Zapping Vestibular Schwannomas: The First Vestibular Schwannoma Treated with the ZAP-X

Stereotactic radiosurgery is a well-documented treatment for small- to medium-sized vestibular schwannomas requiring intervention and tumor control. Refining the advancements made with the CyberKnife, the ZAP-X® Gyroscopic Radiosurgery™ platform features a frameless and self-shielded 3 MegaVolt linear accelerator (LINAC). It was installed at this institute in 2019 in a vault-free room that allows the neurosurgery team of surgeons, radiation oncologists, medical physicists, and nurses and technicians to manage the ZAP-X from inside the room – no shielded observation rooms are needed.

Thus far, the ZAP-x has been used in the treatment of small, round lesions like meningiomas and tumor metastases to the brain. It was first employed in the treatment of a vestibular schwannoma for a 79-year old female patient with a 5-year history of progressive left-sided hearing loss. Her vestibular schwannoma measured 26 × 18 mm, protruded out of the internal acoustic canal into the cerebellopontine angle, and had been growing continuously by ~1 mm per year. Her last audiogram demonstrated non-functional left-sided hearing. The patient's treatment plan was designed with the patient's fused computed tomography and magnetic resonance imaging (CT-MRI) studies. Simulations were conducted by the neurosurgeon, radiation oncologist, and medical physicist, which included alignment tracking and calculations and beam arrangements to ensure reproducibility and accuracy of the setup. The patient was prescribed 25 Gy to the 50% isodose line in 5 fractions over the course of 6 days. The treatment plan used 8 isocenters delivered through 4 collimator sizes: two each from 7.5 mm and 10.5 mm, three from 12.5-mm, and one from 15-mm collimators. Each fraction used 288 beams across 310 beam paths with 36 beams per path with a plan to deliver 5550 MU in just over 4 minutes. The ZAP-X delivered 27,732 MU to the patient across all 5 fractions over 20 minutes. The target coverage was 95.7%, irradiating the tumor isodose volume of 5.38 mL.

The ZAP-X has the potential to bring stereotactic radiosurgical treatment options to more treatment centers, making it accessible to providers and patients without the additional constraints seen in other stereotactic radiosurgery devices. The self-shielded ZAP-X design precludes the need for the shielded vaults that have traditionally provided radiation safety.

Fig. 1 An axial T1-weighted contrast-enhanced brain MRI indicates a possible vestibular schwannoma measuring 26 mm × 18 mm. MRI, magnetic resonance imaging.

Publication History

Article published online:
12 February 2021