Beam control system and output fine-tuning for safe and precise delivery of FLASH radiotherapy at a clinical linear accelerator

IntroductionWe have previously adapted a clinical linear accelerator (Elekta Precise, Elekta AB) for ultra-high dose rate (UHDR) electron delivery. To enhance reliability in future clinical FLASH radiotherapy trials, the aim of this study was to introduce and evaluate an upgraded beam control system and beam tuning process for safe and precise UHDR delivery.Materials and MethodsThe beam control system is designed to interrupt the beam based on 1) a preset number of monitor units (MUs) measured by a monitor detector, 2) a preset number of pulses measured by a pulse-counting diode, or 3) a preset delivery time. For UHDR delivery, an optocoupler facilitates external control of the accelerator’s thyratron trigger pulses. A beam tuning process was established to maximize the output. We assessed the stability of the delivery, and the independent interruption capabilities of the three systems (monitor detector, pulse counter, and timer). Additionally, we explored a novel approach to enhance dosimetric precision in the delivery by synchronizing the trigger pulse with the charging cycle of the pulse forming network (PFN).ResultsImproved beam tuning of gun current and magnetron frequency resulted in average dose rates at the dose maximum at isocenter distance of >160 Gy/s or >200 Gy/s, with or without an external monitor chamber in the beam path, respectively. The delivery showed a good repeatability (standard deviation (SD) in total film dose of 2.2%) and reproducibility (SD in film dose of 2.6%). The estimated variation in DPP resulted in an SD of 1.7%. The output in the initial pulse depended on the PFN delay time. Over the course of 50 measurements employing PFN synchronization, the absolute percentage error between the delivered number of MUs calculated by the monitor detector and the preset MUs was 0.8 ± 0.6% (mean ± SD).ConclusionWe present an upgraded beam control system and beam tuning process for safe and stable UHDR electron delivery of hundreds of Gy/s at isocenter distance at a clinical linac. The system can interrupt the beam based on monitor units and utilize PFN synchronization for improved dosimetric precision in the dose delivery, representing an important advancement toward reliable clinical FLASH trials

An electrical impedance model for deep brain stimulation of Parkinson's disease

Deep brain stimulation is an accepted technique for the treatment of Parkinson's disease. Deep brain stimulation affects the electrical functions of neurons and several explanations are available to describe this treatment modality. Such as depolarization blockade, synaptic inhibition, synaptic depression and stimulation induced modulation of pathological network activity. An electrical impedance model of the treatment area around the electrodes in the brain is determined to further increase the understanding of deep brain stimulation. This model shows the contact impedance between the electrodes and the tissue, the extra cellular resistance, the intra cellular resistance of the neurons and the effect of deep brain stimulation on the treated area in the brain, which in this case is stimulation of the sub thalamic nucleus. The generated electrical field near the electrodes is high enough to perform an electropermeabilization of the cell membranes. This is modelled as a cell membrane capacitance in series with a resistance. The resistance is the consequence of electropermeabilization of the cell membranes. Many observed parameters that occur during deep brain stimulation is reduced tremor activity, influence on speech, the same effect as a lesion, increased axon activity downstream, followed by durations of nerve silence

Impedance spectra of tumour tissue in comparison with normal tissue; a possible clinical application for electrical impedance tomography

Electrical characteristics of living tissues have been investigated for a long time in the search for further methods to complement the traditional investigations of pathology and physiology. Tumour tissue has been shown to exhibit a larger permittivity and conductivity than normal tissues. This might be associated with the fact that tumour cells have a higher water content and sodium concentration than normal cells, as well as different electrochemical properties of their cell membranes. To our knowledge only a few contributions on this subject have been published. This study describes an additional application on measurements of the complex impedance of tumour and normal tissues, in order to compare the impedance features of the two tissue types. The tissue sample is placed in a measuring cell in which the temperature is controlled. The measuring cell is connected to an impedance meter able to measure the complex impedance in terms of real and imaginary part curves for frequencies from 1.5 kHz to 700 kHz. The four-electrode principle is used with the current injected by the outer electrodes and the voltage difference measured between the inner electrodes. The current can be altered up to 1 mA. The instrument can be calibrated with known resistance and capacitance networks connected to the input of the instrument in order to minimize the measurement errors. The calibration routine uses a polynomial adaptation and can be applied interactively. Measurements performed by the instrument show promising results. Preliminary results show that this method can be extended to a new application for detection of tumour tissue by electrical impedance tomography (EIT)

Beam Adjustments for Unflattened X-ray Beam Modes for an Elekta Synergy Linear Accelerator

A flattening filter free x-ray beam mode was added to an Elekta linear accelerator by placing a flat copper plate in the filter carousel and recalibrating the electron steering servo, gun servo and dosimetry system. Machine configurations were saved onto a separate hard disk in order to remain separate from the clinical configuration. Profile measurements with a Schuster BMS diode array show that the beam is stable and start up performance is similar to the normal filtered beam

A simple test device for electrometers

Electrometers used in dosimetric instruments need to be checked regularly in order to measure and maintain the prescribed dose to the patient. This paper describes the function of a simple but accurate test device for electrometers. A number of electrometers have been tested and compared with calibrations performed by the Swedish Standard Dosimetry Laboratory (SSDL). The device can be used to test current, resistance, voltage and charge measurements. The charge can be conducted to the electrometer in different ways, and the input resistance of the electrometer can also be determined. The calibration factors obtained by the device are in good agreement with results obtained from calibrations at the SSDL

The influence of air humidity on an unsealed ionization chamber in a linear accelerator

The safe and accurate delivery of the prescribed absorbed dose is the central function of the dose monitoring and beam stabilization system in a medical linear accelerator. The absorbed dose delivered to the patient during radiotherapy is often monitored by a transmission ionization chamber. Therefore it is of utmost importance that the chamber behaves correctly. We have noticed that the sensitivity of an unsealed chamber in a Philips SL linear accelerator changes significantly, especially during and after the summer season. The reason for this is probably a corrosion effect of the conductive plates in the chamber due to the increased relative humidity during hot periods. We have found that the responses of the different ion chamber plates change with variations in air humidity and that they do not return to their original values when the air humidity is returned to ambient conditions

Improvements in The Hardware of The Lund Impedance Tomography System

The objective of this study is to improve the quality of the hardware of the existing Lund impedance tomography system. Improvement in the current generator and different isolation proposals are presented

Modifying a clinical linear accelerator for delivery of ultra-high dose rate irradiation

Objectives: The purpose of this study was to modify a clinical linear accelerator, making it capable of electron beam ultra-high dose rate (FLASH) irradiation. Modifications had to be quick, reversible, and without interfering with clinical treatments. Methods: Performed modifications: (1) reduced distance with three setup positions, (2) adjusted/optimized gun current, modulator charge rate and beam steering values for a high dose rate, (3) delivery was controlled with a microcontroller on an electron pulse level, and (4) moving the primary and/or secondary scattering foils from the beam path. Results: The variation in dose for a five-pulse delivery was measured to be 1% (using a diode, 4% using film) during 10 minutes after a warm-up procedure, later increasing to 7% (11% using film). A FLASH irradiation dose rate was reached at the cross-hair foil, MLC, and wedge position, with ≥30, ≥80, and ≥300 Gy/s, respectively. Moving the scattering foils resulted in an increased output of ≥120, ≥250, and ≥1000 Gy/s, at the three positions. The beam flatness was 5% at the cross-hair position for a 20 × 20 and a 10 × 10 cm2 area, with and without both scattering foils in the beam. The beam flatness was 10% at the wedge position for a 6 and 2.5 cm diametric area, with and without the scattering foils in the beam path. Conclusions: A clinical accelerator was modified to produce ultra-high dose rates, high enough for FLASH irradiation. Future work aims to fine-tune the dose delivery, using the on-board transmission chamber signal and adjusting the dose-per-pulse

Correction for Ion Recombination in a Built-in Monitor Chamber of a Clinical Linear Accelerator at Ultra-High Dose Rates

In the novel and promising radiotherapy technique known as FLASH, ultra-high dose-rate electron beams are used. As a step towards clinical trials, dosimetric advances will be required for accurate dose delivery of FLASH. The purpose of this study was to determine whether a built-in transmission chamber of a clinical linear accelerator can be used as a real-Time dosimeter to monitor the delivery of ultra-high-dose-rate electron beams. This was done by modeling the drop-in ion-collection efficiency of the chamber with increasing dose-per-pulse values, so that the ion recombination effect could be considered. The raw transmission chamber signal was extracted from the linear accelerator and its response was measured using radiochromic film at different dose rates/dose-per-pulse values, at a source-To-surface distance of 100 cm. An increase of the polarizing voltage, applied over the transmission chamber, by a factor of 2 and 3, improved the ion-collection efficiency, with corresponding increased efficiency at the highest dose-per-pulse values by a factor 1.4 and 2.2, respectively. The drop-in ion-collection efficiency with increasing dose-per-pulse was accurately modeled using a logistic function fitted to the transmission chamber data. The performance of the model was compared to that of the general theoretical Boag models of ion recombination in ionization chambers. The logistic model was subsequently used to correct for ion recombination at dose rates ranging from conventional to ultra-high, making the transmission chamber useful as a real-Time monitor for the dose delivery of FLASH electron beams in a clinical setup