Optimal Volume of the Residual Tumor to Predict Long-term Tumor Control Using Stereotactic Radiosurgery after Facial Nerve-preserving Surgery for Vestibular Schwannomas

Abstract

Background: Intended subtotal resection (STR) followed by adjuvant gamma knife radiosurgery (GKRS) has emerged as an effective treatment option for facial nerve (FN) preservation in vestibular schwannomas (VSs). This study aimed to identify the optimal cut-off volume of residual VS to predict favorable outcomes in terms of both tumor control and FN preservation.

Methods: This retrospective study assessed the patients who underwent adjuvant GKRS for residual VS after microsurgery. A total of 68 patients who had been followed up for ≥ 24 months after GKRS were included. Tumor progression was defined as an increase in tumor volume (TV) of ≥ 20%. House-Brackmann grades I and II were considered to indicate good FN function.

Results: The median residual TV was 2.5 cm³ (range: 0.3-27.4). The median follow-up period after the first adjuvant GKRS was 64 months (range: 25.7-152.4). Eight (12%) patients showed tumor progression. In multivariate analyses, residual TV was associated with tumor progression (P = 0.003; hazard ratio [HR], 1.229; 95% confidence interval [CI], 1.075-1.405). A residual TV of 6.4 cm³ was identified as the cut-off volume for showing the greatest difference in progression-free survival (PFS). The 5-year PFS rates in the group with residual TVs of < 6.4 cm³ (54 patients) and that with residual TVs of ≥ 6.4 cm³ (14 patients) were 93.3% and 69.3%, respectively (P = 0.014). A good FN outcome was achieved in 57 (84%) patients. Residual TV was not associated with good FN function during the immediate postoperative period (P = 0.695; odds ratio [OR], 1.024; 95% CI, 0.908-1.156) or at the last follow-up (P = 0.755; OR, 0.980; 95% CI, 0.866-1.110).

Conclusion: In this study, residual TV was associated with tumor progression in VS after adjuvant GKRS following STR. As preservation of FN function is not correlated with the extent of resection, optimal volume reduction is imperative to achieve long-term tumor control. Our findings will help surgeons predict the prognosis of residual VS after FN-preserving surgery.

Keywords: Facial Nerve; Gamma Knife Radiosurgery; Tumor Volume; Vestibular Schwannoma.

Fig. 1. Flow chart showing the numbers of included and excluded patients.

A total of 68 patients were enrolled for the analyses. NF = neurofibromatosis, GKRS = gamma knife radiosurgery.

Fig. 2. Examples of postoperative changes in the residual tumor on serial MRIs. Preoperative (A), immediate postoperative (B), and 6-month postoperative (C) T-1 weighted contrast-enhanced MRI of right VS. The thin tumor capsule covering the facial nerve was left during the surgery (white arrowhead). Postoperative image taken at 6 months showing progressive closure of the tumorectomy cavity, and the residual tumor was changed to a shape more suitable for gamma knife radiosurgery (white arrow). Preoperative (D), immediate postoperative (E), and 6-month postoperative (F) MRI of left VS. The residual tumor (black arrow) left around the porus acusticus was observed on 6-month postoperative images, but it was not visualized on immediate postoperative imaging.

MRI = magnetic resonance imaging, VS = vestibular schwannoma.

Fig. 3. Box and whisker plots of residual TV distribution according to tumor progression The boxes indicate the 25th and 75th percentiles. The whiskers indicate the minimum and maximum values, dots indicate the outliers, asterisk indicates extreme values, and thick horizontal lines indicate the median value.

TV = tumor volume.

Fig. 4. Sensitivity and specificity analysis. Analyses were performed using the cut-off value of residual TV to predict tumor progression.

TV = tumor volume, CI = confidence interval. ^*P = 0.021; 95% CI, 0.546–0.960; area under the curve, 0.753.

Fig. 5. Kaplan-Meier plot of PFS according to residual TV cut-offs. (A) The 5-year PFS rates in the group with residual TVs < 6.4 cm³ (54 patients) and that with residual TVs ≥ 6.4 cm³ (14 patients) were 93.3% and 69.3%, respectively (P = 0.014). (B) The 5-year PFS rates in the group with residual TVs < 4.4 cm³ (43 patients) and that with residual TVs ≥ 4.4 cm³ (25 patients) were 93.8% and 78.6%, respectively (P = 0.049). (C) The 5-year PFS rates in the group with residual TVs < 2.3 cm³ (31 patients) and that with residual TVs ≥ 2.3 cm³ (37 patients) were 96.8% and 81.7%, respectively (P = 0.096).

PFS = progression-free survival, TV = tumor volume.