Verification and validation of software and electronic hardware for a new or updated pencil beam dose delivery system (PBDDS) is a major undertaking. Many exception cases must be tested. Access to the physical hardware is at a premium and liable to unscheduled interruptions. Key components such as the accelerator, magnets and ion chambers may not respond in a predictable way. It is slow and may be harmful to deliberately invoke certain fail conditions.
We have addressed this problem by developing simulators that closely reproduce the known nominal behavior of these prime components, providing multiple analog and digital signals that connect directly to the actual PBDDS. The PBDDS receives and executes a treatment map as usual. An accelerator simulator unit receives commands from the PBDDS as would the real accelerator and responds accordingly. Ionization chamber simulators coordinate with the accelerator simulator to create multiple current readings with appropriate spatial and temporal profiles.
Controlled errors of almost any characteristic can be introduced at specific places in the irradiation. It is then simple to correlate the response of the PBDDS to these introduced errors. This is not intended to replace completely exception testing on the real system, but it permits a much increased range of cases to be run and evaluated in a reasonable time using an automated procedure.
We describe in detail the hardware that has been developed for simulation and how it is integrated into a complete simulation system for dose delivery.
The simulator system comprises a standard set of control electronics and associated software for a PBDDS. In some cases some beam transfer line controls may also be included. Then to match these devices are hardware simulation electronics that mimic the response of sensors such a ionization chambers, Hall probes, power supply monitors and so on. The primary simulation devices are:
The ICSIM is a flexible set of analog and digital interfaces controlled by an embedded processor. It provides 128 current outputs to represent ionization chamber strip electrode signals, two currents that represent integral plane signals, a voltage representing a Hall probe readout, a high voltage output representing the loopback sense from an ionization chamber, plus other voltages for signals such as temperature, pressure and humidity readbacks.
The ICSIM outputs are updated based on the inputs every 110 µsec. For example, if a map calls for a particular spot charge, energy and position the ICSIM will receive the resulting control signals for scan magnet setting from the PBDDS and beam energy and current settings from the ACCELSIM. The ICSIM will compute the magnet settings using the configured magnet calibration functions including effects such as saturation and hysteresis, and do a voltage-limited slew to the new settings, while providing outputs to simulate magnet power supply Hall probe readbacks. It will compute beam positions on ionization chambers and send out patterns of currents that represent the signals from ionization chamber strip electrodes and integral plane electrodes. The total charge in the signals are a function of the beam current, beam energy, chamber high voltage, temperature and pressure. The currents that would be integrated on chamber strips are pre-computed in normalized form and placed in a lookup table, then simply scaled in width and amplitude in real time.
Noise and anomalous conditions can be introduced at will, or as part of an automated test sequence. Many typical anomalous beam shapes can be produced by simple addition of two Gaussians.
Incorrect high voltage on ionization chamber electrodes is an insidious failure mode in real systems. The ICSIM allows an arbitrary high voltage to be returned to the PBDDS on the loopback connection. This allows high voltage absence, sagging and instability to be simulated.
The completed simulator provides all the control connections required by the PBDDS, so any maps that can be run on a real system can be run on the simulator. The simulator devices as well as the PBDDS devices get copies of the map so that errors can be introduced as deviations beyond map limits expressed in clinical or device units, and present for specified durations.
Testing can comprise running an automated sequence of maps with the simulator programmed to introduce particular errors at known points so that correct response of the PBDDS can be checked. Errors can also be introduced asynchronously by adjusting the simulation configuration, or by manual switching of digital and analog signals, while a map is running. Signal breakout connectors are provided so that time-critical events can be logged with test equipment such as digital oscilloscopes.
Proton therapy is well established for the treatment of localized cancer tumors, especially when there are vital organs close by. This healthy tissue sparing makes it the method of choice for maintaining quality of life and especially for treating children. The proton beam delivery is fast and painless.
Proton therapy centers worldwide use Pyramid sensors, electronics, and software. Pyramid offers products for complete systems, sub-systems, or components. For more information see our Pencil Beam Scanning Brochure
The ionization chamber or ion chamber is conceptually the simplest of all gas-filled radiation detectors, and is widely used for the detection and measurement of ionizing radiation. It is the preferred device for proton therapy dose and beam position monitoring due to a combination of robustness, gain stability, very low material budget in the beam path and availability of large continuous sensing areas.
Pyramid offers a wide range of ionization chambers for proton and heavy ion therapy applications. Advanced electrode coating and patterning allows the creation of precise strip and pixel patterns for position readout. A wide range of sensitive areas and electrode gaps is provided to suit different system geometries and beam currents.