A total of 664 patients which underwent surgery of BM were included in the study cohort. Sex was roughly equally distributed with 51% male (n = 337) and 49% female (n = 327). Mean patient age was 61 years and the BM most frequently originated from the lung (44%, n = 294) followed by melanoma (25%, n = 163) and breast cancer (11%, n = 74) as depicted in Table 1. In most patients a singular BM was present (50%, n = 336), whereas 2–4 lesions were present in 33% and > 10 lesions in 8.6% of the patients (see Table 1).

The most frequently used intraoperative tools were intraoperative neuronavigation (89%) and intraoperative ultrasound (78%). Not surprisingly, neuromonitoring (19%) and intraoperative MRI (4%) were used less often as shown in Table 1. Most of the surgeries were resections (96%) via one craniotomy (89%), while 6% (n = 40) of the patients underwent two and four patients underwent three craniotomies during the same surgery. Biopsy only cases accounted for 4% (n = 27) and 19% of cases were reoperations with previous BM resections (see Table 2, Overall).

78% of cases were located supratentorial, whereas 22% were located infratentorial (n = 145, see Table 2, Overall). Of the supratentorial lesions, 71% had a convexity and 29% a subcortical/deep location. 11% (n = 76) BM were located in the central region, which is comprised of the pre-, post- para- and subcentral gyrus [21]. Online resource 2 and Fig. 1 illustrate the gyral localization of the resected BM in the study cohort. Roughly half of the lesions were either located on the right or left side (50% vs. 44% respectively) while the minority were in the midline (2.6%, n = 17).

Anatomical BM location. BM brain metastases

The occurrence of any AE at discharge was recorded in 8% (n = 53, see Online resource 3) of the cases, most of which were recorded as CDG 1 and 2 (see Online resource 3), meaning that these AEs did not require any invasive interventions, but only pharmacological or no treatment. Severe AE requiring invasive interventions (CDG 3) or ICU stay (CDG 4) were present in only 1.9% (n = 12) and the mortality rate was 0.6% (n = 4, CDG 5). New neurological deficits occurred in 2% of the entire study cohort (n = 13, see Online resource 3), followed by urinary tract infections, pulmonary artery embolism (each 0.8%, n = 5), postoperative hemorrhage and pneumonia (each 0.6%, n = 4).

Stratifying the study cohort according to the occurrence of major AEs (defined as CDG ≥ 2 or new neurological deficits at discharge), Table 2 shows the occurrence of major AEs for the different anatomic and surgical parameters. Cases with major AEs were more frequently located infratentorial (40% vs. 20%, p ≤ 001, Pearson’s Chi-squared test) or in the midline (8% vs. 2%, p = 0.013, Fisher’s exact test; see Table 2). Major AEs were not associated with deep-seated lesions, the number of craniotomies or emergency operations (see Table 2). Figure 2 shows the postoperative change in clinical outcome scales for the two groups: Patients who experienced an AE had higher mRS scales at discharge compared to admission (see Fig. 2B) compared to patients who did not experience any AE (see Fig. 2A). In addition, considering the relative changes in mRS (see Fig. 2C) and KPS (see Fig. 2D) between discharge and admission, more patients with an AE suffered from a worsening of mRS (44%) and KPS (49%) compared to patients without AEs (11% each, p < 0.001 for mRS and KPS, Pearson’s chi-squared test).

Clinical outcome scale changes at discharge. Percentages of mRS scores at admission (upper row) and discharge (lower row) for patients without AE (A) and for patients with AE (B). Changes of mRS (C) and KPS (D) at discharge relative to admission stratified for the occurrence of AE. AE at discharge were considered as CDG ≥ 2 or new neurological deficits. AE adverse event, mRS modified Rankin Scale, KPS Karnofsky Performance Status

In univariate logistic regression analysis, ASA status (OR 1.6, 95% CI 1.01–2.54, p = 0.044), tumor location in the midline (OR 3.94, 95% CI 1.04–12.3, p = 0.037) and cerebellar tumor location (OR 3.44, 95% CI 1.73–6.95, p = 0.005) were significant predictors of postsurgical major AEs (see Online resource 4). Furthermore, in a subsequent multivariate logistic regression analysis, cerebellar BM location (OR 5.46, 95% 2.31–13.8, p = 0.001), reoperation (OR 2.31, 95% 1.07–4.81, p = 0.033, see Table 3) and central region tumors (OR 3.03, 95% 1.03–8.60) were all associated with the occurrence of postoperative AEs. Looking at overall survival (OS), Kaplan–Meier curve analysis showed significant differences in OS between patients with major AEs and the ones without AEs (p = 0.044, log-rank test, see Online resource 5). Additionally, patients with higher BM load (n ≥ 5 BM) showed significantly impaired OS rates compared to patients with fewer BM (n < 5; p < 0.001 log-rank test, see Online resource 6).