Radiant energy – Radiation controlling means
Reexamination Certificate
2001-06-21
2004-04-06
Wells, Nikita (Department: 2881)
Radiant energy
Radiation controlling means
C250S492300, C250S492100, C315S502000, C378S065000
Reexamination Certificate
active
06717162
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for treating a target volume with a particle beam, in particular a proton beam.
The present invention also relates to a device for carrying out said process.
The field of application is the proton therapy used in particular for the treatment of cancer, in which it is necessary to provide a process and device for irradiating a target volume constituting the tumor to be treated.
STATE OF THE ART
Radiotherapy is one of the possible ways for treating cancer. It is based on irradiating the patient, more particularly his or her tumor, with ionizing radiation. In the particular case of proton therapy, the radiation is performed using a proton beam. It is the dose of radiation thus delivered to the tumor which is responsible for its destruction.
In this context, it is important for the prescribed dose to be effectively delivered within the target volume defined by the radiotherapist, while at the same time sparing as far as possible the neighbouring healthy tissues and vital organs. This is referred to as the “conformation” of the dose delivered to the target volume. Various methods which may be used for this purpose are known in proton therapy, and are grouped in two categories: “passive” methods and “active” methods.
Whether they are active or passive, these methods have the common aim of manipulating a proton beam produced by a particle accelerator so as to completely cover the target volume in the three dimensions:the “depth” (in the direction of the beam) and, for each depth, the two dimensions defining the plane perpendicular to the beam. In the first case, this will be referred to as “modulation” of the depth, or alternatively modulation of the path of the protons into the matter, whereas, in the second case, this will be referred to as the shaping of the irradiation field in the plane perpendicular to the beam.
Passive methods use an energy degrader to adjust the path of the protons to their maximum value, corresponding to the deepest point in the area to be irradiated, associated with a rotating wheel of variable thickness to achieve modulation of the path (the latter device thus being referred to as path modulator). The combination of these elements with a “path compensator” (or “bolus”) and a specific collimator makes it possible to obtain a dose distribution which conforms closely to the distal part of the target volume. However, a major drawback of this method lies in the fact that the healthy tissues downstream of the proximal part outside the target volume are themselves also occasionally subjected to large doses. Furthermore, the need to use a compensator and a collimator which is specific to the patient and to the irradiation angle makes the procedure cumbersome and increases its cost.
Moreover, in order to broaden the narrow beams delivered by the accelerator and the beam transport system, so as to cover the large treatment areas required in radiotherapy, these methods generally use a system composed of a double diffuser. However, the protons lose energy in these diffusers, and large irradiation fields at the greatest depths are therefore difficult to obtain unless an “energy reserve” is rendered available by using an accelerator which delivers protons, the energy of which is much higher than that required to reach the deepest areas inside the human body. Now, it is well known that the cost of such accelerators capable of supplying protons increases proportionately with the energy. Despite these drawbacks, passive methods have been widely used in the past and are still widely used today. An example of a passive method which may be mentioned is the “double diffusion” method which is well known in the prior art.
The aim of “active” methods is to solve some, or occasionally even all, of the problems associated with passive methods. In point of fact, there are several types of active methods. A first series of active methods uses a pair of magnets to scan the beam over a circular or rectangular area. This is the case, for example, of the methods known as “wobbling” and “raster scanning”. According to some of these methods, the scanned beam is modulated by a path modulator similar to those used in the passive methods. Fixed collimators and path compensators are again used in this case. According to other methods, the volume to be treated is cut into several successive slices, corresponding to successive depths. Each slice is then scanned by the beam, with the aid of the two scanning magnets, so as to cover an area, the contours of which are adapted to the shape of the tumor to be treated. This shape may be different for each of the slices to be treated and is defined using a variable collimator composed of a plurality of movable slides. An example of this type of method is known from W. Chu, B. Ludewigt and T. Renner (
Rev. Sci. Instr
. 64, pp. 2055 (1993)). By means of these methods, large irradiation fields may be treated, even at the deepest points of the volume to be treated. However, according to certain embodiments based on these methods, it is occasionally still necessary to use a bolus and a compensator. In the case of methods which involve cutting into slices, a better conformation is obtained between the dose delivered and the volume to be treated, for each slice. However, it is necessary, for each irradiation slice, to adapt the multi-slide collimator to the contour of the cross section of the volume to be treated. Needless to say, the quality of the conformation will depend on the “fineness” of the cutting into slices.
In order to dispense with the need to use compensators and collimators, even multi-slide collimators, and to obtain the best possible conformation of the dose delivered to the volume to be treated, a second series of active methods uses scanning magnets to define the contour of the area to be irradiated, for each irradiation plane, and performs three-dimensional cutting of the volume to be treated into a plurality of points. As with the first family of active methods, the movement of the beam along the longitudinal dimension, in the direction of the beam, will take place either by modifying the energy in the accelerator, or by using an energy degrader. Said degrader may be located at the accelerator exit or, on the contrary, in the irradiation head, close to the patient. After cutting the volume to be irradiated into numerous small volumes (“voxels”), each of these volumes is delivered the desired dose using a fine beam scanned in the three dimensions. The specific collimators and other compensators are no longer necessary. An example of implemention of this principle is known from E. Pedroni et al. (
Med. Phys
. 22(1) (1995)). According to this embodiment, the dose is applied by scanning, in the three dimensions, a “spot” produced by a narrow beam. This technique is known as “pencil beam scanning” The superposition of a very large number of these individual dose elements, delivered statically, makes it possible to obtain a perfect conformation of the dose to the target volume. According to this embodiment, the change in the position of the spot is always made with the beam switched off. The fastest movement of the spot is made using a deflector magnet (the “sweeper magnet”). The movement along the second scanning axis is made using a degrader (“range shifter”), located in the irradiation head, which allows the spot to be scanned depthwise. Finally, the third direction is covered by means of the movement of the table on which the patient is supported. The position and dose corresponding to each spot are predetermined using a computing system for planning the treatment. During each movement of the beam, that is to say each time the spot is moved, the beam is interrupted. This is done using a magnet having as purpose to divert the beam in a direction other than that of the treatment (“fast kicker magnet”).
This embodiment of the “active” methods provides a solution to the problems encountered by the other techniques mentioned above, and makes it possible to obtain the best possible
ION Beam Applications S.A.
Wells Nikita
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