Patent Number: 047605901
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the graph shown in FIG. 1, curve A is characteristic of cobalt. The depth of the tissue to be treated has been plotted (in centimeters) as abscissa and the radiation dose has been plotted as ordinates, standardized to 100 with respect to the maximum radiation. The main characteristics of this cobalt type radiation can be clearly seen: maximum dose at 5 mm; PA1 skin dose 85%; PA1 depth reached with 50% of the maximum dose; 12 cm. Such characteristics are interesting for they corresond to certain pathological situations where a tumour is essentially located in depth while having ramifications in the surface tissues. In other cases, however, where the tumour is for example better localized in depth, the practitioner will rather choose a curve of type B, very similar to the cobalt curve but with a skin dose reduced by about half. The invention provides this possibility, by means of a particle (electron, in the example) accelerator simplified by the fact that the HF supply power is fixed therein once and for all at a predetermined level (all the conventional power adjustment electronic systems, generally acting on the modulator, are no longer needed) and a set of switchable targets and/or filters at the output of said accelerator for choosing a characteristic of the beam in accordance with a type A or B curve, using simple mechanical selectors supporting the targets and/or filters. Thus a target-filter combination may be provided restoring the radiation curve A and one or more other combinations restoring one or more type B curves, more or less "staggered in depth" and all having the advantage of a relatively low skin dose. In FIG. 2 the end part has been shown of an average power electron accelerator 11 (of about 4.5 MeV electrons). This accelerator is of quite conventional design and this is why it has not been described in detail. It may be formed for example by a modulator driving a magnetron, which is coupled by a wave guide to a cavity stack 13 forming a linear accelerating structure. This accelerator comprises a main axis 14 which also represents the path of the accelerated electrons. On leaving the accelerator, the electron beam bombards a target, which generates a photon beam. This latter is defined by a collimator 15. In a first embodiment of the invention, the accelerator comprises a mobile support 16 comprising several targets 17, 18 each with a main axis of symmetry 19. The path of support 16 passes in front of the output of the accelerator and positioning means, shown schematically for example by two stops 20 between which support 16 may move, are provided for aligning any axis 19 with the main axis 14 of the accelerator. In this system, the characteristics of the photon beam conforming to a curve A or B are entirely determined by the choice of the material forming the target and the dimensional characteristics thereof. In the embodiments shown in FIG. 3, where similar structure elements bear the same reference numbers, a single target 22 has been provided disposed at the output of accelerator 11 and centered on its main axis 14. Furthermore, a mobile support 23 comprises several filters 24, 25 each having a main axis of symmetry 26. As before, the path of support 23 passes in front of the target and in the vicinity thereof whereas positioning means (20a in the example) are provided for aligning the axis 26 of any filter with the main axis 14 of the accelerator. The filters 24, 25 have a dual role. On the one hand, they allow the spectral components of the photon beam to be fashioned, by attenuating them differently. They have then an energy filtering function which determines a curve of type A or B since the nature of the target is fixed a priori. Furthermore, they have an equalizing function because of their form, allowing directional attenuation of the beam so as to obtain the uniform distribution of the dose at the level of the patient. In fact it is known that in an accelerator the strength of the beam decreases the further away from axis 14. Consequently, in a way known per se, filters 24 and 25 will have a pyramidal shape preferably substantially conical. In the example shown in FIG. 3, support 23 is made essentially from lead. It comprises cavities 28 housing the conical shaped filters. The embodiment shown in FIG. 4, in which elements of similar structure bear the same reference numbers comprises a support 30 having several targets 32, 33 and several filters 34, 35. Support 30 is caused to move opposite the output of accelerator 11. It is essentially made from lead and comprises two stages. The upper stage (the closest to the accelerator) is pierced with holes 36 housing the targets 32 and 33 whereas the lower stage comprises, as in the case of FIG. 3, cavities 28 housing the filters 34 and 35. The holes and cavities are such that the main axis of symmetry of target 32 merges with the main axis of symmetry of filter 34 and so that the main axis of symmetry of target 33 merges with the main axis of symmetry of filter 35. Furthermore, as before, positioning means (stops 20B) are provided for immobilizing support 30 in positions such that any of the axes common to the targets and filters may be aligned with the main axis 14 of the accelerator. The embodiment shown in FIG. 5 is only distinguished from the preceding one in that it comprises two independent supports 40, 41. Support 40 contains several targets 32a, 33a each having a main axis of symmetry whereas support 41 contains several filters 34a, 35a each having a main axis of symmetry. The positioning means (stops 20c) with which supports 40 and 41 cooperate allow the axis of symmetry of any filter and the axis of symmetry of any target to be aligned with the main axis 14 of the accelerator. With respect to the embodiment shown in FIG. 4, the number of target-filter combinations is doubled with the same number of targets and filters. Depending on the beam characteristics desired, the filter may be made from different materials, more especially tungsten, lead, copper, titanium, stainless steel or graphite. In the examples which have just been described, the supports are drawers with rectilinear movement. As mentioned above, they are made essentially from lead but they will advantageously comprise steel slides (not shown). In the simple case, shown, the mechanical handling system may be manual. If it is desired to have more than two targets and/or filters, motor driven solutions may be adopted with remote control and servo-controlled positioning, all these handling systems being within the scope of a man skilled in the art. Positioning control may also be provided by means of microcontactors and a microprocessor logic system monitoring the state of these contactors. FIGS. 5 and 6 illustrate another type of rotary turret mobile support 50. The axis of rotation 51 of this support is offset from axis 14 of the accelerator so that the targets and/or filters may be positioned in alignment with this axis 14. In the example, the support 50 has two stages, one comprising the targets 52 and the other the filters 53. FIG. 7 shows another type of possible mobile support having a general cross shape 55. This support is servo-controlled for movement in a double slide system (not shown) defining two rectilinear and perpendicular directions of movement. The cross may thus support up to five targets and/or filters. The determination of the dimensions of the targets and filters as well as the choice of the materials used will be most often determined experimentally. By way of example, with reference to FIG. 1, and considering an incident beam of electrons of about 4 MeV, the curve A or "cobalt curve" may be obtained by using a flat tungsten target, 2 mm in thickness and a conical filter with a height of 12 mm and a base diameter of 25 mm. Curve B may be obtained by using a target comprising a 1 mm layer of tungsten and a 1 mm layer of copper as well as a stainless steel conical filter with a height of 16 mm and a base diameter of 25 mm.