Making a Varian Linear Accelerator Vault OBI (On-Board Imaging) Ready
Linear accelerators have two different components that allow for images of the patient to be made while the patient is on the linear accelerator:
1. MV Imaging Devices. These devices, also referred to as EPIDs (Electronic Portal Imaging Devices), produce an image of the patient by the use of the mega voltage (MV) beam of radiation that the linear accelerator emits to treat the tumor(s).
2. KV Imaging Devices. These devices produce clearer, x-ray images of the patient through the use of an x-ray tube that is mounted onto the linear accelerator.
Both MV and KV imaging devices use amorphous silicon panels to produce their images.
Many older linear accelerators that were not originally equipped with KV imaging systems were replaced with linear accelerator systems that include a KV imager. Varian’s KV imaging device is referred to as the On-Board Imager or “OBI” system.
This article discusses what is needed to convert a vault from one which houses an MV-only device to one that can accommodate an OBI machine.
Items needed for a vault to upgrade to OBI-ready status:
1. Extra conduit for OBI cables. Varian linear accelerator systems with OBI require additional cables to connect the OBI system to the console/computer control area. These cable, like all the other linear accelerator cables, run underneath the ground in conduits (tunnels) that are made specifically to house these cables. If the vault does not include conduits for OBI cables, there is the option of running the cables overhead, in the ceiling space above, provided the cables are long enough. Another option is to share the existing conduits and bunch up all the cables inside. Otherwise, the concrete will have to be cut and dug out to provide for the additional conduits needed. It is recommended that facilities currently building vaults to accommodate non-OBI machines, go ahead and design the vault to accommodate these future upgrades.
2. 480V power for OBI (main linac takes 208V). The OBI system requires 480V power, whereas the linear accelerator uses 208V power. Most vaults in the U.S. will have city power running to it of either 208V or 480V. If the vault is only 208V, a simple solution is to purchase a dual-output power conditioner to “step-up” one output to 480V to go directly to the OBI unit (the 208V would continue to go to the linear accelerator). If a facility already has a single output power conditioner for their linac, they can simply purchase a separate “step-up” transformer to bring the power to the OBI. However, its recommended that a smaller 480V in, 480V out, power conditioner is purchased and placed between the step-up transformer and the OBI. This will ensure protection of the machine and the OBI from power surges, ground faults, loops, and other electrical charges (put in layman’s terms, it acts essentially as an enormous surge protector, the same concept as when you plug your home electronics into a “power strip”!).
3. X-ray on warning light (in addition to “Beam On” linac warning light). Most states require an additional warning light with the addition of this X-ray component. This does require some electrical wiring from the equipment to this outside light. Here is an example:
4. OBI breaker panel (in addition to main breaker panel). It is usually recommended to purchase the Varian recommended GE OBI breaker panel. When the linear accelerator has an integrated generator (which most newer models have), the panel should be installed just outside the vault by the control area directly adjacent to the main circuit breaker. If the machine happens to have a stand-alone generator behind the linear accelerator stand, then the OBI breaker should be installed on the rear wall of the vault. The majority of machines have integrated OBI generators.
5. Power Conditioner with UPS System to protect OBI. In addition to a power conditioner, a UPS (uninterrupted power supply) system can provide protection against power outages and unstable power generation. Power outages can materially damage linear accelerators and other components, much like they can other electronic devices. UPS units take up a lot of space and will often need to be located in the electrical room to accommodate the dimensions and noise. They also tend to produce noise that may disturb patient care if not properly housed outside the vault in the electrical room.
6. Form 2579. Because the OBI is an X-ray source, your installer will need to file with the FDA form 2579 for installations in the United States. This is very common and any licensed installer will be capable for handling this for you.
By attending to the details, your clinic or treatment room can quickly be transformed to house newer and more advanced technologies to better serve your patients, staff, and community. If ROS can be of help in your plans for upgrades, please reach out to one of our qualified team members.