augmented reality-assisted navigation system for CT-guided biopsies and concluded that this technology could significantly improve procedural efficiency and reduce the radiation dose.¹⁴ Some smartphones or tablet-based AR navigation platforms have demonstrated high accuracy, reduced procedure times, and fewer intermediate CT scans.¹⁴ Faiella E et al., compared the outcome of 80 patients who underwent CT-guided biopsy of ground glass opacities of the lung with an augmented reality infrared navigation system with a group of patients who underwent biopsies with standard CT guided technique and found significantly reduced procedural time, radiation dose administrated to patients and complications rate among the group who had CT biopsies with an augmented reality infrared navigation system.¹⁵ Amiras, D and concluded that augmented reality-based simulation of CT-guided biopsies could be used for training.¹⁶

Furthermore, Wen wu and colleagues found that 3D printing technology may be helpful in CT-guided procedures. They used preoperative CT and MRI scanning data to print the individualised puncture guide plate using 3D printing technology to guide the needle biopsy of acetabular tumours and found that this process makes the operation simpler and more precise.¹⁷ Apart from this, laser-assisted angle selection systems are available in some CT scanners to help CT-guided procedures, and they have been found to improve the accuracy of needle placement.¹⁸

3.2.2 Ablation

Ablation in musculoskeletal tumours is a well-established percutaneous minimally invasive technique to treat primary or metastatic bone or soft tissue lesions. This can be used as a first-line curative therapy, an adjuvant (combined with radio/chemotherapy or surgery) or a palliative therapy. The aim is to destroy the tumour cells using either chemical, thermal or non-thermal energy.

3.2.3 Chemical ablation

Chemical agent, commonly a sclerosant, is delivered into the tumour tissue/cavity under the image guidance. This treatment option is commonly used for vascular malformation or a cystic tumour such as aneurysmal bone cyst (ABC).

Sclerotherapy has emerged as a promising alternative to curettage for ABC. In this procedure, an 11G bone biopsy needle is introduced into the lesion under image guidance and the contents of the lesion are aspirated to decompress the cyst. An attempt is made to break the locules to achieve free communication within the cyst cavity, followed by injecting an iodinated contrast to identify any vascular communication or communication with adjacent structures, such as the spinal canal in vertebral lesions. Following this, the sclerosant is injected into the cyst cavity. Various sclerosing agents such as alcohol, polidocanol, sodium tetradecyl sulphate, doxycycline and human albumin foam mixture have been used. This procedure requires multiple sessions. Consolidation of the lesion can be seen as early as three months following the procedure (Fig. 5).

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Fig. 5 Sclerotherapy for ABC- Proton density fat saturated axial MRI image (a) demonstrating an aneurismal bone cyst in the femoral head with fluid levels (arrow). CT images demonstrating needle position in the lesion (b) following aspiration of the contents, contrast material was injected into the lesion followed by sclerosants (c).

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3.2.4 Thermal ablation

Thermal ablation intends to bring about tissue damage in the tumour by subjecting the tumour cells to extremes of temperature. Radiofrequency ablation (RFA), microwave ablation, laser ablation and MR guided high intensity-focused ultrasound uses heat to achieve this, whereas cryoablation exposes the tumour to freezing temperature to bring about coagulation necrosis. Among these, RFA and cryoablation are the most commonly used and studied.

3.2.5 Radiofrequency ablation (RFA)

In the RFA procedure, the tumour tissue is treated to a high temperature (45–95 °C), resulting in the permanent death of tumour cells due to coagulation necrosis of the cellular proteins. Commonly this is achieved by a monopolar RFA device/circuit where an electrode (which acts as the cathode) is placed in the lesion after drilling the cortical bone with an 11 or 13G coaxial system which acts as a cathode and a grounding pad placed usually on patient's thigh. The current which flows between the cathode and anode produces frictional heat in the tumour. The intact cortical bone acts as a good thermoinsulator and prevents heat dissipation to the surrounding tissues to achieve high intratumoral temperature.

Complications of RFA are less frequent. Post-procedural pain is common. Apart from this, there is a potential risk for a pathological fracture if the lesion is sizable. Thermal injury to the surrounding structures, such as nerves and skin, may occur, and a safety distance of 10 mm is recommended. It may be challenging to achieve this distance occasionally, and in these situations, thermoprotective techniques are utilised to create a barrier for heat transmission. Some of these techniques include thermal monitoring in adjacent tissues, injection of a non-ionic solution often mixed with non-ionic contrast, and pneumo dissection by injecting CO2 in perineural/epidural space.

RF ablation is used as a curative option in benign tumours such as osteoid osteoma, osteoblastoma and chondroblastoma and as a palliative option for pain relief in painful bony metastasis (Fig. 6).

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Fig. 6 RFA of osteoid osteoma – Axial proton density fat saturated images demonstrating marrow oedema in the proximal femur (a). Corresponding T1 axial image (b) demonstrates a small cortical lesion (arrow). Axial CT image at this level confirms osteoid osteoma (c). CT guided RFA was performed, image (d) demonstrating the position of the electrode within the lesion.

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3.2.6 Some of the newer concepts tried in RFA

Bipolar radiofrequency ablation-. This system involves placing two electrodes with tips close to each other, and the electrical current is dissipated between the tips of these electrodes, abating the tissues in between. The bipolar system does not require grounding pads. Compared to a monopolar system, a bipolar system is faster, requires less energy, and provides a more defined, predictable and larger ablation zone, thus making it safer and more effective, particularly in abating a tumour in the spine or close to a vital structure.

Internally cooled electrodes -Dehydration and charring of the tissues adjacent to the tip of the electrode is one of the technical challenges faced in RFA, which can limit heat dissipation and hence reduce the ablation zone. Internally cooled ablation electrodes are designed to address this issue. Internally cooled electrodes are designed to minimise tissue charring and contain an internal lumen with circulating fluid which cools the tip of the electrode. This can be used with both monopolar and bipolar systems.

Ablation systems with built-in thermocouples – one of the disadvantages of RFA is the inability to assess the ablation zone accurately. RF systems with thermocouples are devised to show the ablation margin by identifying the temperature changes in the ablation zone.

3.2.7 Cryoablation

In this procedure, the death of the tumour tissue is achieved by the use of the freezing temperature. Cryoablation works based on Joule - Thomson effect, which states that a rapidly expanding pressurised gas results in a drop in temperature. A low temperature below −20° Celsius damages the cells by crystallisation of water molecules, interrupting local microcirculation and leading to apoptosis. Cryoprobes with different ablation zone sizes and geometry are available, which are used to achieve low temperatures. One or more cryoprobes are placed in the tumour tissue under image guidance, covering the entire ablation zone. The first freezing cycle is achieved by passing argon gas, followed by thawing using an infusion of helium gas. After this, one more session of freezing and passive thawing is done. The ice ball can be monitored using imaging such as ultrasound or CT, thus allowing greater control of the ablation zone.

Cryoablation, unlike RFA, can be used in larger tumours as it produces a wider ablation zone. A combination of cryoprobes can be used to achieve an overlapping ablation zone of varying geometry. However, it is more expensive and time-consuming.

Cryoablation is used to treat bony lesions such as metastasis, osteoblastic lesions, lesions with a significant soft tissue component and soft tissue lesions such as an extra-abdominal desmoid tumours. Cryotherapy can be combined with cement augmentation for osseous lesions involving weight-bearing bones, e.g., tibia, femur and acetabulum (Fig. 7).

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Fig. 7 CT guided Cryoablation – T1 and Proton density fat saturated axial images of the pelvis demonstrating a left posterior sacral chondromyxoid fibroma (a and b). Coronal and sagittal reformatted CT images (c and d) demonstrating position of the cryoprobes within the lesion.

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Similar to the heat sink effect in RFA, achieving a required low temperature in a lesion with high vascularity or a lesion close to a significant vessel might be challenging due to flowing warm blood. Injury to the adjacent structures such as muscles (cryomyositis), nerves or skin may occur. Cryomyositis refers to an injury of the muscles near the ablation zone. On MRI, this appears as an oedema-like signal on fluid-sensitive images. This is seen in the majority of the cases following cryoablation and is rarely symptomatic, requiring treatment with anti-inflammatory medications. Cryoshock is a rare inflammatory response due to the release of inflammatory mediators. Apart from this, skin necrosis, skin burns, osteomyelitis, insufficiency/pathological fractures, and foot drop are potential complications.

Injury to the adjacent vital structures such as nerves, spinal cord vessels, and the bowel is one of the important complications of cryoablation. Among them, nerves are difficult to visualise on commonly used image-guidance modalities such as CT and fluoroscopy, exposing them to greater risk. In nerve injuries, motor function is affected earlier than sensory function.

Various thermoprotective techniques are used to reduce the occurrence of these complications, which can be categorised as passive or active. Passive thermoprotective techniques are used to monitor for injuries, such as temperature monitoring, electrostimulation, evoked potentials (motor/sensory) and thermocouples. Active thermoprotective techniques act by tissue displacement, including hydro dissection, carbon dioxide injection, warming and myelography for the spine.

3.2.8 Microwave ablation (MWA)

In this technique, tumour is exposed to an electromagnetic field through one or more antennae inserted in the lesion. This forces dipoles in the lesion to align in the direction of applied magnetic field which results in production of the heat. MWA can bring about coagulation necrosis faster and can have a larger ablation zone than RFA.

3.2.9 Laser ablation

In Laser ablation, tumour tissue is delivered with infrared light through the inserted optical fibres, resulting in coagulation necrosis. Laser ablation is MRI-compatible, and, in this technique, energy is delivered precisely and more predictably into the lesion.

3.2.10 MR guided high intensity focused ultrasound

This is a non-invasive technique where ultrasound waves are focused on the target lesion, and absorption of the US waves in the lesion produces heat resulting in tumour destruction. MR thermometry can be used to monitor the temperature of surrounding tissues.^19,20

3.2.11 Augmentation procedures

Pathological fractures are complications encountered in primary or metastatic bone lesions. With the increasing lifespan of patients with primary malignancy, events related to metastatic bone lesions, such as fractures, are causing a considerable amount of morbidity to the patient impacting the quality of life. These are difficult to treat with conventional open surgery due to the disease extent, complex locations and poor general condition of the patients. Pathological fractures may also occur following the ablation of a bone lesion which transforms the lesion into a cavity filled with necrotic tumour tissue weakening the bone. Image-guided percutaneous osseous augmentation procedures such as Cementoplasty can be beneficial in these situations to improve the strength of the involved bone, reduce the risk of pathological fractures, and treat some of the select fractures. They can be planned as prophylactic/palliative procedures or combined with therapeutic procedures such as ablations or cryotherapy.

Cementoplasty aims to achieve consolidation of weakened bone. This technique is often used in vertebral fractures or in lesions of the weight bearing bone. In this procedure, image guidance is used to optimally position the needle. Following this a bone cement, generally Polymethyl methacrylate (PMMA) is injected, ensuring an optimal filling of the cavity or lesion. Traditionally, fluoroscopy or CT are used to guide the needle position. CT is generally preferred as it allows precisely placing the needle at the required location within the lesion, visualizing cement distribution and early detection of cement leak (Fig. 8).

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Fig. 8 CT guided Cryoablation – Axial CT image demonstrating a lytic lesion in the left acetabulum (a). Axial CT post cementoplasty shows lesion augmented with cement (b) (arrow). .

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PMMA consolidates in the tumour cavity/tissue by polymerisation, which is an exothermic process resulting in increasing the local temperature (>50%), which may have an ablation effect on the tissue. Although optimal filling of the cavity/lesion is desired to achieve adequate consolidation and provide stability, it has been shown that even injection of a minimal amount of PMMA can provide pain relief, possibly by heating effect and causing injury to the nerve endings.

A common indication for augmentation procedures includes benign lesions such as cystic lesions, geodes, and aggressive haemangiomas and malignant lesions such as metastasis and myeloma. It can be combined with ablation procedures for lesions which are large enough or located in weight-bearing bones, predisposing them to fracture.^19,20

3.2.12 Future applications of image-guided interventional procedures in musculoskeletal oncology

Apart from the above-mentioned applications of future applications of image-guided interventional procedures in musculoskeletal oncology, combination of other interventional techniques, tumour embolization, neurolysis may allow pain management, palliation and stabilisation of MSK tumours. Advances in Digital Technology, imaging technology, development of consensus guidelines such as Delphi studies, Clinical research and collaboration between sub-specialities will allow a cross-specialty management of such patients.