Ultra-thin high dose iridium source for remote afterloader

An ultra-thin iridium source is used for the treatment of cancerous tissue, particularly in areas of the human body, such as the brain, where minimization of trauma to adjacent tissue is a high priority. The source is formed of a relatively pure iridium seed encapsulated in the end region of a relatively pure unitary platinum delivery wire. The relatively pure iridium source is irradiated to a high activity level (e.g. 10 curie) even though having a short overall length (e.g. 10 millimeter) and ultra-thin diameter (e.g. about 1/8 millimeter). The platinum delivery wire defines a substantially uniform ultra-thin cross section of approximately 0.5 millimeter diameter. The iridium source is formed within the unitary platinum delivery wire without resort to welding or other inherently unreliable attachment systems. Delivery of the high activity pure iridium source to treatment area is achieved using remote afterloader equipment.

BACKGROUND OF THE INVENTION 
The present invention relates to radiation treatment of cancerous tissue 
internally within the human body. More specifically, the present invention 
pertains to a high activity/high dose iridium source adapted for remote 
afterloader implantation in living tissue. 
In general the irradiation of tissue as a means to reduce or eliminate 
malignancy has been known for many years. The specific procedure and 
equipment required for any given treatment, however, varies according to 
numerous factors including the location of the cancer within the body, its 
level of development and spread, and the age and condition of the patient. 
For example, Wappler U.S. Pat. Nos. 2,322,902 and 2,439,438 and Packer U.S. 
Pat. Nos. 3,458,365, relate to the manufacture of radium and xenon gas 
"seeds" which may be used to treat cancer by physically implanting the 
seeds at the cancer site in the patient's body. As described in the Packer 
patent, these implants may be of low dosage, thereby facilitating the 
permanent implantation thereof as well as permitting safe handling by 
doctors and other personnel during surgical installation. This form of 
irradiation treatment is limited to those regions of the body in which 
placement of the seeds can be effected without undue trauma to adjacent 
normal tissue. 
Another known cancer treatment procedure involves the insertion of a 
radioactive source through a guide tube, which may be an elongate needle 
or catheter. The present invention relates generally to this radiation 
source placement technique. 
While the use of a guide tube (needle or catheter) offers relatively safe 
and easy access to many parts of the body, it is not without its 
limitations. First, the placement of the guide tube, while less invasive 
than alternative direct surgical placement, nevertheless can traumatize 
tissue along its path of insertion. This is particularly true for 
treatment of, for example, brain tumors, where a hole to receive the guide 
tube must be drilled through the cranium and brain tissue. Any drilling of 
brain tissue causes irreparable damage thereto and consequently such 
drilling is desirably kept to a minimum. 
Second, for low intensity radiation treatments, the source of radiation 
must remain resident for extended periods of time, often in excess of 
several days. As the patient will not normally be hospitalized during this 
entire extended treatment cycle, the guide tubes must be inserted, 
positioned, and terminated such that the patient may undertake much of his 
normal daily routine. 
Known indirect guide tube products typically employ an iridium/platinum 
alloy of approximately 30% iridium. These products are often used in 
connection with low intensity, source-resident type procedures, that is, 
where the source remains in the body for extended durations. Known iridium 
alloy sources used for such irradiation generally exhibit activity levels 
between about 0.5 and 25 millicurie/centimeter. (The overall activity 
level thus varies as a function of the length of the active region, which 
may be several centimeters.) 
Alternatively, iridium/platinum alloy sources of proportionately larger 
diameter have been fabricated to achieve the higher radiation activity 
levels necessary for short term treatment procedures. As noted, placement 
of these larger diameter sources results in correspondingly increased 
damage to normal tissue. 
The smallest source assemblies heretofore used have been about 1 millimeter 
in diameter. 
Furthermore, known guide tube sources are fabricated by welding or 
otherwise affixing an iridium alloy source to the end of a steel "fish" or 
delivery wire. Insertion of these relatively large sources can be 
frustrated where the guide tube has been oriented, for medical reasons, 
along a curved contour. In fact, stresses induced during source insertion 
have been known to break the source from the delivery wire at its point of 
welded attachment. This breakage problem is particularly acute where it is 
expected that the iridium source will be reused for a number of surgical 
procedures. Repeated flexure ultimately causes joint fatigue and failure. 
Radioactive iridium is a common source of irradiation for cancer treatment 
because it has a convenient half life and emits gamma rays of suitable 
energy. The useful isotope is iridium 192, which has a half life of about 
74 days. This is sufficiently long to permit use of the source at some 
time and distance from its creation. It emits gamma rays of a number of 
useful energies in the range of hundreds of KeV and less than about 0.5 
MeV. One such gamma ray energy is about 484 KeV. This is energetic enough 
to pass out of the guide tube and through adjacent tissue to irradiate the 
tumor but is not so energetic as to reach more remote parts of the body to 
the detriment thereof. That is, the radiation can be relatively 
concentrated at the tumor without destroying too much healthy tissue. 
A problem with certain prior art iridium guide tube sources has been 
attributed to their fabrication. Iridium 192 is produced by the 
irradiation of iridium 191 in a nuclear reactor. At least partly because 
of their size and shape, the sources were assembled with respective 
delivery wires before irradiation, which resulted in the irradiation of 
these wires, rendering them radioactive as well, with undesirable half 
lives and energies. 
SUMMARY OF THE INVENTION 
The present ultra-thin iridium guide tube source overcomes many of the 
above limitations by utilizing a high activity iridium core member secured 
within the end of a substantially nonradioactive delivery wire. 
In accordance with the present invention, the radioactive core member is 
made by irradiating a natural iridium member with thermal neutrons in a 
nuclear reactor. Natural iridium is 33% iridium 191 and 67% iridium 193. 
Iridium 191 has a substantial thermal neutron cross section and captures 
thermal neutrons with a substantial probability to form iridium 192, which 
is radioactive with a half-life of 74 days. Iridium 193 has a lesser 
thermal neutron cross section and, hence, is not so readily made 
radioactive. Further, the iridium 194 that is formed has a half-life that 
is very short relative to that of iridium 192 and, hence, rapidly decays 
within a time short relative to the half-life of iridium 192 to a 
negligible level relative to the remaining iridium 192. 
The present invention also contemplates a more concentrated source in order 
that high radioactivity may be provided in a smaller diameter. To this end 
the core member is made of relatively pure iridium rather than the 30% 
iridium/platinum of the prior art. By relatively pure iridium is meant a 
purity of at least about 90%, where the remainder is material that 
following the neutron irradiation becomes negligibly radioactive relative 
to the iridium 192 within a time after neutron irradiation short relative 
to the half life of iridium 192. Negligibly radioactive means that the 
energies of the radiation are negligibly small relative to the radiation 
from iridium 192 and the rate of disintegration is negligibly small 
relative to the disintegration of iridium 192. The relatively pure iridium 
core is activated in a thermal neutron flux to a desired activity, which 
may be at least 10 curies. This permits a full radiation treatment of a 
few minutes duration with a thin guide tube. The magnitude of the 
activating thermal neutron flux is not critical; however, it should be 
sufficient to achieve the desired degree of radioactivity well within the 
half-life of iridium 192 lest the iridium 192 decay almost as fast as it 
is formed, resulting in costly wasteful neutron flux. A time of a few 
weeks has proven practical. 
The core member is fabricated by encapsulating it in the end of a unitary 
delivery wire prior to neutron irradiation. As the delivery wire is then 
itself irradiated by neutrons, it is essential that it be made of material 
that is negligibly radioactive relative to the iridium 192 within a time 
after neutron irradiation that is short relative to the half-life of 
iridium 192. This may be material that has a thermal neutron capture cross 
section negligible relative to that of iridium 191, so as not to become 
very radioactive in the first place. It may be material that becomes 
radioactive with a half-life so short relative to that of iridium 192 that 
any radioactivity substantially decays away while leaving iridium 192. It 
may be material that provides radiation of such low energy relative to the 
radiation from iridium 192 as to have substantially no penetration and, 
hence, substantially no physiological effect. It may be material with all 
these properties. In addition it must have the necessary structural 
qualities as to permit driving the core member through the guide tube and 
protecting it from abrasion or breakage, assuring the integrity of the 
source so that no parts thereof are left in a patient's body. In 
accordance with the present invention, platinum has proven to have 
suitable properties. 
The core and delivery wire assembly made in accordance with the present 
invention can be and is made of smaller diameter than has previously been 
achieved successfully, that is, substantially smaller than 1 millimeter in 
diameter, preferably less than about 0.7 millimeters, with the core member 
suitably smaller, preferably less than about 0.2 millimeters. An assembly 
of about 0.5 millimeters in diameter has proven practical in reaching 
tumors in previously inaccessible regions, such as behind the eye. In such 
an assembly the core member was about 0.125 millimeters in diameter. Such 
fine assemblies may be formed by inserting a larger diameter core member 
in a uniform larger diameter platinum wire along its axis and drawing the 
assembly down to the desired diameter. 
There are no welds or other junctions of unpredictable quality to break 
when urging the present ultra-narrow source through tight radius curves. 
The narrow diameter of the present delivery wire, in addition to its 
suitability for minimum tissue damage ultra-fine guide tube insertion, is 
more flexible and therefore better adapted for treatment procedures 
requiring a convoluted catheter routing. 
These and other aspects and advantages of the invention will be further 
apparent from the following detailed description, particularly when taken 
in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates the high dose iridium/platinum radioactive cancer 
treatment source 10 of the present invention. Source 10 includes a 
relatively pure iridium core member or seed 12 formed, as discussed in 
more detail below, in the end of a unitary elongate relatively pure 
platinum delivery wire 14. 
Iridium enhancement is not required. Natural iridium, comprising the 
isotopes of atomic weight 191 and 193, may be used. Natural iridium is 
approximately one-third iridium 191, which is activated in a nucleaar 
reactor by thermal neutron flux to iridium 192, which has a principal 
energy level of 484 KeV and a half-life of 74 days. 
Fabrication of the core member 12 using relatively pure iridium 
advantageously permits activation of the source to a high dose level while 
simultaneously maintaining the core 12 and delivery wire 14 at the 
smallest possible diameter, thereby minimizing tissue damage caused by 
guide tube or catheter placement. 
As used herein, high dose signifies an activity level on the order of 10 
curies. A 10 curie source is suitable for rapid, nonresident treatment 
sessions generally in the order of from one to ten minutes. For example, a 
10 curie iridium 192 source (at 484 KeV) will provide a 3000 rad dose in 
ten minutes. Such a dose may be divided into four sessions to minimize the 
adverse effects of an unduly rapid treatment profile. In any event, 
radiation treatments are preferably effected by the temporary and short 
duration placement of the requisite guide tubes within the patient as 
contrasted with the long-term, multiple day treatment process of 
alternative low dose techniques. 
The iridium core 12 is activated by placing the entire source 10, including 
the unitary delivery wire 14, within a thermal neutron flux field. Flux 
field residence required for a desired activation level depends upon the 
neutron flux density and the purity and mass of the iridium core. A 
shorter residence is required upon use of the preferred relatively pure 
iridium source. 
By far the greatest expense associated with a high activity radioactive 
source is the cost of neutron flux activation. As this expense is directly 
proportional to the residence or exposure time of the iridium source to 
the neutron flux field, substantial cost benefits accrue by using a 
concentrated relatively pure iridium source with its correspondingly lower 
irradiation residence time requirement to achieve the desired activation 
After activation the source 10 must be maintained in a shielded enclosure 
or "safe" when not in use. Due to its high activity level, the source 
cannot safely be handled by doctors or other personnel, and, therefore, 
patient treatment is achieved by linking the guide tubes or catheters to a 
remotely actuated apparatus commonly known as an afterloader (not shown). 
The afterloader serves to reposition the source remotely from the safe to 
the site of treatment within the patient and, thereafter, to withdraw and 
replace the source in the safe. 
Referring again to FIG. 1, a relatively pure iridium core suitable for a 10 
curie activation intensity can advantageously be formed of narrow diameter 
and of suitable overall length. Specifically, the iridium core 12 is 
preferably about 0.125 millimeters in diameter and about 10 millimeters in 
length. A longer iridium core, e.g., 20 millimeters, would advantageously 
reduce the number of discrete positions or steps required for any given 
treatment session. Iridium cores in excess of one centimeter, however, 
encounter manufacturing limitations. Further it is often desirable to more 
nearly approximate a point source, although at least about 5 millimeters 
is preferred. 
The iridium core 12 is fabricated in the end of a platinum delivery wire 
14, preferably spaced inwardly about 1 millimeter from the distal end 16 
thereof. Platinum is preferred by reason of its mechanical strength and 
flexibility as well as its relatively short radioactive half-life upon 
thermal neutron irradiation (about 14 days), and its relatively low 
thermal neutron capture cross section. Unlike the prior art stainless 
steel delivery wires, platinum wire, when exposed to thermal neutron flux, 
is substantially nonradioactive relative to the irradiated iridium in a 
time short relative to the half-life of iridium 192. The wire is made of 
relatively pure platinum in the sense that any other elements present do 
not weaken the structural properties of the platinum and when exposed to 
thermal neutron flux are substantially nonradioactive relative to 
irradiated iridiation in a time short relative to the half-life of iridium 
192. 
The platinum delivery wire may be of any convenient length consistent with 
the surgical procedures contemplated and with the requirements of the 
remote afterloader equipment with which the iridium source is intended to 
be used. A typical length is 2.1 meters. 
As previously noted, one of the principal objectives of the present 
invention is to provide a high dose iridium source of the smallest 
possible diameter In this connection, the platinum wire 14 of the present 
source is made less than 1 millimeter and preferably about 0.5 millimeter 
in diameter. Although delivery wires of narrower diameter are 
contemplated, the present 0.5 millimeter system represents a practical 
lower dimensional limit in view of present manufacturing technology. 
The source assembly 10 may be fabricated by conventional mechanical or 
laser drilling techniques whereby a hole to receive the iridium core 12 is 
drilled in the distal end of the platinum wire 14. However, in view of the 
extremely small diameter of the present source and, further, the 
difficulty in working with pure iridium, it is preferred to fabricate the 
present source by a drawing process whereby the iridium is disposed in a 
larger platinum wire and drawn to the desired smaller diameter. 
It will be appreciated that the above described source 10 provides for the 
rapid nonresident treatment of cancerous tissue through the incorporation 
of a physically small, high activity pure iridium source into a platinum 
delivery wire of correspondingly narrow diameter. In this manner, access 
to and treatment of cancer tissue in remote and sensitive regions of the 
body may be effected through the use of extremely fine needles, or 
catheters or other guide tubes with a minimum of damage to surrounding 
normal tissue. It will be further appreciated that the present source is 
integral, that is, a unitary delivery wire 14 is formed over the active 
iridium element 12 without resort to welds or other bonding systems. This, 
in turn, allows for routing of the source through irregular and tightly 
contoured catheters with substantially less danger of source separation, 
separation which is known to occur in conventional systems at welded 
junctions between the delivery wire and the active radiation source.