2 - A Technical Overview of the CyberKnife System

Simulation software for medical linear accelerators that can be used in a teaching environment was developed. The components of linear accelerators were modeled to first order accuracy using analytical expressions taken from the literature. The expressions used constants that were empirically set such that realistic response could be expected. These expressions were programmed in a MATLAB environment with a graphical user interface in order to produce an environment similar to that of linear accelerator service mode. The program was evaluated in a systematic fashion, where parameters affecting the clinical properties of medical linear accelerator beams were adjusted independently, and the effects on beam energy and dose rate recorded. These results confirmed that beam tuning adjustments could be simulated in a simple environment. Further, adjustment of service parameters over a large range was possible, and this allows the demonstration of linear accelerator physics in an environment accessible to both medical physicists and linear accelerator service engineers. In conclusion, a software tool, named SIMAC, was developed to improve the teaching of linear accelerator physics in a simulated environment. SIMAC performed in a similar manner to medical linear accelerators. The authors hope that this tool will be valuable as a teaching tool for medical physicists and linear accelerator service engineers.PACS number: 87.55Gh, 87.56bd

It is generally known that the use of high-energy electron linear accelerators (LINACs) in radiotherapy medical treatments may generate secondary neutrons, mainly via photonuclear (gamma, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> ) giant dipole resonance reactions of incident photons with all the heavy materials present inside the gantry and along the beam line. A detailed knowledge (i.e., fluence energy distribution) of this parasite radiation, which is approximately isotropic and not confined within the primary LINAC beam field, would be of great interest to estimate the associated radiological risk for the patient and the working staff. It has been shown, in this study, that our recently developed passive Bonner sphere system, using pure gold activation foils as central detectors, is well adapted to measure neutron spectra at pulsed and intense mixed n-gamma fields with high-energy photon component. This system was used to characterize the neutron field around a new generation medical electron LINAC. Two measurement positions (isocenter and maze entrance) inside the treatment room of this facility, with the machine operating in Bremsstrahlung photon mode, were chosen. The obtained specific <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">198</sup> Au saturation activities were processed by means of the NUBAY unfolding code, which performs a Bayesian estimation of a parameterized spectrum, to derive the final neutron spectra. Another unfolding method (MAXED), based on the maximum entropy principle and which may depend to some extent on considered the initial guess or default spectrum, was also applied to check the robustness of the NUBAY solutions as well as to carry out a sensitivity analysis to confirm their stability and to corroborate the associated uncertainties on their output results. Also presented are the obtained integral quantities, in terms of neutron fluence and ambient dose equivalent rates normalized to the primary LINAC photon dose, together with an estimation of their associated uncertainties due to the unfolding code used.

Radiotherapy of moving targets, such as lung, liver or prostate tumors, is prone to treatment errors due to the uncertainties in the target position. Minimizing such error is critical for radiotherapy treatments, especially hypofractionated SBRT treatments. On‐board real time imaging verification and tracking of moving targets has become one of the most exciting areas in radiotherapy. The tremendous developments in this area enable us to improve targeting accuracy and reduce the healthy tissue toxicity, which paves the road to further margin reduction and dose escalation in conventionally fractionated or SBRT treatments.Over the past few years, several real‐time imaging techniques have been clinically implemented, including techniques using ionizing radiation, such as x‐ray based imaging, and radiation‐free techniques such as MRI, imaging based on implanted electromagnetic transponders and optical imaging. The x‐ray imaging techniques are widely accessible in clinics, although they have limited soft tissue contrast and contribute to patient exposure. Recently, MRI has gained wide interests in radiotherapy due to its better soft tissue contrast and no radiation dose. Imaging based on electromagnetic transponders and optical imaging have also gained popularity in many clinical scenarios due to their unique capabilities.Based on these imaging techniques, various tracking approaches have been developed. The room mounted orthogonal 2D x‐ray system (CyberKnife system) was the first medical device capable to perform real‐time target tracking by combining image guidance with robotically targeted radiation delivery. In the Vero system, tumor tracking is achieved by combining image‐guidance with a two‐dimensional pivot assembly of the linear accelerator. In gantry‐based linear accelerators, target motion can be tracked in real time by repositioning the MLC based on the signal from x‐ray images or electromagnetic transponders implanted.The rapid development of new imaging and tracking technologies raises new challenges in revolutionizing our treatment procedures. It becomes important to understand the functionality, limitations and clinical impact of each technique so that they can be chosen wisely for different clinical settings. The goal of this educational session is to compare and contrast different imaging and tracking techniques implemented in CyberKnife, Vero, MRI based, and gantry‐based radiotherapy machines. The general concepts of real‐time imaging and tracking, from imaging the target to adapting the treatment, will be reviewed for each modality. While discussing the technical aspects of the systems including the overall uncertainties and residual imaging and tracking errors, emphasis will be placed on the clinical implementation and workflow. The increased patient dose due to x‐ray imaging and quality assurance procedures will be described. Additionally, the impact of tracking on treatment volume and margin reduction will be discussed. The session will close with an outlook at future developments.Learning Objectives: Understand the opportunities and challenges of real time imaging and tumor tracking. Understand the advantages and limitations of different imaging and tracking techniques, as well as the clinical implementation of each technique. Understand the clinical impact of real time tumor tracking and future directions in this area. Supported by National Institutes of Health, Varian Medical System, Accuray Inc.

High energy x-rays from a medical linear accelerator (LINAC) is used to treat cancer patients in radiotherapy department. The medical linac can produce bremsstrahlung x-rays from 4MV to 18MV and the commonly used linac produces 6MV, 10 MV and 15MV photons for treating major parts of the cancer patients. The photo neutron production during the radiation treatment is one of the major issues which have to be addressed with high impact because of the high LET nature associated with it. Photo neutrons are produced from several high atomic number (Z) elements present in the linac head mostly by the (ɣ, n) reaction. The flattening filter used in medical linac to produce uniform intensity distribution (made up of either tungsten, steel, lead, uranium, aluminum or a combination) is one of the photo neutron sources in the linac head. Advanced radiotherapy treatments like Intensity Modulated Radiotherapy (IMRT) and Volumetric Arc Therapy (VMAT) doesn't require a flat radiation field, and hence the flattening filter could be removed. This is called flattening filter free beam (FFF beam)1, 2. The removal of the filter will result in a forward peaked photon beam, which is very advantageous for these kinds of treatments3, 4. The purpose of this work was to find out the difference in the percentage of photo neutron production in a flattening filter free (FFF) and flattening filter (FF) beam using CR39 Solid State Nuclear Track Detector (SSNTD) from an Elekta Versa HD machine. The neutron fluence was measured at different positions with FFF and FF mode and the results were tabulated. The neutron fluence was lower for the FFF mode of operation than the FF mode. The reduction ranges from 2.27% to 44.22%.

Background:One of the most significant Intensity Modulated Radiation Therapy treatment benefits is a high target to normal tissue dose ratio. To improve this advantage, an additional accessory such as a compensator is used to deliver doses. Compensator-based IMRT treatment is usually operated with an energy higher than 10 MV. Photoneutrons, which have high linear energy transfer and radiobiological effectiveness, are produced by colliding high-energy photon beams with linear accelerator structures, then they deliver the unwanted doses to patients and staff. Therefore, the neutron energy spectra should be determined in order to calculate and reduce the photoneutron risk. Objective:We have conducted a comprehensive and precise study on the influence of brass compensator thickness and field size on neutron contamination spectrum in an Elekta SL 75/25 medical linear accelerator with and without the flattening filter by Monte Carlo method. Material and Methods: MCNPX MC Code version 2.6.0 was utilized to simulate the detailed geometry of Elekta SL 75/25 head components based on Linac’s manual. This code includes an important feature to simulate the photo-neutron interactions. Photoneutrons spectrum was calculated after the Linac output benchmarking based on tuning the primary electron beam. Results and Conclusion: Based on the Friedman and Wilcoxon nonparametric tests results (P<0.05), photoneutron fluence directly depends on the field size and compensator thickness. Moreover, the unflattened beam provides lower photoneutron fluence than the flattened beam. Photoneutrons fluence is not negligible in compensator-based IMRT treatment. However, in order to optimize treatment plans, this additional and unwanted dose must be accounted for patientss.

Purpose: The goal of image‐guided radiotherapy is to deliver the planned conformal radiation dose precisely to the target tumour volume and minimize dose to nearby normal tissue. To further this goal, integration of a linear accelerator (LINAC) with an MRI has been proposed. An obstacle to the integration of a LINAC with an MRI is the RF interference between the two devices. Our measurements indicate that LINAC induced RF occurs in the MHz range, with wavelengths on the order of 10s of meters, necessitating the RF measurements to be made in the near field. The purpose of this work is to develop and validate a method of measuring RF field patterns using standard dipole antennas both in near and far fields. Method and Materials: To validate the measurement technique, near and far field electric (E) and magnetic (H) field patterns were measured using specialized E and H field probes from two dipole radiation sources, and compared to the theoretical values. Antennas and probes were mounted in controlled geometry using a specially made wooden stand. The wave impedance (E/H) was also calculated and compared to theory. Results: For E and H fields, angular and radial field strength patterns demonstrate compliance with theory in both the near and far fields. The E field measurements oscillate around the theoretical falloff as a function radial distance, these oscillations are predominantly a result of reflections from the room structures. Our room simulations are consistent with this interpretation. Wave impedance in the near field agrees with theoretical predictions. Conclusion: A technique to measure the RF field strength in the near and far fields has been demonstrated. This technique is suitable for measuring RF field values in the vicinity of a medical LINAC enabling quantification of RF interference between a LINAC and MRI.

Although optical imaging of electron beams and X-rays from medical linear accelerators (LINAC) is a possible method for dose distribution measurements, it has been limited to two-dimensional (2D) projection images. For the precise measurement of an optical image of electron beams and X-rays, three-dimensional (3D) imaging is desired. To measure 3D dose distributions, we conducted imaging of electron beams and X-rays using a plastic scintillator plate set in a water phantom. When this plate was immersed in the water phantom, irradiation with electron beams or X-rays was carried out from along the plate's sides. Optical images of the scintillator plate were acquired using a charge-coupled device (CCD) camera from the side during irradiation with electron beams and X-rays. Measurements were conducted at 6 MeV, 9 MeV and 12 MeV for electron beams and at 6 MV and 10 MV for X-rays. The imaging system was set on the bed of the LINAC and moved at 10-mm steps perpendicular to the beam direction to acquire a set of sliced optical images of the beams. A set of these sliced images were stacked and interpolated to form 3D optical images. For the 3D images, after the correction of the Cherenkov-light component in the images, the relative depth and lateral doses were evaluated. From the relative depth doses of electron beams, the half-value depths could be evaluated within an error of 1.3 mm. Lateral widths could be evaluated within an error of less than 2 mm parallel to the plastic scintillator and less than 6.5 mm perpendicular to it. From the relative depth doses of X-rays, the average difference between the measured value and that by a planning system was within an error of 2 %. Lateral widths could be evaluated within an error of less than 0.68 mm parallel to the plastic scintillator and less than 2.6 mm perpendicular to it. We confirmed that 3D imaging of electron beams and X-rays using plastic scintillator plate is feasible and is a promising method for measuring dose images at any position.

Cobalt-60 Machines and Medical Linear Accelerators: Competing Technologies for External Beam Radiotherapy

Surgical resection is the preferred treatment for locoregional recurrence of differentiated thyroid cancer (DTC). However, some recurrences are unresectable because of their aggressive invasion or severe adhesions. On the other hand, stereotactic radiotherapy (SRT) enables high-dose irradiation to target lesions, and its usefulness for various cancers has been reported. The objective of the present study was to investigate the feasibility and efficacy of SRT as salvage treatment for locoregional recurrence of DTC. Between August 2011 and December 2017, 52 locoregional recurrent lesions in 31 patients with recurrent DTC were treated by SRT using the CyberKnife system. Information on the adverse events associated with SRT was retrospectively collected from the patients' medical records. Of the 52 lesions, 33 could be evaluated for therapeutic effectiveness by follow-up CT, and response was assessed using the RECIST criteria. Twenty-five patients had papillary carcinoma, 5 had follicular carcinoma, and 1 had poorly differentiated cancer. SRT was delivered in one to 20 fractions, and the median dose was 30Gy (range 15-60Gy). Adverse events were not frequent, but 1 patient developed bilateral vocal cord palsy that required emergent tracheostomy. The median follow-up period of 33 lesions was 14months (range 1-54months). Complete response, partial response, stable disease, and progressive disease were seen in 10, 11, 9, and 3 patients, respectively. The 3-year local control rate was 84.6%. SRT using the CyberKnife system was found to be a feasible and effective treatment to suppress the growth of locoregional recurrence of DTC.

This book is the updated successor to The Physics of Radiotherapy X-Rays from Linear Accelerators, published in 1997 by three well-respected radiotherapy physicists from Australian centres. Time marches on and so does the subject. The last decade has been a time of rapid and exciting change for radiation oncology and radiotherapy physics and this new book aims to update the existing areas and to cover new ones. It is much bigger than the original, up from 492 to 905 pages. The title and content has gained electrons, which always seemed like a major omission from the first version. The title has dropped 'from Linear Accelerators', and the authors have added a short section on kV x-rays and a short section on protons. There is also a nod towards brachytherapy in the beam modelling chapter. The authors have gained in seniority and roles, one becoming a Senior Product Manager at Tomotherapy, a company and a commercial treatment machine not in existence at the time of the last book. This alone gives a direct illustration of pace of change over the period!The immediately obvious changes in content are new chapters: on electron beam properties of course, but also on special procedures covering stereotactic radiosurgery, conventional linac intensity modulated radiation therapy (IMRT) and tomotherapy, on quality assurance in radiotherapy, including IMRT, image-guided radiotherapy (IGRT) and in vivo dosimetry, on patient immobilization and image guidance, and on radiation protection and room shielding. Other chapters have been split or rearranged, e.g., there are now separate chapters on dosimetry of beams and their calibration, whilst the beam modelling chapters have been expanded. There are chapters, or large sub-sections, that at first glance seem the same: on medical linear accelerators, interaction properties of radiation, dosimetry, beam properties, treatment planning, beam modelling, Monte Carlo modelling and tumour and normal tissue response. However, looking in more detail, there are new illustrations, added information, new and updated technologies and methods and updated references. There is a nice central section of colour illustrations. One other change is that the old linked book of problems and solutions (the 'Q book') has now gone. However the authors have updated the questions and answers and incorporated them into the main book (accounting for some of the size increase). The level of information is generally excellent. The chapters often combine information from different areas of the subject to make a comprehensive whole and to give a fuller explanation of their topic. These authors must be good teachers. The original book felt as though it grew from lecture courses and that is positive, as the information is at the right level. This, with the host of illustrations and the problems and answers, make it a good book for students. Whilst it is intended for medical physics students and trainees, it also contains a lot of material of interest for trainee radiation oncologists, dosimetrists and therapy radiographers (radiation therapists, etc). However those already in practice, including anyone who wants a quick update, will enjoy this book in the areas it covers. Remember it is not intending to cover all of radiotherapy physics, but what it sets out to cover, it generally covers well. If I am asked to choose highlights, it would be difficult, but I would select the overviews of some of the new technologies and also the updated sections on Monte Carlo and on tumour and normal tissue response and their practical applications, as these are not often done so well, or at the right level, in student texts. Sometimes the coverage is a little light, but this is a difficult balance. However the book does provide a wealth of references to follow up. Despite my praise of the illustrations, there are a few which are not well-reproduced and in which the presentation of information and the fonts are variable. However these are small niggles. I would recommend this book to students, trainees and staff members. Finally, I note with approval and renewed regret that the authors have dedicated the book to John Cameron, the founder of Medical Physics Publishing and the catalyst without whom many such excellent books would not necessarily have come to exist.

Object. The purpose of this work was to investigate the targeting and dosimetric characteristics of a linear accelerator (LINAC) system dedicated for stereotactic radiosurgery compared with those of a commercial gamma knife (GK) unit. Methods. A phantom was rigidly affixed within a Leksell stereotactic frame and axial computerized tomography scans were obtained using an appropriate stereotactic localization device. Treatment plans were performed, film was inserted into a recessed area, and the phantom was positioned and treated according to each treatment plan. In the case of the LINAC system, four 140° arcs, spanning ± 60° of couch rotation, were used. In the case of the GK unit, all 201 sources were left unplugged. Radiation was delivered using 3- and 8-mm LINAC collimators and 4- and 8-mm collimators of the GK unit. Targeting ability was investigated independently on the dedicated LINAC by using a primate model. Measured 50% spot widths for multisource, single-shot radiation exceeded nominal values in all cases by 38 to 70% for the GK unit and 11 to 33% for the LINAC system. Measured offsets were indicative of submillimeter targeting precision on both devices. In primate studies, the appearance of an magnetic resonance imaging—enhancing lesion coincided with the intended target. Conclusions. Radiosurgery performed using the 3-mm collimator of the dedicated LINAC exhibited characteristics that compared favorably with those of a dedicated GK unit. Overall targeting accuracy in the submillimeter range can be achieved, and dose distributions with sharp falloff can be expected for both devices.

Quantifying Imaging Radiation to Patient from Optimized Procedures with LIVE System during SBRT Lung Treatment

Purpose: Recently two AAPM task groups have identified the need for more detailed data on the energy and angular distribution of bremsstrahlung X‐ray photons produced from the target of medical linear accelerators. Previous studies had focused on primary photon beam toward the patient. On the other hand, X‐ray photons produced at large angles are responsible for neutron activations in the surrounding of the accelerator head and for the secondary radiation dose to the patient outside the treatment volume. This paper presents our effort to create a database on angular X‐ray energy and fluence distributions from a medical linear accelerator outside the primary beam direction. Method and Materials: Parameters for a Varian Clinac 2100C operated in 6‐ and 18‐MV were modeled using the MCNPX code. Energy and 360° angular distributions of photons and electrons emanating from radiotherapy targets were determined using MCNPX. A tally sphere was placed around the target in order to characterize the full 4π emittance of these particles from the target. The tallies were binned according to energy and the directional cosine from a fixed vector pointing perpendicular to the target face. Results: Both the 6‐ and 18‐MV photon beams are mostly emitted in forward direction and are predominately in low energies. A low energy peak in the backward direction is also seen in the 6‐MV distribution. The electron distribution emanating from the target for the 6‐MV platform is highly backward directed, while the electron distribution from the 18‐MV platform is mostly low energy and has a forward and backward directed contribution. Conclusion: The results in these studies are consistent with data from previous studies. A detailed database on angular X‐ray energy and fluence distributions from medical linear accelerator outside the primary beam direction will allow the sources of the secondary photonuclear interactions to be located and mapped.

The aim of the present study is to determine and compare initial treatment costs of microsurgery, linear accelerator (LINAC) radiosurgery, and gamma knife radiosurgery in meningioma patients. Additionally, the follow-up costs in the first year after initial treatment were assessed. Cost analyses were performed at two neurosurgical departments in The Netherlands from the healthcare providers’ perspective. A total of 59 patients were included, of whom 18 underwent microsurgery, 15 underwent LINAC radiosurgery, and 26 underwent gamma knife radiosurgery. A standardized microcosting methodology was employed to ensure that the identified cost differences would reflect only actual cost differences. Initial treatment costs, using equipment costs per fraction, were €12,288 for microsurgery, €1,547 for LINAC radiosurgery, and €2,412 for gamma knife radiosurgery. Higher initial treatment costs for microsurgery were predominantly due to inpatient stay (€5,321) and indirect costs (€4,350). LINAC and gamma knife radiosurgery were equally expensive when equipment was valued per treatment (€2,198 and €2,412, respectively). Follow-up costs were slightly, but not significantly, higher for microsurgery compared with LINAC and gamma knife radiosurgery. Even though initial treatment costs were over five times higher for microsurgery compared with both radiosurgical treatments, our study gives indications that the relative cost difference may decrease when follow-up costs occurring during the first year after initial treatment are incorporated. This reinforces the need to consider follow-up costs after initial treatment when examining the relative costs of alternative treatments.

Context:Introduction of dual-layer multileaf collimator (MLC) radiotherapy linear accelerators into clinical practice is an important development in advanced external beam radiotherapy. A method of delivering comparable high-resolution fluences with a single-layer MLC is presented.Aims:The aims of this study are to present new algorithms and approaches to define high-resolution hypermodulated fluences, obtain orthogonal decomposition of fluences, and deliver them on a linear accelerator with single MLC from two perpendicular collimator settings.Materials and Methods:High-resolution fluences were defined using Monte Carlo (MC) calculation. A novel use of a limited-memory, bounded, Broyden–Fletcher–Goldfarb–Shanno algorithm was used to decompose such fluences to ones deliverable with a pair of fields with mutually orthogonal collimator settings. Such a technique, here named cross motion leaf calculator (XMLC), is compared against single sliding window (SSW) technique typically used in intensity-modulated radiation therapy (IMRT). An electronic portal imaging device (EPID) is used, and the results were compared with gamma analysis. Furthermore, MC was used to determine dose distributions for computed tomography images of ten head-and-neck cancer patients.Results:Gamma analysis (3%, 3 mm) against ideal fluence is considerably more favorable to XMLC (94% ± 4%) versus SSW (76% ± 5%). Furthermore, the dose–volume histogram (DVH) analysis showed that XMLC enables delivery of fluences superior to that of IMRT and these results in clinically relevant enhancements in DVH results.Conclusions:At the time of writing of this study, there were more than 12,000 medical linear accelerators in clinical use, and XMLC can prove itself useful wherever linac is equipped with MLC but cannot delivery latest techniques, such as volumetric modulated arc therapy.