25.1 Introduction

Over the past 30 years, stereotactic radiosurgery (SRS) has emerged as an accepted treatment option for patients with small-to-moderated-sized vestibular schwannomas (VSs). Over this time period, changes to VS SRS have included the use of magnetic resonance imaging (MRI) for dose planning, improved dose planning software, lower radiation doses, and an appreciation that cochlear radiation dose may affect hearing outcomes.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur The most important modification in SRS technique was marginal dose reduction from 18–20 to 12–13 Gy, but the other alterations have also contributed to improved patient outcomes. Cranial nerve morbidity after contemporary VS SRS is markedly lower when compared to early studies, and facial nerve outcomes and early hearing preservation rates have been shown to be significantly better after SRS compared to surgical resection.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur Based on the efficacy and safety of SRS for properly selected VSs, more than 100,000 patients have undergone VS SRS. However, misinterpretation of the follow-up imaging after VS SRS is not uncommon and physicians may prematurely conclude that SRS has failed, leading to patients having unnecessary and inappropriate surgery. In this chapter, management recommendations are presented based on the imaging characteristics of VS after SRS.

25.2 Methods of Tumor Size Assessment

After VS SRS, patients are requested to have follow-up clinical evaluation to assess cranial nerve function, audiometric testing for patients with pre-SRS serviceable hearing, and MRI to determine tumor size, evaluate for edema in the adjacent brain, and monitor for ventricular enlargement. Although each center will follow its own protocol after VS SRS, we typically request follow-up at 6, 12, 24, and 48 months, and then biyearly after the procedure. However, many patients live a long distance from the center that performed SRS, so review of clinical information and MRI from local care providers is not uncommon.

Two methods are commonly used to assess tumor response after VS SRS. Both techniques rely on comparison of the tumor size at the time of SRS to follow-up examinations. Although computed tomography (CT) is necessary in some patients who cannot undergo MRI, the vast majority of patients are followed with serial MRI. The first method is based on the comparison of linear measurements of the cerebellopontine angle (CPA) component in the right–left (x-plane), anterior–posterior (y-plane), and superior–inferior (z-plane).s. Literatur In most cases, the intracanalicular tumor follows the CPA tumor response and is not formally compared. Tumor size is generally classified as unchanged, decreased, or increased. For most series, a change of 2 mm or greater in average tumor diameter is needed for tumors to be considered decreased or increased in size. As noted by Linskey et al,s. Literatur the change in tumor volume necessary for tumors to be significantly changed compared to the time of SRS must account for potential measurement errors and the initial tumor size. For example, tumors with an average diameter < 1 cm must increase approximately 100% in volume before tumor enlargement can be reliably identified, whereas tumors > 2 cm in average diameter need only a 25% increase to safely state that the tumor has enlarged. Linear measurement comparisons are simple and are the most common technique utilized in both clinical practice and published VS studies.

The second method is based on comparison of tumor volumes at the time of SRS to follow-up MRI.s. Literatur ^,​ s. Literatur This technique is more sensitive to small changes in tumor size than comparison of linear measurements, but it is more time consuming, requiring segmenting (outlining) the tumor on post-SRS to determine the tumor volume. Accurate volumetric determination is closely correlated to MRI slice thickness, especially for smaller tumors. In most series, tumor volume is defined as unchanged or stable if it falls within a range of 10 to 15% of the pre-SRS volume, so that tumors that change more than 10 to 15% are defined as enlarged or regressed. To date, quantitative volumetric analysis is used primarily as a research tool that more accurately defines tumor response after SRS, but its day-to-day value in the clinical management of patients remains unclear.

25.3 Expected Imaging Changes after Vestibular Schwannoma Radiosurgery

The imaging changes seen after SRS of VS were first recognized by Georg Norén during his time at the Karolinska Institute, Stockholm, Sweden.s. Literatur ^,​ s. Literatur He noted that the majority of VS will exhibit “swelling” with a loss of central enhancement and an increase in tumor size. This pseudoprogression (also described as tumor expansion) is generally noted 3 to 12 months after SRS, and is followed by the tumor regaining its central enhancement on later imaging with a subsequent stabilization or reduction in tumor size (Fig. 25‑1 ).s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur The average increase in tumor volume during this period has been reported from 23 to 27%,s. Literatur ^,​ s. Literatur with the majority of tumors regressing to less than the pre-SRS volume on serial imaging (Fig. 25‑2 ). Previously, we described three patterns of tumor volume changes in VS patients whose tumors enlarged after SRS.s. Literatur Type 1 patients showed eventual tumor regression to either the same or reduced volume. Type 2 patients had tumors that increased and remained larger but did not show progressive enlargement (Fig. 25‑3 ). Type 3 patients showed progressive tumor enlargement on serial studies denoting treatment failure. Table 25‑1 shows the results for a number of studies on VS SRS. The range of treatment failure after VS SRS is between 2 and 12%, depending on the criteria used at each center.