Method for automatically indexing cast field shaping blocks to a radiation therapy treatment tray and a cooling tray for cast blocks

A method is described for indexing a polyfoam block in which a metal field shaping block is cast to both the cutting tray of a hot wire cutting system and the treatment tray of a radiation therapy treatment machine. The cast metal field shaping block is automatically correctly oriented for treatment. A cooling tray for cooling the casting is also described.

BACKGROUND OF THE INVENTION 
It is conventional practice in radiation therapy to cast beam-forming field 
shaping blocks specifically individualized to each patient. The eutectic 
alloy field shaping blocks are usually cast in a polyfoam block such as 
polystyrene which has been especially cut to make the mold. One method of 
cutting the field shaping block molds is to place the patient X-ray a set 
distance from a pivot which simulates the radiation source and to place a 
block of rigid polyfoam a set distance between the X-ray and the pivot and 
to cut the polyfoam with a hot wire cutter which follows either a pivoted 
rigid rod or a long wire cable as the operator traces the outline of the 
designated treatment areas on the X-ray. 
The voids thus created in the polyfoam are then filled with a molten 
eutectic alloy which cools to become one or more field shaping blocks. The 
blocks are then placed on a treatment tray in the beam of the radiation 
therapy machine, such as an accelerator or cobalt unit. The field shaping 
blocks are manipulated into position on the treatment tray to shield the 
areas of the patient which are to be protected from radiation. 
While this current practice of defining the radiation beam is certainly 
better than the older practice of simply arranging rectangular lead blocks 
on the treatment tray, the whole process leaves much to be desired, and it 
leads to many cumulative inaccuracies because the process described above 
cannot orient field shaping blocks with sufficient accuracy for use with 
modern high-powered, well-collimated treatment machines. Even slight 
errors in orientation can seriously undermine the physician's intent by 
allowing some areas to go untreated while others are seriously 
over-exposed. Furthermore, in casting the blocks, conventional cooling 
trays do not provide uniform pressure in holding the polyfoam block, and 
they permit spillage to occur. 
Accordingly, it is an object of this invention to provide an improved 
system of indexing and orienting accurately formed cast field shaping 
blocks in radiotherapy treatment apparatus. It is another object to 
provide a cooling tray for cast field shaping blocks. 
DESCRIPTION OF THE INVENTION 
The invention provides an indexing system which automatically ensures 
correct placement of the field shaping eutectic alloy block in the 
radiotherapy treatment machine in a position exactly analogous to that of 
the cavity in which the field shaping block was cut from the styrofoam 
block in the hot wire cutting machine using, advantageously, the hot wire 
cutting system described in my copending patent application, Ser. No 
658,435, filed Oct. 5, 1984, now U.S. Pat. No. 4,601,224 issued July 22, 
1986. 
Unless the the cast alloy shielding blocks are correctly situated upon the 
treatment tray, all the accuracy in cutting has been wasted. It is 
therefore paramount to orient the shielding blocks in the correct position 
on the radiation treatment tray. Accordingly, it is a purpose of this 
invention to index the polyfoam block to the cutting tray used in the 
cutting system and to the treatment tray used in the treatment machine in 
such manner that the shielding blocks cast within the polyfoam block will 
automatically be correctly oriented on the treatment tray. The shielding 
blocks can be fastened to the tray in correct position before the polyfoam 
is removed. 
A method is described of indexing to a treatment tray used in a radiation 
therapy treatment machine having a radiation source and a patient 
positioned for treatment in line with the radiation source, a polyfoam 
block having a cavity cut in it by cutting apparatus having a phantom 
radiation source and a patient X-ray film positioned in line with the 
phantom source, the cavity being filled with a cast metal field shaping 
block. The polyfoam block containing the cast metal field shaping block is 
indexed to the treatment tray of the radiation therapy treatment machine 
with respect to the radiation source and positioned patient identically to 
the indexation of the polyfoam block to a block support or cutting tray in 
the cutting apparatus with respect to the phantom radiation source and 
patient X-ray film. The polyfoam block may be indexed to the treatment 
tray by an arrangement of fasteners, the arrangement of fasteners being 
identical to an arrangement of fasteners fastening the polyfoam block to 
the block support or cutting tray in the cutting apparatus.

A polyfoam block such as polystyrene foam is used for cutting the cavities 
in which the shielding blocks used in radiation therapy are cast because 
it has a high degree of dimensional stability. An indexing method which 
capitalizes upon this stability and rigidity is desirable. Accordingly, a 
convenient method to index the polyfoam block to the cutting machine tray 
and the treatment machine tray is to deform an unused area of the polyfoam 
block in such manner that the polyfoam block will index to the cutter tray 
and to the treatment tray by interlock to the deformation. 
Any of several practical and simple arrangements are suitable for this 
"lock and key" index system. For example, both the cutter tray and the 
treatment tray may have a series of raised pins which indent the polyfoam, 
thus assuring precise location. Alternatively, the arrangement of FIGS. 
1-5, in which like parts are similarly numbered, offers the advantage of 
increased mechanical stability because it holds the foam block firmly in 
place. 
In FIG. 1 there is shown vertical column 5 supporting a cutter tray 4 which 
holds a block of polyfoam 2 shown with a cut pattern forming a cavity 3 
and cutter wire 1. The polyfoam block 2 is held in the tray with two or 
more thumb screws 7 which pass into the polyfoam block 2 through holes 6 
in the cutter tray 4. 
The dimensional stability of the rigid polyfoam makes it ideal for molding 
and indexing the shielding blocks. However, because the polyfoam is 
lightweight, inelastic, and easily crushed or deformed, it has long 
presented great difficulties in the casting process itself. 
While the polyfoam is light in weight, the eutectic alloy is very heavy, 
approximating the weight of lead. It is consequently necessary to hold the 
polyfoam mold firmly against a cooling surface while the molten alloy is 
poured into the cavity in the polyfoam or the alloy will displace the 
polyfoam, spill out, and spoil the casting. While various clamps and 
weights have previously been devised to immobilize the polyfoam, none have 
proven truly satisfactory, because either they crush the polyfoam, or they 
do not maintain the polyfoam in uniform, intimate contact with the cooling 
surface. The conventional method is to tape shut the "closed" end of the 
mold cavity. This method not only wastes time, but it also greatly retards 
cooling. 
In order to eliminate these problems, it is essential to provide uniform 
pressure on the polYfoam so that the casting is not distorted, yet the 
pressure must be sufficient to intimately immobilize the polyfoam block 
against the cooling surface. Furthermore, the cooling surface must conduct 
heat rapidly and efficiently so that the eutectic alloy solidifies rapidly 
and forms its own seal as the remainder of the alloy is poured. 
A cooling tray of this invention for cooling a field shaping block cast in 
a cavity in a polyfoam block comprises a generally flat heat-sink base 
means for supporting the polyfoam block on an upper surface thereof, a 
spaced pair of clamp support means affixed on the cooling tray for 
embracing respectively opposite sides of the polyfoam block disposed in 
the space therebetween, a pair of angle clamps each comprising first and 
second flanges disposed at right angles to one another, hinges pivotly 
supporting the first flanges of the angle clamps on the clamp support 
means with the second flanges thereof disposed over the space between the 
clamp support means, and spring means for biasing the angle clamps to 
pivot about the hinges and normally biasing the angle clamps to pivot so 
that the second flanges thereof are forced downwardly toward the space 
between the clamp support means, whereby to force the second flanges 
downwardly against the polyfoam block supported on the tray between the 
clamp support means. 
FIG. 2 shows a cooling tray 12 which holds the polyfoam block 2 while the 
metal casting is poured and cooled. The cooling tray 12 has a heat-sink 
cooling surface such as brushed or etched aluminum, and is shown with a 
block of polyfoam 2 immobilized upon it by means of two or more angle 
clamps 13. Angle clamps 13 seize polyfoam block 2 along its edges and 
distribute clamping pressure uniformly without distortion. Clamps 13 
rotate to exert even pressure by means of long springs 14 connected to 
angle clamps 13. The free ends of springs 14 fit into slots 15 in the 
opposite angle clamps 13 or clamp supports 18 which are rigidly attached 
to cooling tray 12 and hinged with hinges 16 to angle clamps 13. Springs 
14 provide great mechanical advantage for ease of operation and high, 
exactly reproducible clamping pressure, while the geometry of the rotating 
angle clamps 13 resists twisting and uneven pressure, unlike conventional 
screw or lever clamps. 
With the polyfoam molding block 2 thus securely immobilized, it is 
necessary to cool the eutectic alloy as rapidly and efficiently as 
possible so that the alloy becomes "self-sealing" and cannot leak out of 
the lower "closed" end of the cavity 3 in the polyfoam block 2 which rests 
upon the cooling surface of tray 12. Accordingly, the surface of the 
cooling heat-sink tray 12 should be brushed or etched in order to increase 
surface area and improve thermal transfer. When the alloy is poured into 
cavity 3 the alloy begins to solidify instantly and provides its own seal. 
Castings can thereby be produced very rapidly with little difficulty, and 
cooling time can be reduced from hours to minutes. 
In FIG. 3 the polyfoam block 2 with the metal casting 9 in place is shown 
ready for attachment to the treatment tray 8 by means of fasteners, shown 
as thumb screws 7', which pass through holes 6' in treatment tray 8. 
FIG. 4 shows the polyfoam block 2 in place on the treatment tray 8, 
fastened by thumb screws 7' through holes 6' in the treatment tray. The 
holes 6' in the treatment tray 8 are situated identically to the holes 6 
in the cutter tray shown in FIG. 1. The cut cavity is shown already cast 
with the eutectic alloy casting 9 which has been attached to the treatment 
tray 8 by means of the screws 10. 
FIG. 5 shows the treatment tray 8 with the polyfoam block removed and the 
casting 9 left in correct location by means of screws 10. The now unused 
indexing holes 6' are shown for clarity. 
Of course, many such mechanical indexing arrangements are possible, but the 
essential feature is that the polyfoam block must index to both the cutter 
tray and the treatment tray in such manner that the cast field shaping 
block will occupy the same location on both trays. In the illustrated 
example, this was done with an identical pattern of holes and screws in 
both the cutter and treatment trays; the pattern is thus located and the 
field shaping blocks oriented with perfect accuracy. In this manner the 
relationship between the hot wire pivot position (phantom radiation 
source), the cut polyfoam block on the cutting tray and the patient X-ray 
film, in a hot wire cutting system, is identical to the relationship 
between the radiation source, the cast metal field shaping block on the 
treatment tray and the patient during treatment using a radiation therapy 
treatment machine. 
In use, the treatment machine operator slides the cast metal block secured 
to the treatment tray into the treatment machine and the metal block then 
needs no further manipulation to ensure proper orientation or placement 
for treatment because the polyfoam block into which the metal block was 
cast was identically indexed to both the cutting and treatment trays. 
For some procedures it is necessary to use a cast block of paraffin or 
other material rather than the cast eutectic alloy (or other cast metal) 
block. The apparatus and method described above are suitable for casting 
and use of paraffin blocks as well as for metal blocks. 
Variations and modifications of this invention can be effected within the 
spirit and scope of the invention as described above and as defined in the 
appended claims.