Dosimetric Comparison of Dedicated Radiosurgery Platforms for the Treatment of Essential Tremor: Technical Report.

Essential tremor (ET) is one of the most common adult movement disorders. As the worldwide population ages, the incidence and prevalence of ET is increasing. Although most cases can be managed conservatively, there is a subset of ET that is refractory to medical management. By virtue of being "reversible", deep brain stimulation (DBS) of the ventral intermediate nucleus (VIM) of the thalamus is one commonly accepted intervention. As an alternative to invasive and expensive DBS, there has been a renaissance in treating ET with lesion-based approaches, spearheaded most recently by high-intensity focused ultrasound (HIFU), the hallmark of which is that it is non-invasive. Meanwhile, stereotactic radiosurgical (SRS) lesioning of VIM represents another time-honored lesion-based non-invasive treatment of ET, which is especially well suited for those patients that cannot tolerate open neurosurgery and is now also getting a "second look". While multiple SRS platforms have been and continue to be used to treat ET, there is little in the way of dosimetric comparison between different technologies. In this brief technical report we compare the dosimetric profiles of three major radiosurgical platforms (Gamma Knife, CyberKnife Robotic Radiosurgery, and Zap-X Gyroscopic Radiosurgery (GRS)) for the treatment of ET. In general, the GRS and Gamma Knife were shown to have the best theoretical dosimetric profiles for VIM lesioning. Nevertheless the relevance of such superiority to clinical outcomes requires future patient studies.

X-ray Target Shielding and Leakage Radiation Inside the Treatment Capsule of the Zap-X.

The ZAP-X represents the first-of-its-kind "self-shielded" therapeutic radiation device, which by novelty, challenges regulators to accommodate it within the existing regulatory framework for radiation protection. To facilitate informed regulatory interpretation, X-ray radiation leakage from the ZAP-X was measured inside the shielded treatment capsule at the level of the patient and X-ray target plane. Measurements were performed on a clinically commissioned system calibrated for reference conditions to deliver 1cGy/MU. Radiation was measured with a FLUKE 451 survey meter and a RadCal ionization chamber as both exposure and dose and presented as a percentage of the system reference dose. Measurements were taken at thirteen locations, eight in the patient plane and five in the X-ray target plane. The results showed a maximum X-ray leakage of 0.000986% in the patient plane and 0.000907% in the target plane. These results are 30 - 100 times lower than existing recommendations as referenced by IEC guidelines standard 60601-2-1 (2020) for radiotherapy linear accelerators (LINACs). Although most conventional LINACs apply a safety factor of 2-5 to the design of collimator shielding and patient dose sparing, the ZAP-X delivers less than 10% of the patient whole body dose compared to this standard, originating from the X-ray target. Even though the ZAP-X intensity modulated radiation therapy (IMRT) factor is significantly higher than conventional Linacs, the absolute dose originating from leakage radiation remains lower by 25. The amount of unintended dose received by the patient's body distant from the isocenter is of interest from the perspective of both clinical and radiation safety. As this whole-body dose is decreased, the resulting treatment-related cancer incidence and mortality rates are decreased accordingly.

Characterization of the ZAP-X® Peripheral Dose Fall-Off.

Various small-field radiation dose detectors were systematically compared and their impact on measured beam performance of the ZAP-X® dedicated stereotactic radiosurgery system (ZAP Surgical Systems, Inc., San Carlos, CA, USA) was determined. Three Physikalische Technische Werkstaetten (PTW) diodes, i.e., the microSilicon, the microDiamond, and the Stereotactic Radiosurgery (SRS) diode detectors of (PTW-Freiburg, Freiburg, Germany), as well as Gafchromic™ External Beam Therapy 3 (EBT) film (Ashland, Inc., Wilmington, DE, USA), were used and compared to arrive at a recommended standard for this critical component of small-field beam measurements. Beam profiles, including the dose fall-off region near the edge of the beam, were measured with the PTW diodes and EBT3 film and subsequently contrasted. The impact of detector physical and dosimetric characteristics on the results of the measurements was investigated and compared with film measurements. The beam penumbra was used to quantify the dose fall-off. The measurement acquired with the diodes and film showed the most significant differences in the fall-off region near the field edge. The film-based measurements clearly showed the steepest dose gradient verified by the penumbra value of 1.21 mm, followed by the SRS diode with 1.60 mm, the microSilicon diode with 1.67 mm, and the microDiamond diode with 1.83 mm. A clear correlation of each detector's sensitive area with the penumbra was found, with the microDiamond detector at 2.2 mm diameter sensitive area having the largest penumbra, followed by the microSilicon and SRS diodes. Beam measurements for the purposes of system characterization or treatment planning system beam data acquisition depend, to a large extent, on detector characteristics. This is especially true for small-field dosimetry performed during stereotactic radiosurgery beam measurements. Careful consideration should be practiced which allows for the measurements to represent true beam characteristics and minimize the impact of the detector on the measurements. We conclude that film should be considered the reference method for such measurements with the ZAP-X due to its smallest physical measurement resolution of 23.1 µm. Potential drawbacks to this methodology are the need to calibrate the film relative to the dose and possible problems with saturation and non-linear film response for very high and very low optical densities.

Self-Shielding for the ZAP-X®: Revised Characterization and Evaluation.

The ZAP-X® is a newly designed, self-contained, and first-of-its-kind self-shielded therapeutic radiation therapy device dedicated to brain and head and neck stereotactic radiosurgery (SRS). By using an S-band linear accelerator (linac) and employing integrated minimal but sufficient shielding, the ZAP-X does not typically require a radiation bunker. At the same time, the self-shielded features of the ZAP-X are designed for more consistency of radiation protection, reducing the risk to radiation workers and others potentially exposed from a poorly designed or constructed radiotherapy vault. This study postulates that a radiosurgical system can be self-shielded, such that it produces radiation exposure levels deemed safe to the public while operating under a full clinical workload. The goal of self-shielding is achieved under all but the most exceptional clinical conditions. This work is intended to serve as guidance for the radiation safety evaluations of future ZAP-X treatment operations, following local or regional applicable regulatory requirements, and utilizing the unique provision of all or most of the required shielding material as an integral part of the device.

Concept for a Fan-beam Computed Tomography Image-guided Radiotherapy Device.

Many advancements taking place in the field of radiation therapy come in the form of increasingly powerful devices and specialized treatments that aim to increase precision, visualization, and facility throughput. Although these devices are very effective at their respective roles within radiotherapy, they are expensive and require specialized vaults to shield the public and the radiation worker from the ionizing radiation. A proposed device, known as the Simple XRT, is designed to circumvent the inherent drawbacks of the current devices. The Simple XRT uses a 6 MV linear accelerator that utilizes diagnostic quality computed tomography (CT) image guidance. Simple XRT will serve as a cost-effective device for treating most cancer indications.

Characterization of a Novel 3 Megavolt Linear Accelerator for Dedicated Intracranial Stereotactic Radiosurgery.

The purpose of this article is to investigate and characterize from a physics perspective the Zap-X (ZAP Surgical Systems, Inc., San Carlos, CA), a new, dedicated self-contained and self-shielded radiosurgery system, focusing on beam energy and performance, leakage, radiation safety, dose delivery accuracy, regulations, quality assurance, and treatment planning. This investigation is required to establish the mechanical and overall performance specifications of the system and to establish baseline parameters for future clinical usage. The applied methods include measurements of energy, focal spot size, beam performance, dosimetry, beam data, treatment planning system, leakage radiation, acceptance testing, and commissioning. The results of the characterization reveal a 3 megavolt (MV) linear accelerator (linac) with a focal spot size of 2 mm, a dose rate of 1,500 MU/min at the isocenter with a dose linearity of 3%, a beam penumbra of less than 3 mm, and beam symmetry of less than 2%. Beam performance, as well as dosimetry characteristics, are suitable for intracranial radiosurgery. It can be concluded that the system was found to meet safety, accuracy, and performance requirements widely accepted in the radiation oncology and radiosurgery industry. Furthermore, the system was shown to meet the practical, clinical needs of the radiosurgery community.

Correction: Characterization of a Novel Revolving Radiation Collimator.

[This corrects the article DOI: 10.7759/cureus.2146.].

Correction: Self-Shielding Analysis of the Zap-X System.

[This corrects the article DOI: 10.7759/cureus.1917.].

Ventricular Tachycardia: A Treatment Comparison Study of the CyberKnife with Conventional Linear Accelerators.

The work described here compared the available technical solutions for the treatment of ventricular tachycardia with stereotactic body radiation therapy. Due to the complexity of target motion during cardiac and pulmonary motion as well as the several proximate radio-sensitive structures of the tracheobronchial tree and esophagogastrointestinal tract, four potential candidates for this treatment were identified: Accuray CyberKnife (Accuray Incorporated, Sunnyvale, California, United States), Varian TrueBeam (Varian Medical Systems, Palo Alto, California, USA), Elekta Infinity (Elekta, Stockholm, Sweden), and Varian Edge (Varian Medical Systems, Palo Alto, California, USA). All four treatment modalities were evaluated for their ability to deliver a conformal, homogeneous dose to most of the target volume, to spare nearby and distant critical and sensitive anatomical structures as well as for treatment efficiency. It was found that conventional linear accelerator technology was superior in their ability to spare distant critical structures and deliver treatments efficiently while the CyberKnife showed superiority in sparing nearby critical structures more aggressively by creating larger dose gradients at the periphery of the target volume. Both treatment modalities were similar in their ability to cover the entire target with the prescription dose, conform that dose to the target volume, and deliver a homogeneous dose.

Artificial Intelligence in Medicine and Radiation Oncology.

Artifical Intelligence (AI) was reviewed with a focus on its potential applicability to radiation oncology. The improvement of process efficiencies and the prevention of errors were found to be the most significant contributions of AI to radiation oncology. It was found that the prevention of errors is most effective when data transfer processes were automated and operational decisions were based on logical or learned evaluations by the system. It was concluded that AI could greatly improve the efficiency and accuracy of radiation oncology operations.

Characterization of a Novel Revolving Radiation Collimator.

Introduction The ZAP-X is a novel self-contained and first-of-its-kind self-shielded therapeutic radiation device dedicated to brain and head and neck radiosurgery. By utilizing a 2.7-MV linear accelerator and incorporating a design in which a beam stop and major mechanical elements serve a radiation shielding function, the Zap-X does not typically require a radiation bunker. The unique collimator design of the Zap-X is especially critical to the performance of the overall system. The collimator consists of a shielded tungsten wheel oriented with its rotational axis perpendicular to the beam's central axis; the goal of this design is to minimize radiation leakage. Beam selection is accomplished by rotating the wheel within its tungsten-shielded housing. We investigated radiation leakage from the Zap-X collimator to determine its compliance with internationally accepted standards using direct radiation measurements. Materials and methods To measure collimator leakage in the plane of the patient, equidistant measurement stations were defined in a plane perpendicular to the central beam axis (cax) 1 m from this axis (1 m from the radiation focal spot). To measure leakage alongside and adjacent to the accelerator, equidistant measurement stations were located 1 m from the cax along a line parallel to the cax in the plane of the collimator wheel and along a line parallel to the cax 90 degrees offset from the first line of stations. Results Radiation leakage emanating from the collimating head of the linear accelerator in the patient plane ranged between 4.0 and 10.4 mR. Radiation along the linear accelerator (1000 R delivered in the primary beam) varied between 1.7 and 6.8 mR and constituted between 0.00017% to 0.00068% of the primary beam. The former radiation originated from X-ray target leakage, while the latter is produced directly by the linear accelerator and both contributed to the overall leakage radiation that would reach a patient. Discussion Due to the large diameter of the Zap-X tungsten collimator wheel and the massive Zap-X tungsten cylindrical collimator shield, the overall patient leakage is 0.00104% of the primary beam at a 1-m distance from the beam central axis in the patient plane. Leakage radiation in the patient plane is limited by the International Electrotechnical Commission (IEC) to 0.1% of the total primary radiation. Radiation leakage along the linear accelerator and the collimator housing was determined to be 0.00068% of primary radiation intensity. This leakage value is lower than the 0.1% leakage limit stipulated by IEC by more than a factor of 100. Conclusions Typically, an MV radiation therapy system minimizes exposure by utilizing a combination of device and structural shielding. However, the Zap-X has been uniquely designed to minimize the need for structural shielding. Our results indicate radiation leakage from the collimator meets internationally accepted standards as defined by the IEC.

Self-Shielding Analysis of the Zap-X System.

The Zap-X is a self-contained and first-of-its-kind self-shielded therapeutic radiation device dedicated to brain as well as head and neck stereotactic radiosurgery (SRS). By utilizing an S-band linear accelerator (linac) with a 2.7 megavolt (MV) accelerating potential and incorporating radiation-shielded mechanical structures, the Zap-X does not typically require a radiation bunker, thereby saving SRS facilities considerable cost. At the same time, the self-shielded features of the Zap-X are designed for more consistency of radiation protection, reducing the risk to radiation workers and others potentially exposed from a poorly designed or constructed radiotherapy vault. The hypothesis of the present study is that a radiosurgical system can be self-shielded such that it produces radiation exposure levels deemed safe to the public while operating under a full clinical workload. This study summarizes the Zap-X system shielding and found that the overall system radiation leakage values are reduced by a factor of 50 compared to the occupational radiation limit stipulated by the Nuclear Regulatory Commission (NRC) or agreement states. The goal of self-shielding is achieved under all but the most exceptional conditions for which additional room shielding or a larger restricted area in the vicinity of the Zap-X system would be required.

Radiosurgical Treatment Verification Using Removable Megavoltage Radiation Detectors.

Introduction Accurate dose delivery is critical to the success of stereotactic radiosurgery. Unfortunately, verification of the accuracy of treatment delivery remains a challenging problem. Existing radiosurgery delivery paradigms are limited in their ability to verify the accurate delivery of radiation beams using data sampled from the beam after it has traversed the patient. The Zap-X Radiosurgery System (Zap Surgical Systems, San Carlos, CA) addresses this issue by implementing a fully integrated treatment delivery system that utilizes a factory commissioned megavoltage (MV) imager to measure the transmitted beam. The measured intensity is then compared with an expected value in order to confirm that treatment is proceeding as expected. The purpose of this study was to evaluate a prototype system and investigate the accuracy of an attenuation model used in generating the expected transmitted intensity values. Methods A prototype MV imager was used to measure transmitted beam intensities at various exposure levels and through several thicknesses of solid water. The data were used to evaluate imager linearity and model accuracy. Results Experimental results indicate that a quadratic attenuation model is appropriate for predicting beam attenuation and that the imager exhibits excellent dose linearity. Conclusions The MV imager system is shown to be capable of accurately acquiring the data needed to confirm treatment validity.

Cone Beam CT vs. Fan Beam CT: A Comparison of Image Quality and Dose Delivered Between Two Differing CT Imaging Modalities.

A comparison of image quality and dose delivered between two differing computed tomography (CT) imaging modalities-fan beam and cone beam-was performed. A literature review of quantitative analyses for various image quality aspects such as uniformity, signal-to-noise ratio, artifact presence, spatial resolution, modulation transfer function (MTF), and low contrast resolution was generated. With these aspects quantified, cone beam computed tomography (CBCT) shows a superior spatial resolution to that of fan beam, while fan beam shows a greater ability to produce clear and anatomically correct images with better soft tissue differentiation. The results indicate that fan beam CT produces superior images to that of on-board imaging (OBI) cone beam CT systems, while providing a considerably less dose to the patient.

Analysis of Dose Distribution in the Heart for Radiosurgical Ablation of Atrial Fibrillation.

In a treatment planning study, radiosurgical treatment plans designed to produce lesions on the left atrium were created using two different methodologies. In one, structures in the heart (mitral valve and coronary arteries) were designated as critical structures while this was not done in the second plan. The treatment plans that were created were compared with standards for heart dose used when treating spine tumors. Although the dosage for the whole heart greatly exceeded the dose standards, when only the dose to the ventricles was considered, the plan where the mitral valve was spared was very close to the dose standards. The ventricles received a substantially higher dose in the plan where the mitral valve was not a critical structure. Although neither treatment plan was delivered, this study demonstrated the feasibility of treating the heart while minimizing dose to the ventricles.