The Spot-Scanning technique developed at PSI
With the new treatment technique at PSI a pencil beam (7mm diameter) of protons is regulated by computer controlled magnets in such a way that the high-dose spot can be positioned very precisely, for an exactly specified period of time and at any desired location within a tumor. By superimposing many individual spots – more than 9000 within a volume of one liter– we can impose the desired radiation dose uniformly within a tumor, with the dose being individually monitored for each single spot. This enables extremely precise and homogeneous irradiation, ideally adapted to the shape of the tumor, which is in most cases irregular.
We refer to this dynamic, three-dimensionally conforming radiation therapy procedure as the Spot-Scanning technique. Made a reality for the first time at PSI, it is an international breakthrough, making it possible for a higher radiation dose to be applied to a tumor, with reduced damage to the surrounding healthy tissue for most of the irradiated tumors.
The magnet controlled proton beam can be applied from almost any direction. With the Spot-Scanning technique and the rotatable beamline system, we achieve the optimum conformation of the dosage to the shape of the tumor.
By using large kicker-magnets, we can direct the protons to the Gantry and to the patient, into the tumor. Step by step, the high-dose spot scans the tumor in all three spatial dimensions. The penetration depth of the proton-spots is controlled by a system of moving plastic plates. These movements last only a few milliseconds. They are repeated, layer by layer, corresponding to the shape of the tumor, by slowly moving the patient in an individually moulded couch in the beam.
The Gantry 1, which weighs more than 100 tonnes, can be rotated as one unit about its central axis, and therefore enables irradiation to be carried out from any desired direction. For all its complexity and enormous weight, the unit still needs to be extremely precise and stable. Depositing exactly the right dose in the tumor very homogeneously calls for the utmost precision in delivering every single spot, with regard to both, its location and duration. The extremely short reaction times of the facility which this requires proved to be an extraordinary technical challenge. It takes less than four minutes to irradiate a volume of 1 liter, but during this time more than 9000 spots are deposited, every single one with a dose precision of 1%.
In order to treat moving tumors (i.e. in the chest and lung area) also with such a high accuracy, the treatment technique is being developed further (see research and development and the project PROSCAN).