Primary investigator has changed his employee and the country of residence.

Background: The aim of the study was to assess the safety and effectiveness of stereotactic brain tumour biopsy (STx biopsy) guided by low-field intraoperative MRI (iMRI) in comparison with its frameless classic analogue based on a prospective randomized trial.

Patients are prospectively randomized into a low-field iMRI group and a control group that undergo a frameless STx biopsy. The primary endpoints of the analysis are: postoperative complication rate and diagnostic yield, and the secondary endpoints: length of hospital stay and duration of operation.

INTRODUCTION Stereotactic brain biopsy (STx biopsy) offers a relatively straightforward, accurate and safe method of obtaining diagnostic tissue. Frameless computer-based neuronavigation is now widely used in brain tumor surgery. It has many advantages over frame-based techniques and provides similar accuracy to the rigid frame. One of the methods applied to improve diagnostic yield and safety is the usage of intraoperative magnetic resonance imaging (iMRI), proposed by Bernays et al. in 2002.

Besides the obvious indications given in previous papers, including STx biopsy of very small, deeply localized or cystic lesions, iMRI guidance is particularly useful in two cases. The first one is when a satisfactory 3D volume modality cannot be obtained during preoperative high-field diagnostic MR imaging. When using iMRI there is no need to perform preoperative 3D imaging of any kind and manually register the patient's head in a neuronavigation system before the operation, which helps to significantly improve the workflow. In our study the total amount of patients with insufficient neuroimaging - admitted from outside medical centers - is over 30%. The second case is for teaching purposes - frameless iMRI guided STx biopsy, is relatively uncomplicated and technically straightforward and can be introduced as the first procedure during training in neurosurgical intraoperative imaging.

Although the usefulness of ultra-low-field iMRI in STx biopsy was subsequently confirmed by other authors, according to our knowledge no previous published studies have compared iMRI to preoperative MRI for brain tumor biopsy according to evidence-based medicine guidelines (EBM). Though this method has been subject to slight criticism, it has been consequently applied in neurosurgical daily practice in recent years.

The aim of our study was to verify the safety and effectiveness of the STx biopsy guided by low-field iMRI in comparison with its frameless classic analogue basing on a prospective randomized parallel-group, controlled trial. In the current paper we present the study design and results of the interim analysis.

MATERIAL AND METHODS

Patients Patients who are - following contemporary recommendations - scheduled to undergo STx biopsy, are prospectively recruited for the study. Each patient sign a written consent to participate in the study.

Inclusion criteria The inclusion criteria were as follows: male and female patients ≥ 18 years with supratentorial brain tumor scheduled to undergo STx biopsy. The estimated number of patients needed to reveal the difference of over 5% between primary endpoints' - diagnostic yield and complications ratio - at the level of significance 0.05 and power 80% was 465 per each arm.

Exclusion criteria Patients unable to provide informed consent and those with metal implants which could prevent or influence the head MR study were excluded from the study.

Allocation Patients were prospectively allocated by minimization according to demographic (gender, age) and epidemiologic data (preoperative Eastern Cooperative Oncology Group Performance Status-ECOG- a scale providing information about neurological and social status of a patient with oncological disease, maximum tumor diameter, presence of contrast enhancement, independent risk factors of hemorrhage - basal and thalamic localization and preoperative diabetes) into the iMRI and the control group.

Intervention After being transferred to the operating room each patient was sedated with an intravenous infusion of Remifentanil with passive oxygen therapy and monitoring of vital functions. Additionally, the sites of head holder pins and skin incision were anaesthetized with 1% Lignocaine. All biopsies in both groups were performed via a 6 mm burr-hole with the use of the Vertec system (Medtronic Navigation, Louisville, CO, USA). A passively navigated side cut 2.2 mm biopsy needle was used. All operations were performed by one of the three first authors.

In the iMRI-guided group the head of each patient was immobilized with a 3-pin iMRI-compatible head holder. The PoleStar N20 iMRI system (Medtronic Navigation, Louisville, CO, USA) with a 0.15-T constant magnet was used in all procedures. Subsequently, after the patient's positioning, the preoperative reference examination was routinely carried out (T1+gadolinum, T2 or FLAIR weighted - depending on the pathology, axial 4 mm scans). Images were automatically transferred into the neuronavigation system (StealthStation, Medtronic Navigation, Louisville, CO, USA). The entry point, target and optimal biopsy trajectory were then defined by the operator on the basis of the obtained iMRI images. Serial tissue samples (4 from the central and another 4 from the marginal part of the tumor) were collected according to the modified protocol described by Shooman et al., which made use of intraoperative histopathological examination obsolete. Following each operation, a control iMRI (T1-weighted, axial, 4 mm scan examination) was routinely performed to confirm and document the proper targeting and - as proposed by Bernays et al. - to exclude postoperative hyperacute intraparenchymal bleeding.

A frameless STx biopsy was performed for each patient from the control group with the use of a neuronavigation system. The entry point, target and optimal biopsy trajectory were defined by the operator before the operation on the basis of the preoperatively obtained high-field MR images with the use of a neuronavigation workstation (Cranial 5, StealthStation Application Software, Medtronic Navigation, Louisville, CO, USA). Following surgery the specimens were sent for independent histopathological analysis.

Postoperative care Postoperative care was conducted according to standard protocols and clinical guidelines. A postoperative follow-up head CT was subsequently performed 4 to 6 hours after each procedure. All patients were followed up with a clinical examination 2 weeks postoperatively performed by an independent and blinded for the allocation investigator.

Data collection Demographic and epidemiological data were collected prospectively. The primary endpoints were: the ratio of acute postoperative complications and the diagnostic yield. The presence of acute postoperative complication was noted if any of following findings was noted: (wound site infection up to two weeks after the operation, a new neurological deficit developed up to 24 hours following the operation and present in a follow up clinical examination 2 weeks postoperatively), intraparenchymal hematoma with radiological or clinical signs of the intracranial expansion.) and the diagnostic yield. The diagnostic yield was expressed according to the literature as a percentage of patients in whom the histopathological diagnosis was possible on the basis of the biological material obtained during the operation. Secondary endpoints included: the preoperative (LOSpre), postoperative (LOSpost) and total length of hospital stay (LOS) as well as the preparation (Tprep), operation (Top) and total operating room (TOR) time. The LOS and T were routinely measured and recorded in the central hospital files by the independent staff.

Inclusion Criteria:

• male and female patients ≥ 18 years
• supratentorial brain tumour
• scheduled to undergo STx biopsy

Exclusion Criteria:

• patients unable to provide informed consent
• metal implants which could prevent or influence the head MR study

A postoperative follow- up head CT was subsequently performed 4 to 6 h after each procedure. All patients were followed up with a clinical examination 2 weeks postoperatively performed by an blinded for the allocation investigator.