fig1 a shows two graphs . the horizontal axis is graduated in energy units , kev , whereas the vertical axis is in arbitrary units of intensity . the upper graph is the energy spectrum for a 50 kv miniature x - ray source as might be employed for brachytherapy within a resection cavity in the patient &# 39 ; s breast . the lower is the computer modeled attenuated energy spectrum after the radiation of the upper graph has passed through a 50 % silver attenuator , i . e ., the energy is reduced by half . fig1 b shows the same upper graph as in fig1 a , but in the lower plotted curve , the radiation has passed through a 50 % attenuator of iron . note that the spaces between the upper and lower graphs of the two figures are different . in fig1 a where the preferred silver attenuator has been used , the change in intensity ( comparing before to after attenuation ) is , to a substantial degree , uniformly distributed with respect to energy , even recognizing the discontinuity ( the k energy edge ) centrally . in fig1 b , the changes in intensity level are much greater at lower energies than at higher energies . this observation reflects the differences between the preferred materials and other materials as described above . another representation of these differences is illustrated in fig2 . the horizontal axis represents the penetration depth in water after attenuation , and the vertical axis is relative dose intensity , normalized at unity . all attenuators are sized for a 50 % intensity reduction . in fig2 , the computer modeled radial dose functions for several attenuator materials are compared with unattenuated behavior . ( the graph is corrected for and does not show the 1 / r 2 relationship , r being the penetration distance after exiting the attenuator .) from top to bottom , the curves represent attenuation by water , iron , silver and molybdenum . the bottom curve is unattenuated . note that water and iron , although normalized initially at the left , are much less attenuated up to and at 2 cm depth . by comparison , silver and molybdenum follow the unattenuated curve more closely . in other words they are more proportional to the unattenuated intensity profile , as described in the previous discussion . fig3 a depicts a source catheter 10 positioned near a section of bone 11 . the bone represents a radiation sensitive structure , another example of which might be the patient &# 39 ; s skin . bone and skin are often encountered in brachytherapy practice within the range of target tissue , and if measures cannot be taken to protect them from overdose , brachytherapy cannot proceed . the catheter 10 shown in fig3 a has a radiation source 12 positioned within its lumen , and in accordance with the invention an elliptical mask 14 ( preferably of silver or molybdenum ) is positioned on the catheter surface . the catheter 10 has been manipulated such that the mask 14 creates a solid angle shadow 16 in the emitted radiation which encompasses the bone , thus reducing the dose intensity to which the bone 11 is subjected . fig3 b is a transverse section view of the apparatus and anatomy shown in fig3 a , taken through the mask 14 . in fig3 c , a sheath 17 ( sometimes referred to as a catheter element herein ) is interposed over the source 12 and its catheter 10 . the sheath 17 is inserted into a source guide of an applicator , such as a balloon applicator . an elliptical mask 18 is positioned on the sheath and the elements positioned so that the mask creates a solid - angle shadow 16 ( fig3 d ) similar to that shown in fig3 a . with the mask on either the catheter or the sheath , the treatment plan is otherwise unaltered , simplifying the planning and treatment problem for the therapist yet protecting the bone 11 . fig3 d shows this embodiment in cross section , revealing the sheath 17 carrying the mask 18 and surrounding the catheter 10 with the source 12 . a catheter element carries the attenuating mask in both cases , fig3 a / 3 b and fig3 c / 3 d ; in the one case the catheter element is the body or shaft of the catheter , normally fixed relative to the source , and in the other it is the sheath surrounding the source catheter , capable of manipulation independent of the source . in the simplest case the sheath can be fixed to the applicator or immobile relative to the applicator once put in place , although it is normally preferable that the sheath with mask be capable of manipulation . note also , it is possible to place a mask directly on an applicator shaft ( although this would be difficult on a balloon applicator ) but in situ adjustment of the mask position is preferable . the mask could be placed on a balloon surface if desired . masks of this invention are generally quite thin if for x - ray brachytherapy , perhaps on the order of 0 . 001 inch thickness . they can be of any suitable size and fastened to the surface of the catheter or sheath extrusion by bonding or other means known to those of skill in the art . the geometry of the mask 14 will determine the geometry of the solid angle shadow when radiation from the source 12 is projected through the mask . multiple masks can be nested or combined , or custom masks created to accommodate patient specific situations , or to allow source manipulation within the catheter while the mask remains fixed in relation to the balloon or at risk structure . it is clear that , in order for the shadow to be positioned effectively and provide the desired protection , the direction of the at - risk structure and its location must be captured during the imaging process , and that information incorporated in the treatment plan . the catheter or sheath carrying the mask must then be positioned , and if necessary manipulated during treatment in conjunction with the source , to effect the desired protection . this may require a catheter or sheath manipulator and a source manipulator , both responsive to a central controller to provide the necessary coordination . such a controller driven manipulator system is pictured in fig5 . if independent control of source and catheter or sheath is not required , the manipulator apparatus may be simplified in a manner well understood by those of skill in the art . where a region of skin comprises the at - risk structure which needs to be protected , assuring proper placement of the mask , hence the shadow , may be simple . it is well known that tissue transmits visible light , and where the sections of tissue are relatively thin , it transmits very well . before the source catheter is placed in the resection cavity , a light source , for example an led source in the visible light range , can be inserted into the applicator ( and within the sheath 17 if included ), and can be used visually to indicate proper placement . by adjusting the mask on the sheath until the illumination on the skin is satisfactory ( e . g . an illuminated ring darker at center due to the mask position ), proper positioning under the skin is obtained , and sensitive skin areas are accurately protected . in fig3 c the reference number 13 , with dashed lead line , indicates that this can be considered the light source behind the mask , placed in the sheath 17 as a step preceding insertion of the source catheter 10 ( the light source 13 is removed prior to insertion of the radiation source catheter ). fig3 e shows the illuminated ring 22 discussed above , with a central dimmer region 24 . note that the light ring will generally not be as well - defined as illustrated in the drawing . dependent in part on the shape of the mask , the light ring will typically be an approximate ellipse , although in some cases it could be an approximate circle . the led source is positioned where the radiation source will be positioned once the light source is removed . if greater accuracy is required , visible - light sensors ( or other sensors consistent with the wavelength of the radiation from within the catheter element ) can be placed on the skin . feedback from their output can be used to adjust the mask position until a satisfactory solution is obtained . the procedure for using the light source , preferably an led , is first to insert the light source ( prior to insertion of the radiation source ) into the lumen of the applicator . the light source will be carried at the end of a wire or other insertion device . the light source is moved through the same positions prescribed by the treatment plan for the radiation source , and the method preferably includes finding the brightest light spot showing through the skin , indicating the closest position the source will assume relative to the skin surface . next the sheath is inserted into the applicator , and the light is put back in the same position or positions that are of interest , within the sheath . this can be done by hand , using depth markers , or by a controller . typically the light source and the sheath carrying the mask will be moved through the positions of the treatment plan , and at each location the orientation of the mask can be adjusted . or , there may be a single or several particular locations at which the light is brightest and which might be the only locations of concern . in that case the mask can be manipulated to optimum only at those locations . note also that a camera can be used during the light manipulation and / or during the light and mask manipulation , to make a record . although it is possible to manipulate the sheath and mask accurately using a controller and without a light source , the use of the light source is advantageous in verification that the correct skin location ( s ) will be protected . alternatively , a string of mosfet radiation sensors can be used to assure that the shadow is properly oriented toward the at - risk structure . these sensor readings may then be incorporated into the plan , becoming the basis for subsequent control and manipulation . such a string can be used alternatively as a safety device to interrupt emissions if the dose at the at - risk structure exceeds the maximum allowable dose . fig4 shows a matrix of an exemplary set of catheters / sheaths with masks for one level of attenuation . a complete set will have several percentage levels of attenuation available , for example , 20 %, 30 % and 40 %. transverse ( or circumferential ) and longitudinal angles are specified as descriptive of the shadow which will be formed by the projected radiation . different sets or permutations can be chosen to suit the procedure ( s ) of interest , or for personal preference . fig5 depicts two manipulators 40 and 42 operating in tandem , which when controlled by a central controller , can cooperate to deliver a treatment plan . in the preferred embodiment where the source is secured at a fixed position in the applicator , the manipulator 40 controls the catheter 20 ( and source mounted , for example , on the end of a power cable , neither shown , which are positioned within the catheter ), and the manipulator 42 independently controls the sheath 17 that holds the mask 44 . in an alternate embodiment where the source and catheter are not joined , a sheath is unnecessary , and the manipulator 40 controls the source position while the manipulator 42 controls the position of the catheter , now including the mask . in either embodiment , the servomotors 50 control axial translation of the manipulator sleds 52 riding on parallel rails 54 , while servomotors 56 control rotation of the collets 60 which grip the appropriate elements within through two transmission belts 58 , all driven in a coordinated manner by the controller ( not shown ). if it should prove desirable independently to control each of the source ( with cable ), the catheter and the sheath , a third manipulator can be positioned in series with the manipulators 40 and 42 . see fig6 for a block diagram illustrating the third manipulator 38 . on the other hand , if the application at hand requires fewer degrees of freedom than as depicted , the appropriate portions of the manipulation apparatus may be eliminated , as will be obvious to those skilled in the art . with the methods of this invention , radiation sensitive structures lying with the range of target tissue or nearby the resection cavity can be protected , and brachytherapy will then be indicated for a greater proportion of cancer patients . other means and apparatus than the embodiments described will be apparent to those of skill in the art , but are deemed within the scope of the invention as defined in the following claims .
attenuating masks for selective attenuation of radiation , particularly from an x - ray source in brachytherapy , employ materials having atomic number in a specific range , so as to attenuate radiation dose intensity generally proportionally through a range of distance from the source . some preferred attenuation materials are silver and molybdenum . a set of small masks can be provided for a particular design of catheter that holds a radiation source , each mask having a size and shape to define a different solid angle of masking , so that in a brachytherapy procedure , sensitive tissue such as bone or skin can be protected by proper selection of and positioning of the attenuating mask , to reduce the dose intensity to that tissue .