Blood loss during MSTS remains challenging, although minimally invasive surgical techniques have greatly improved. Blood loss varies greatly depending on the primary tumour of spinal metastases, surgical approaches, and operative time [2]. The average volume of blood loss has been reported as 1,418 mL (713.3-3,120 mL) in open and conventional spine surgery for vertebral metastases and 745 mL (184-1,320 mL) in minimally invasive spinal metastatic tumour surgery [14]. In the present study, the average volume of blood loss was 392.5 ± 270.61 mL, which is less than that reported in other studies. Eleven patients received ABT with an average transfusion volume of 280 ± 293.08 mL. The prevalence of anaemia in patients with cancer is reported to be approximately 40% [15] due to nutritional deficiency, chronic disease, blunted response to erythropoietin, bone marrow suppression either due to cancerous cells or as a side effect of chemotherapy/radiotherapy, and other causes [16]. Transfusion requirements are usually larger than expected. Thus, the investigation of IOCS+LDF is valuable for patients.
There is still controversy around the clinical safety of using salvage blood in oncological surgeries, despite literature establishing its safety. LDF is a filter device based on a membrane-like filter material used to remove leucocytes from the blood. The mechanisms underlying the removal of tumour cells are physical interception and charge adsorption based on cell size. The size of white blood cells is approximately 7-20 µm. The RLDF refers to a white blood cell filter with a pore size of 40 µm. Gray et al. conducted a prospective cohort study and found no significant difference in progression-free survival between preoperative autologous donation and IOCS-LDF groups undergoing prostatectomy [17]. Patients who undergo partial hepatectomy for colorectal cancer metastases can safely receive a transfusion of filtered autologous blood, which is not associated with an increased risk of recurrence or a higher mortality rate [18]. Patients who received a salvaged blood transfusion required significantly lesser amounts of allogeneic blood, and their survival rates and disease progression remained lower or similar to that in control patients. Furthermore, there are many studies on the clinical safety of salvage blood used in oncological surgeries, including gynaecological [19], hepatobiliary [20, 21], gastrointestinal [22], urological [17], and pulmonary [23] surgery. However, there are only a few studies on IOCS+LDF reinfusion in patients with MSTS. Gakhar et al. observed that transfusion of intraoperatively salvaged blood did not result in disseminated metastatic cancer in MSTS (level of evidence IV) [24].
In this study with 20 patients, tumour cells were detected in the blood samples of 12 patients (60%) after IOCS+RLDF processing (S4), whereas it was found in venous blood before surgery (S1) in 14 (70%), in blood from the operative field (S2) in 16 (80%), and in blood after IOCS (S3) in 13 patients (65%). There was no significant difference between S4 and S1 or between S4 and S2, although a downward trend appeared after IOCS+RLDF processing (Fig. 4). This may imply that RLDF could not completely eliminate tumour cells. With the aim to discover more effective methods, Mei et al. reported a 3-4 log reduction in leucocytes using MLDF with a pore size of 12-18 µm [25]. RLDF-treated samples were subsequently inoculated in nude mice, 67% of which developed tumours. In contrast, no tumour cells were found in MLDF-treated samples, and no solid tumours were observed in inoculated nude mice [25]. Therefore, it is considered that MLDF with mannitol-adenine-phosphate solution had higher filter efficiency, but further clinical research is warranted. Thus, we explored MLDF with a pore size of 18 µm to filter spinal metastasis tumour cells from known primary epithelial tumours. The number of tumour cells was significantly lower in the samples after MLDF processing (S5) than in the operative field (S2). We also confirmed that MLDF was more effective in eliminating tumour cells and safer than RLDF. In this study, MLDF was more efficient for filtering tumour cells, which suggests that it may have great prospects for managing blood salvage in oncologic surgery.
Ideally, we try to eliminate all tumour cells from salvaged blood. In our study, IOCS+RLDF markedly reduced the number of tumour cells in patients’ peripheral venous blood, but it did not eliminate them completely. However, MLDF achieved a clearance rate of zero. Karczewski et al. demonstrated that 62% of the tumour cells in blood underwent lethal trauma, whereas all the remaining tumour cells displayed morphological changes, after being processed with the IOCS device [26]. Kumar et al. reported that the tumour cells that pass through the IOCS device are morphologically altered and become nonviable and that they lose their ability to form new metastatic deposits [27]. Similar results were observed in other studies [7, 19]. Therefore, we assume that IOCS+MLDF would be an effective strategy to destroy and eliminate malignant cells and that salvaged autologous blood can be safely reinfused to the patient.
Strength and limitations of this study
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MLDF was applied for the first time on patients of MSTS in China.
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The salvaged blood used in this study was not reinfused to patients because of the limitations imposed by ethical issues.
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We failed to clarify the viability and functionality of the tumour cells because there were too few residual tumour cells to evaluate.