Advantages of MWA for the treatment of bone tumors
The main finding of our study is that MWA can effectively relieve pain and improve the quality of life of patients with bone tumors, including benign or malignant bone tumors, and a significant difference was observed between preoperative and final follow-up measures. Reducing pain and improving quality of life are the primary goals in the treatment of bone tumors by multidisciplinary care teams [19]. With particular reference to ablation, MWA is a local thermal ablation technique that uses a rapidly oscillating electromagnetic field to cause water molecules to rotate, resulting in frictional heating [6]. Depending on the procedure, the surgeon can place an ablation antenna in the tumor to induce coagulation necrosis in the target tissue under imaging guidance. In recent years, MWA has been shown to be a safe and clinically efficacious treatment for liver cancer, lung cancer and other clinical fields [11]. However, clinical evidence associated with the application of MWA for the treatment of bone tumors has been limited to some small series [11, 13, 16,17,18, 20,21,22]. In a prospective study by Prud’homme et al. [13], thirteen cases of osteoid osteoma of the extremities were treated by MWA under CT guidance. During the follow-up period, the overall success rate was up to 92.3% (12/13), and almost all patients experienced total pain relief. Recently, Deib et al. [21] performed the largest retrospective study of painful extraspinal osseous metastases and myelomatous tumors treated with MWA and BC. They concluded that MWA is a promising, safe, and effective treatment for painful spinal metastasis that can result in both a reduction in pain and a degree of local control over the disease process. Moreover, in a retrospective study, Khan et al. [22] reported on the use of MWA to treat 102 painful spinal metastases (69 patients), which included spinal lesions in 12 patients. Local tumor control was achieved in all patients, and significant pain palliation was achieved at 2–4 weeks and 20–24 weeks following the procedure. Our results are in line with those of the above mentioned previous study and further confirm the excellent curative potential of MWA.
Application of MWA in benign bone tumors
Clinically, percutaneous RFA is a well-established gold standard for treating osteoid osteoma and other benign bone lesions [6, 8, 9, 12, 14, 19]. In RFA, a needle electrode is inserted into a target zone, and heat generated by dielectric heating at the needle tip causes coagulation necrosis of the bone lesion. However, poor thermal conduction through bone is a limiting factor in RFA. In tissues with a higher impedance, such as bone, there is a reduction in energy deposition from RFA, which, in turn, leads to a smaller temperature increase and a potential increase in the treatment failure rate [12, 16, 21]. Interestingly, the characteristics of MWA can overcome the limitations of RFA, leading to a considerably improved power efficiency and the rapid coagulative necrosis of tumor cells. In addition, MWA can produce faster heating, higher intralesional temperatures, and less susceptibility to both heat-sink and charring effects. It is also fairly insensitive to the intrinsic high impedance of bone, especially in the case of osteosclerotic tumors, as it allows deeper thermal penetration than other modalities [6, 21, 22]. According to animal models, Brace et al. [23] established that the application of MWA is more advantageous in bone tissue with high impedance. Based on these advantages, we successfully applied MWA in the treatment of benign bone lesions, including osteoid osteoma, osteoblastoma, enchondroma, osteofibrous dysplasia and nonossifying fibroma. In detail, MWA alone was performed in 14 patients (6 with osteoid osteoma, 5 with osteoblastoma, 2 with osteofibrous dysplasia, and 1 with nonossifying fibroma), and the remaining 21 patients with enchondroma underwent endoscopic MWA combined with IAB. It is noteworthy that enchondroma can most often be adequately treated with intralesional curettage and bone grafting [24]. To reduce the surgical trauma and achieve better local control over the tumor, we designed our endoscopic MWA technique combined with IAB to sustain tumor necrosis and restore structural stabilization. In 2015, Lui et al. [25] reported a technique consisting of endoscopic curettage and bone grafting for treating enchondroma of the proximal phalanx of the hallux; complete incorporation of the bone graft and a good range of motion of the hallux were achieved 3 months after the operation. In our study, with the help of a MAST QUADRANT minimally invasive system, we carried out the MWA and curettage of tumor tissue under direct vision, completely removed the tumor tissue, and repaired the bone defects at the same time. Our results are consistent with the results of preliminary studies evaluating MWA in the treatment of benign bone tumors [13, 16, 17].
Application of MWA in malignant bone tumors
In general, bone metastases commonly occur in patients with advanced disease. The treatment of bone metastases is typically aimed at both pain relief and the preservation of ambulatory functions. Percutaneous MWA is an effective alternative modality for relieving the pain of patients with bone metastases, particularly when patients with metastatic disease are often undertreated for pain [21, 22]. However, MWA alone may render cavity formation and bone mass reduction, resulting in an increased risk of pathological fractures. Meanwhile, some scholars believe that thermal ablation can create a cavity through tissue dissolution rather than tissue displacement alone, leading to cement deposition and hence theoretically minimizing the risk of complications [26]. Therefore, MWA combined with BC may be a useful method for both additional pain relief and structural stabilization in the treatment of bone tumors [27]. Both Deib et al. [21] and Khan et al. [22] have described how MWA can be performed through safe, repeated, short ablation cycles to control the diffusion of the heating zone without diminishing the efficacy of MWA. In the current study, all patients with malignant bone tumors, including bone metastases and multiple myeloma, were treated with the application of BC. Of these, 13 patients with bone metastases and 5 with multiple myeloma were treated with MWA combined with BC, and the remaining 3 patients with bone metastases were treated with endoscopic MWA combined with BC. According to our experience, bone tumors located in the pelvis may present different challenges to the treating physician due to complex anatomical structures. Furthermore, pelvic metastatic tumors may be particularly painful and debilitating [21]. It seems possible that endoscopic MWA combined with BC can be performed through safe and short ablation cycles to control the diffusion of the heating zone without damaging vital nerves or blood vessels. Recently, Fan et al. [28] attempted en bloc MWA in situ to improve the outcome of the treatment of primary malignant pelvic bone tumors, with encouraging oncological and functional results. Our results are consistent with those of previous studies, with immediate pain reduction and quality of life improvement obtained in almost 100% of the patients and maintained at the final follow-up visit.
Nonetheless, how to evaluate changes in bone and soft tissue after RFA or MWA remains one of the most challenging issues for oncologic orthopedic surgeons and radiologists. Recently, Razek and his colleagues reported a series of cases with benign and malignant bone tumors that underwent diffusion-weighted MR imaging (DWI). Those investigators concluded that different tumor tissues have different DWI findings and different apparent diffusion coefficient (ADC) values. Inspired by the discovery of these different ADC values, we believe that DWI may provide essential information for assessing the tissue changes following tumor removal by RFA or MWA [29,30,31,32].
Complications
Percutaneous MWA techniques have been used for the treatment of patients with several benign and metastatic bone lesions, and major complications are infrequent [6, 15,16,17, 21, 22, 27, 33, 34]. Kastler et al. [27] reported the successful treatment of spinal metastases with MWA, without any major complications. Subsequently, in 2017, Kastler et al. [35] also reported the use of a thermocouple technique for real-time temperature control during MWA in the treatment of metastatic bone disease. In their series, the maximum temperature of the thermocouple near the monitored root did not exceed 43 °C, which served as an added safety feature. Furthermore, light sedation or local anesthesia adds to the safety of the procedure because patients may alert the surgeon in the case of pain. Recently, Cheng et al. [34] have shown that ultrasound-guided percutaneous MWA has the advantages of being a real-time, convenient, lower-cost and nonradiative treatment; additionally, complications related to thermal damage (in the form of skin burns), infection and nerve injury did not occur in any patients. Other recent and mostly retrospective studies have shown the same results, with a relatively low rate of complications [11, 15, 16, 21, 22]. Similarly, the results of our study also indicate that MWA is a safe and efficient approach, with only three minor complications related to thermal injury that caused myofasciitis and affected wound healing in the entire study. However, some scholars would have preferred the more diffuse RFA technology over MWA, with poor ablation zone predictability, which may present an increased risk of complications, especially along the antenna. Indeed, early MWA technologies may present greater inherent limitations. In the current study, we utilized an MWA system based on Thermosphere™ Technology, and newer probes have solved this major performance issue, enabling the more precise delivery of energy to the tissue with consequent large, spherical and predictable ablation zones [36].
Limitations to this study must be acknowledged. First, this was a retrospective observation from a single institution in a population of Chinese patients. However, our primary goal was to demonstrate the feasibility and safety of MWA in the treatment of bone tumors, including benign and malignant bone lesions. A higher level of evidence could be achieved by performing a prospective, multicenter trial in the future. Second, this study does not constitute a comparative study, as no other thermal ablation techniques (e.g., RFA, laser ablation, and cryoablation) were observed, and no long-term follow-up data were assessed. In addition, various subtypes and volumes of bone tumors present differences in terms of MWA technique. In regard to these problems, more randomized studies will gradually be conducted in the future.