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Proton Therapy | Treatment Delivery Technology

Figure 1 Depth dose curves for a proton bream (with Bragg peak) and a photon beam.

Proton therapy offers the potential to improve dose conformity and precision in targeting the tumor due to the physical differences between protons and photons. Protons have a range that depends on the energy of the beam, and protons will not progress further than that range. In addition, the dose supplied to tissue is at its greatest point just before the particle’s maximum penetration depth, a point known as the Bragg Peak. The depth of the Bragg Peak depends on the beam energy produced by the accelerator, which can be adjusted to focus within the tumor.

For the irradiation of most treatment volumes a Spread-Out Bragg Peak (SOBP) is used, which is accomplished by superimposing multiple beams of slightly different energies. The Bragg Peak or SOBP ensures a limited dose deposition in the skin region and no dose behind the target volume. This greatly improves dose conformity and allows the treatment of tumors close to vital organs.

Proton therapy delivery requires a dedicated building to house the therapy equipment and combines advanced hardware and software components.

Disclaimer

The proton therapy device technology described here has not been cleared by the US Food and Drug Administration (“FDA”) for clinical use and is in development. The FDA has a unique set of standards for proton therapy devices because the entire proton therapy facility must be built for the specific device. For these types of devices, the FDA allows proton therapy sponsors to discuss projects with potential customers and to engage in contracts/orders to build proton therapy facilities and /or to purchase proton therapy equipment. However, contracts or other engagements specific to clinical practice are strictly prohibited; that is, no engagements or agreements for patient scheduling or physician group practice to use proton therapy equipment are permitted in advance of FDA clearance.

Hardware components

Proton accelerator: 250 MeV Superconducting Cyclotron

The latest 250 MeV (Mega-electron-Volt) Superconducting Cyclotron is a key component of Varian proton therapy delivery. Stable beam current and beam intensity modulation allow precision spot scanning and three-dimensional proton dose distribution for unprecedented precision.

Improved manufacturing techniques mean that the lightweight, compact size helps reduce the total cost of ownership. In addition, superconducting magnet coils result in high extraction efficiency, low energy consumption, excellent reliability, and an overall reduction in operating costs and maintenance.

Unlike some other accelerators, the superconducting cyclotron, behaves in a very linear and predictable manner. This allows for complete automation and eliminates the need for routine operators.

Beam transport system

The proton accelerator is connected to one or multiple treatment rooms by the beam transport system (BTS). The BTS is segmented so that additional treatment rooms can be added to the system later, providing scalability to treat more patients as the clinic grows.

A key element of the BTS is the energy selection system (ESS). The ESS transforms the energy of the beam that the cyclotron produces from 250 MeV particles to the desired treatment energy between 250 MeV and 70 MeV. This translates to a possible range of range of 36.5–4.0 cm in water. Range shifters directly in front of the patient can be used to reduce the range to under 4.0 cm.

The ESS is used to set and verify the beam energy and spread to guarantee optimal treatment precision. The ESS is located close to the particle accelerator and outside the treatment rooms. This eliminates neutron production in the treatment room.

The BTS consists of a beam tube under vacuum and with a variety of magnets. Quadruple magnets are used to focus the proton beam and dipole magnets are used to deflect the beam to the selected treatment room. The magnets of the BTS system are tuned automatically by the control system to accommodate the selected treatment room, energy, and size of the beam.

Treatment Rooms

Varian's proton therapy delivery system is modular and the number of treatment rooms scalable. It is possible to start with a single-room system that can be expanded to a multi-room facility later. Two types of treatment rooms are available, which can be combined in any desired ratio to match patient demand.

Gantry beam rooms are similar to the Varian® Clinac® and Trilogy™ gantries in which the beam can be fully rotated around the isocenter point. The proton gantry in combination with the treatment table offers the flexibility to select any desired beam angle for optimal beam delivery.

Fixed beam rooms offer a horizontal beam that can be optimized for eye, head, and neck treatments or be combined with a versatile treatment table for a variety of dedicated treatments.

Gantry

A gantry is a steel structure that houses the final section of the BTS and the delivery "nozzle." The gantry rotates around the patient, delivering the beam at the desired angle. The combination of a gantry and treatment table offers the flexibility to treat patients from all angles. The unique mechanical design of the Varian gantry—two large roller bearings for support, balanced with counterweights—provides exceptional mechanical rigidity. The operator can select an angular position with a precision of ± 0.1°.

The Varian proton gantry can be equipped with one of two types of beam delivery systems, also called "nozzles." The nozzle is equipped with motion sensors to help ensure patient safety. Alerts sound if these sensors detect if the gantry nozzle and patient come too close and the motion of the gantry and the table is slowed down. Additional contact sensors are hard-wired to emergency brakes.

Standard table

The treatment table support is located in front of the gantry as an integral part of the gantry enclosure. The treatment table, nozzle contour, and diagnostic equipment positioned close to the patient are optimized to achieve a 4-π treatment-angle range as far as possible. All patient tables are equipped with six numerically controlled axes, and bending due to patient weight will be automatically detected and corrected.

 

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