68. FRAMELESS, VAULT-FREE RADIOSURGERY: INITIAL CLINICAL EXPERIENCE WITH THE ZAP-X STEREOTACTIC SYSTEM

The Zap-X is a novel self-contained and self-shielded dedicated radiosurgery system developed and manufactured by ZAP Surgical Systems, Inc. of San Carlos, California. Intended for the stereotactic radiosurgery (SRS) treatment of benign and malignant intracranial and cervical spine lesions, this gyroscopically stabilized 3 megavolt (MV) linear accelerator (LINAC) provides a unique radiosurgical alternative for selected patients. Beginning in January 2019, a total of 38 metastatic lesions in 24 patients were treated in our facility. Radiation prescription doses ranged from 1500–1900 cGy (single fraction) to 2500 cGy (five fractions), with treatment volumes ranging from .04 to 15.3 cc. Daily treatment times averaged 45 minutes or less. Target coverage, dose homogeneity, and conformality were comparable to the existing Gamma Knife, CyberKnife and LINAC-based radiosurgery treatment systems in daily use at our facility. As with other frameless radiosurgery platforms, the Zap-X proved particularly useful in situations where either surgery or single-fraction radiosurgery was considered a less desirable treatment option; or when fractionated radiosurgery was thought to be radiobiologically advantageous. All treatments were completed without complication. At two months post-treatment, all lesions showed a complete or partial response to therapy based on MRI scan. None of our patients experienced treatment-related skin reaction, cognitive deficit, fatigue or steroid dependency. Among patients who had previously undergone Gamma Knife treatment, there was a clear preference for frameless radiosurgery. In our experience, the Zap-X delivery system offers a high-precision, patient-friendly and cost-effective alternative to traditional dedicated radiosurgical platforms.

Evaluation of a new software prototype for frameless radiosurgery of arteriovenous malformations

Background In order to locate an arteriovenous malformation, typically, a digital subtraction angiography (DSA) is carried out. To use the DSA for target definition an accurate image registration between CT and DSA is required. Carrying out a non-invasive, frameless procedure, registration of the 2D-DSA images with the CT is critical. A new software prototype is enabling this frameless procedure. The aim of this work was to evaluate the prototype in terms of targeting accuracy and reliability based on phantom measurements as well as with the aid of patient data. In addition, the user’s ability to recognize registration mismatches and quality was assessed. Methods Targeting accuracy was measured with a simple cubic, as well as with an anthropomorphic head phantom. Clearly defined academic targets within the phantoms were contoured on the CT. These reference structures were compared with the structures generated within the prototype. A similar approach was used with patient data, where the clinically contoured target served as the reference structure. An important error source decreasing the target accuracy comes from registration errors between CT and 2D-DSA. For that reason, the tools in BC provided to the user to check these registrations are very important. In order to check if the user is able to recognize registration errors, a set of different registration errors was introduced to the correctly registered CT and 2D-DSA image data sets of three different patients. Each of six different users rated the whole set of registrations within the prototype. Results The target accuracy of the prototype was found to be below 0.04 cm for the cubic phantom and below 0.05 cm for the anthropomorphic head phantom. The mean target accuracy for the 15 patient cases was found to be below 0.3 cm. In the registration verification part, almost all introduced registration errors above 1° or 0.1 cm were detected by the six users. Nevertheless, in order to quantify and categorize the possibility to detect mismatches in the registration process more data needs to be evaluated. Conclusion Our study shows, that the prototype is a useful tool that has the potential to fill the gap towards a frameless procedure when treating AVMs with the aid of 2D-DSA images in radiosurgery. The target accuracy of the prototype is similar to other systems already established in clinical routine.