the present invention provides devices and methods for delivering a treatment , such as a cancer treatment , into a cavity within the body of an animal . for example , devices and methods having features of the invention may be used to deliver treatments into a biopsy site or into a cavity left after removal of cancerous tissue from within the body of a human patient . vacuum is applied to tissue to enhance contact between a treatment delivery assembly within a body cavity and tissue surrounding the body cavity . a vacuum path around the treatment assembly is provided by devices , systems and methods embodying features of the invention . vacuum may be applied to tissue via one , two , or multiple vacuum ports . a vacuum port may be a port in a vacuum delivery conduit , a hole in a sheath or balloon connected to a vacuum delivery conduit . a fluid permeable wall or portion of a fluid permeable wall may be effective to serve as a vacuum port . fig1 is a perspective view of a system 10 embodying features of the invention illustrating a device 12 having an outer balloon 14 enclosing an inner balloon 16 ( shown in the cut - away portion of the illustration ), a shaft 18 and connector 20 . outer balloon 14 comprises a sheath assembly around inner balloon 16 . outer balloon 14 is thus an example of an enclosure assembly , and forms an enclosure wall around inner balloon 16 . outer balloon 14 comprises at least in part a fluid permeable wall ; as illustrated in fig1 , outer balloon 14 has holes 22 allowing fluid permeation into and out of balloon 14 . in alternative embodiments , an outer balloon 14 may be made of woven or otherwise substantially continuous materials that are fluid permeable . in further embodiments , an enclosure wall or assembly such as an outer balloon may comprise a net , mesh , framework , or other discontinuous structure . holes 22 ( or fluid permeable material ) allow fluids to pass through outer balloon 14 into intermediate space 24 disposed outside inner balloon 16 . intermediate space 24 provides a vacuum path adjacent inner balloon 16 . where at least a portion of outer balloon 14 is disposed adjacent inner balloon 16 , intermediate space 24 is disposed between outer balloon 14 and inner balloon 16 . inner balloon 16 defines an inner lumen 26 , within which a delivery shaft 28 may be at least partially contained . as shown in fig2 , a treatment material 30 may be permanently or transiently disposed within delivery shaft 28 . a probe 32 configured to move within delivery shaft 28 may be used to position treatment material 30 , including to place treatment material 30 into and to retrieve placement material 30 from , within delivery shaft 28 . a vacuum conduit 34 may be part of , or may be contained within , a shaft 18 and operatively connected to intermediate space 24 . shaft 18 may also include or contain an inflation conduit 36 configured to allow passage of inflation fluid into inner lumen 26 . passage of inflation fluid into inner lumen 26 is effective to inflate inner balloon 16 . inflation fluid may be any suitable fluid , either a gas or a liquid , and is typically inert . inflation fluid , where a gas , may be , e . g ., air , nitrogen , carbon dioxide or other gas . inflation fluid , where a liquid , may be water , saline , mineral oil , or other liquid . in some embodiments , an inflation fluid may be effective to absorb radiation to , for example , moderate or adjust a dosage of radiation delivered to a patient &# 39 ; s tissue from radioactive treatment material 30 contained within a delivery shaft 28 . vacuum applied to intermediate space 24 is effective to deliver a treatment within a body cavity 38 within a patient &# 39 ; s body effective to urge surrounding tissue into contact with at least a portion of the surface of the outer balloon 14 . the outer balloon 14 shown in fig1 - 5 is illustrated as a balloon that is configured to permanently or semi - permanently enclose inner balloon 16 or inner balloon assembly . such an enclosure may be partial or complete . it will be understood that the outer surface of a device and of a system embodying features of the invention may also be a sheath 50 configured for deployment over and around an inner balloon assembly 14 . in further embodiments , an enclosure may be , e . g . a net , mesh , framework , or other discontinuous structure . fig2 is a longitudinal cross - sectional view of the system of fig1 taken along line 2 - 2 showing in cross section , for example , the relative positions of treatment material 30 , an inner balloon 16 , and an outer balloon 14 or sheath 50 . fig2 includes cross - sectional views of shaft 18 including views of delivery shaft 28 , vacuum conduit 34 and inflation conduit 36 . fig3 is a transverse cross - sectional view of the system of fig1 taken along line 3 - 3 showing outer balloon 14 and holes 22 therethrough , inner balloon 16 disposed within outer balloon 14 , delivery shaft 28 and probe 32 . fig4 a and 4b show portions of outer balloon 14 and inner balloon 16 as indicated in fig1 , including intermediate space 24 and spacers 40 which serve as separation elements effective to maintain patency of intermediate space 24 even under the influence of vacuum supplied via vacuum conduit 34 . spacers 40 may be part of outer balloon 14 , or of inner balloon 16 , or both . a spacer 40 may be a bump , knob , ridge , or other feature extending inwardly from an inner surface 42 of outer balloon 14 , or extending outwardly from an outer surface 44 of outer balloon 14 . in addition , or alternatively , a spacer 40 may be an object that is placed within intermediate space 24 and is separate from outer balloon 14 and inner balloon 16 . for example , as shown in fig4 a and 4b , spacers 40 may be stand - offs extending from an inner surface 42 of outer balloon 14 and from an outer surface 44 of outer balloon 14 . fig5 a - 5d illustrate the fitting of an outer balloon assembly 46 ( including an outer balloon in the form of a sheath 50 ), over an inner balloon assembly 48 including an inner balloon 16 . fig5 b shows the assembled outer 46 and inner 48 balloon assemblies of fig6 a following placement into a cavity 38 within a breast 52 of a patient and before inflation of the inner balloon assembly 48 . in fig5 c , the inner balloon assembly 48 has been inflated by passage of inflation fluid through inflation conduit 36 , pressing some parts of the outer surface 54 outer balloon assembly 46 into contact with portions of the inner surface 56 of body cavity 38 . note , however , that since most cavities 38 have irregular inner surfaces 56 , there will typically be poor and intermittent contact between outer surface 54 of sheath 50 ( or outer balloon 14 in alternative embodiments ) and inner surface 56 of cavity 38 , as shown in fig5 c . fig5 d shows the assembled outer 46 and inner 48 balloon assemblies of fig5 a following application of vacuum via vacuum conduit 34 to the intermediate space 24 separating the inner balloon 16 and the sheath 50 ( outer balloon 14 ). treatment material 30 is in place within delivery shaft 28 . note that inner surface 56 of cavity 38 has been pulled into intimate contact with outer surface 54 of sheath 50 . such intimate contact configures inner surface 56 into an optimal configuration for the application of treatment by a treatment material 30 . for example , radiation treatment by a radiation treatment material 30 is enhanced by proper positioning of adjacent tissue to provide proper irradiation . irradiation levels may vary widely where the adjacent tissue of tissue cavity 38 is at different , irregular , or improper distances from a radiation source . application of vacuum effective to draw tissue into better contact with device 12 , e . g ., into better contact with outer surface 54 of sheath 50 , is effective to improve the delivery of radiation treatment from a radioactive treatment material 30 . fig6 a illustrates a system embodying features of the invention including a vacuum delivery element comprising an enclosure 60 having ribs 62 configured to partly enclose an inner balloon assembly 48 . vacuum is delivered to intermediate space 24 via vacuum ports 64 operatively connected to vacuum conduit 34 . as shown in cross - section in fig6 b , ribs 62 serve as separation elements effective to provide vacuum paths in the intermediate space 24 between tissue surface 56 and outer surface 44 of inner balloon assembly 48 . methods for treating tissue adjacent a body cavity 38 include methods for delivering a treatment to tissue adjacent a device 12 embodying features of the invention . for example , a method of treating tissue adjacent a body cavity 38 includes contacting tissue adjacent the body cavity 38 with a sheath 50 or an outer balloon 14 , and applying a vacuum via vacuum conduit 34 . the vacuum may be effective to draw adjacent tissue towards and into contact with a sheath 50 or an outer balloon 14 , and so enhance the contact between the outer wall 54 and the tissue . delivery of inflation fluid to an inner balloon 16 via an inflation conduit 36 to inflate inner balloon 16 is effective to enhance contact with adjacent tissue as well , serving to bring outer balloon 14 or sheath 50 closer to tissue than it would be in the absence of inflation of inner balloon 16 . in preferred embodiments , the inner balloon assembly 48 comprises an inflatable treatment delivery device such as a mammosite rts ( proxima therapeutics , inc ., alpharetta , ga . 30005 ) or similar device . methods further include placing a treatment material 30 , such as a radiation source , within the device ( e . g ., by placement within a delivery shaft 28 ). a radiation source , such as a solid radiation source ( e . g ., a brachytherapy seeds ) may be advanced into a delivery shaft 28 with a probe 32 or by other means . other solid treatment materials 30 may similarly be advanced into a delivery shaft 28 with a probe 32 or by other means . a liquid radiation source ( e . g ., iotrex ®, proxima therapeutics , inc ., alpharetta , ga .) may be advanced into a delivery shaft 28 by fluid flow , under the influence of gravity , pressure applied by a syringe or other pressure source , or other means for delivering fluid into a space . similarly , hot liquids and other liquid treatment materials 30 may be introduced into a delivery shaft 28 or an inner balloon 16 ( via inflation conduit 36 ) under the influence of gravity , pressure applied by a syringe or other pressure source , or other means for delivering fluid into a space . some treatment regimens may include periodic or episodic treatment , in which radiation or other treatment is applied for a treatment period , and then the treatment is stopped for a recovery period . such periodic or episodic treatments may be repeated , so that treatment is applied during a first treatment period , stopped during a first recovery period , and then treatment is re - applied for a second treatment period . further treatment periods and recovery periods may also be used as necessary . thus , methods may further include removal of a radiation source or other treatment material 30 from within a delivery shaft 28 , and may further include replacing the treatment material 30 . although a cavity 38 is typically an artificial cavity remaining after removal of tissue at biopsy , surgery , or other medical procedure , a body cavity may be a natural body cavity . for example , devices 12 may be inserted into a bladder for the treatment of bladder cancer . application of suction is effective to enhance contact with a device 12 in such an example as well . such enhanced contact may be effective to improve the delivery of radiation or other treatment , and may be effective to avoid “ hot spots ” ( tissue regions receiving more radiation than is received by neighboring tissue regions ) and is one of the important advantages provided by the present invention . treatment material 30 may include a chemotherapy agent effective to treat cancer or other disease condition of tissue surrounding a body cavity 38 . in preferred embodiments , treatment material 30 includes a radiation source configured to delivery radiation to tissue adjacent a device 12 . thus , treatment material 30 may include a radiation source which may be solid or liquid . a liquid radiation source may include , for example , a liquid containing a radioactive iodine isotope ( e . g ., 125 i or 131 i ), a slurry of a solid isotope , e . g . 198 au , 90 y , 169 yb , or a gel containing a radioactive isotope . liquid radiation sources are commercially available ( e . g ., from iotrex ®, proxima therapeutics , inc ., alpharetta , ga .). a solid radiation source may include brachytherapy seeds or other solid radiation source used in radiation therapy , such as , for example a radioactive microsphere available from the 3m company of st . paul , minn . a solid radioactive source can either be preloaded into a device 12 at the time of manufacture or may be loaded into the device 12 after placement into body cavity 38 of a distal portion of the device 12 . such distal portion preferably includes the outer balloon 14 , inner balloon 16 , and at least a portion of delivery shaft 28 . such a solid radioactive core configuration offers the advantage in that it allows a wider range of radionuclides than if one is limited to liquids . solid radionuclides suitable for use with a delivery device embodying features of the present invention are currently generally available as brachytherapy radiation sources ( e . g ., i - plant ™, med - tec , orange city iowa ). in general , the amount of radiation desired by the physician is a certain minimum amount that is delivered to a site about 0 - 3 cm away from the wall of the body cavity 38 ( e . g ., from where a tumor has been excised ). vacuum applied to intermediate space 24 effects good contact between tissue surrounding body cavity 38 and the wall of the outer balloon 14 or sheath 50 , promoting effective treatment delivery , such as delivery of radiation to surrounding tissue . it is desirable to keep the radiation in the region near the wall of the outer balloon 14 or sheath 50 as uniform as possible to prevent over - exposure to tissue at or near the reservoir wall . it is well known that the absorbed dose rate at a point exterior to a radioactive source is inversely proportional to the square of the distance between the radiation source and the target point . thus , it is possible that the radiation dosage delivered to adjacent tissue may differ from that delivered to tissue disposed at more distal locations . in some instances , penetration of radiation to locations far from a device 12 is not desired . for example , in treating cancers such as bladder cancer , where the neoplastic tissue is generally located on the bladder surface , deep penetration is unnecessary and to be avoided . an inflation fluid may also be a radiation absorbing fluid . for example , an inflation fluid may be an x - ray contrast agent as used in angiography , such as a barium salt ( e . g ., barium sulfate ), water , saline or other such fluid . a radiation - absorbing inflation fluid , which will surround a radiation source placed within delivery shaft 28 , serves to moderate and control the delivery of radiation from the radiation source to surrounding tissue . such moderation and control that is obtained with a radiation - absorbing inflation fluid may aid in avoiding the delivery of an excessive amount of radiation to some portions of the surrounding tissue . thus , in the absence of such a radiation - absorbing inflation fluid , it is possible in some instances that a radiation source sufficient to provide an effective dose at distances removed from a device 12 , would expose tissue that is directly adjacent the wall of the outer balloon 14 or sheath 50 to an excessive radiation dose . such excessive exposure to such tissue near to the device 12 may result in necrosis of healthy tissue necrosis . alternatively , an inflation fluid may contain radioactive elements , either as a liquid or slurry , so that the inner balloon 16 is filled with a source of radiation , providing a fairly uniform source of radiation that is distributed over the volume of the inner balloon 16 . in such embodiments , an inflation fluid thus itself serves as a radiation source , thereby providing well - controlled amounts of radiation to surrounding tissue while minimizing irregularities in the dosages delivered to particular locations . in embodiments of the invention in which an inflation fluid includes a radiation source , a delivery shaft 28 may contain a radiation absorptive material , so that , for example , less volume of radioactive material is required than if the entire volume of a device 12 were filled with radioactive material . such a configuration may be advantageous where a profile exhibiting higher intensity at a tissue surface with lesser penetration is desired . moreover , the outer balloon 14 need not be spherical , yet a uniform profile of radiation delivery is obtainable . experiments reported in williams u . s . pat . no . 5 , 918 , 813 are described as showing that a steeper radial absorbed source gradient can be obtained using a radiation attenuation fluid in an inner chamber of a similar radiation deliver device than otherwise obtains with a device having only a single distensible chamber ( as described in williams u . s . pat . no . 5 , 429 , 582 ).