Note: Descriptions are shown in the official language in which they were submitted.
r \ 2 1 2 ~i 3 7 4
FLEXIBLE RADL~TION PROBE
~i
Field of the I~ention
~; This invention relates to flexible packaging for rniniature metal oxide
semiconductor field efEect transistor (MOSFl~l~ and more particularly to
; ~ 5 packaging for a MOSFET sensor probe used as a radiation dosimeter.
Background of the Invention
~ The use of MOSFETs for detecting radiation is well known. It is
- understood that the threshold voleage of a MOSFET, or as it is sometimes
~` ~ called ~;n insulated gate field effect transistor (IGFEI~ varies with exposure
~; 10 to radiation and therefore provides a useful building block in the design of
dosimeters. The theo~y behind the use of these MOSFET dosimeters has
1~ ~
- been described in a number of papers some of which have been authored by
the inventor. A paper entitled "Radiation Dosimetry with MOS Sensors" by
Ian Thomson, R.E. Thomas a~d L Berndt, published in "Radiatio~ Protection
1S Dosimetry", Vol. 6, No. 1, pp. 121-1Z4, December, 1983, and a paper entitled
"Semiconductor MOSFET DosimetryN, published in the proceedings of the
; Health Physics Society 1988 Annual Mee~ng p~esents the theory behind
MOS~iET dosimeters, as well as experimental results with response to
radiation of different kinds. These papers provide usefill infolmation on the
backBround of this application.
- Various coDfigurations of MOSFETs have been implemented in the prior
art in order to measure the amount of radiation dose received, while at the
same time overcorning the numerous problems which limit the accuracy and
stability of these devices. A recent implementation of a direct reading
dosimeter using IGFETs is described in the inventor's United St~tes Patent
No. 5,117,113 dated May 26, 1992. T~is patent discloses a radiation
dosimeter having a pair of IGF~Ts in~egrated into the same silicon substrate,
- in which each of the transistors is operable in a bias mode and a test mode.
A circuit elemel~t is pro~ided for determining, dunng the test mode, the
difference in the threshold voltages of the transistors, whereby the d~erence
voltage is indicative of the radiation dose, alld a circuit element is provided
~ .
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for continuously switching the traIIsistors between the bias mode and the test
mode, whereby the period of operation of the transistors in the test mode
time period is small in comparlson to the period of operation of the
tra;nsistors in the bias mode.
S There are a mlmber of applications that require the use of a dosimeter
to measure radiation. The sensitivity of a MOSFET to radia~ion is dependent
upon the thickness of the gate o~ide and the gate bias. By placing a small
voltage, example 3 to 10 volts, on the gate of a MOSFET the sensitivity of
the sensor can be enhanced.
MOSFETs of the type disclosed in the U.S. patent 5,117,113 would have
to be mounted on a suitable package or substrate. Generally this tal~es ~e
fonn of a standard IC package such as an 8 pin dual in line package (DIP).
This type of packaging is adequate for the applications such as portable
dosimeters or even badge ~pe dosimeters.
A standard 8 pin DIP packaging is too buL~y in uses such as in vivo
radiation measurement. It is generally desirable for the dosimeter to be
attached to a patient or inserted into a patient. In the latter case, this may
require the use of a catheter where sterile conditions apply. United States
Patent No. 4,976,266 to HufEman et al. discloses a method and apparatus for
`- 20 in vivo radiation measurements which uses a MOSFET dosimeter. A
disad~antage of the Huf~an device is that it requires the separation of the
MOS~ETs by having a single MOSFET in a probe and another matched
MOSF~T outside the probe in order to provide temperature compensation.
- A compensation circuit is connected with this matched unirradiated MOSFET
2 to opsrate at a culTent designed to eliminate temperature dependence of the
device. However the human body has a much higher temperature tha
ambient temperature and it is likely that Huf~an does not achieve the
temperature compensation which is reguired of this type of application since
any ilexing of the catheter, pa~ticularly near the MOSFET, is lLkely to brealc
the lead v~re to the MOSFET. A further disadvantage with the Huf~nan
device is the mounting of the MOSFET within the catheter with all epoxy
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-~ which itself is limited in applicatiom. Further, the MOSFET is rigidly
mounted which further limits the possible uses of the dosimeter. Since the
threshold voltage of the MOSFET increases with cumulative dose and cannot
" be re-set to its original value, the dosimeter is generally discarded after a
number of exposures. ~ the case of culTent MOSF~iT dosimetry systems
when a saturation level of 20,000 cGy is reached the dosimeter is discarded.
This means that for radiotherapy exposures of 200 cGy, the dosimeter may
be used up to 100 times before it is discarded. In the case of higher dose
exposures, such as in bracy~herapy, the dosimeter may be used orlly 20 times
or less.
A major disadvantage of Huf~nan and current devices is that they are not
intended for large scale manufacture and thus the cost of these dosimeters
does not justiy use on a routine basis.
Furthelmore there is a requirement for radiation workers to wear
` 15 dosimeters at their extremities where these ex~remities are lilcely to receive
higber doses than their whole body badges. Examples include technicians
-~ who worlc in the manufachlre of radioisotopes, and technicians in nuclear
medicine departments of hospitals who administer radioisotopes, physicians
and nurses who work in the X-ray beam with fluoroscopy procedures and
nuclear plaIlt workers.
-~ The annual dose allowed to extremities is 50 rems, as opposed to 2 rems
for whole body since the extremities are less susceptible to negative effects
.~
of radiation than the organs in the body. The current reqwrements for lowest
` detectable levels for extremil~ dosimeters is 250 mrem (Q2S rem,~ and the
ma~imum is 10 rem as detelmined by the "US Department of Energy-
`` Standard for the Performance Testing of Extremity Dosimetry" draft May 4,
1991 (Rev 4). (Radiotherapy dose units used earlier were in cGy and we
assume for simplicity that 1 rem = 1 cGy.)
- The most commonly used extremi~ dosimeter is a TID, which is fL~ed
to the wrist or iïngers of ~lhe worker with adhesive tape or special finger ring
~pe holders. Some disadvantages of this approach are:
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- (i) TLD extrernity dosimeters suffer from the same drawbacks as
other TID dosimeters in that they can only be read with a special laboratory
instrument. The crystals must be removed from their holders and manually
handled. This is a relatively expensive slow process and, once read, the dose
information is erased. In addition, identification can be lost once the c~ystal
is removed from its holder which may contain an identification tag. ln some
applications, daily dose readings are carried out, thus requiring the facility to
have twice as many dosimeters as workers since reading is carried out in a
laboratory.
(ii) Extrernity TLD dosimeters are generally too large to wear at the
iïngertips, which is the area of highest dose in most handling operations. A
finger ring or wrist type is most commonly used in this application.
(iii) One of the main types of radiation of interest for extremity
-~ dosimetry is beta particles. These particles travel a short distance in material.
Placement on the wrist or on a i inger ring will not give an accurate measure
of the dose to finger tips or thumbs. rhe dose at the wrist, for example, may
be orders of magnitude dif~erent from that at the thumb.
(iv) Normal TLD crystals are too thick to adequately measure all
energies of be~a partiLcles. The performance of T~ dosimeters is, therefore,
not uniform with different types of radiation such as beta particles and
X-rays. There also exists the need for a dosimeter which can be attached to
a person's extremities (e.g. fingers, hands, head,) to measure personal
radiation dose.
There therefore still e~nsts a need for a miniature dosimeter that is
capable of being used, for example, in vivo radiation measurement or which
can be bent into any configuration to conform the sensor to its measuring
environment. The sensor must also be capable of compensating for a wide
variation in temperahlre and must also be sufficiently cost effective to justifyits large scale use.
There are also times when it may be inconvenient to have ma~y wires
leading from the flexible circuit to direct reading circuitry. For ~tance, in
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s
radiotherapy, it is desirable to keep material on the patient to a minimum.
At present thennoluminescence TLDs are used to measure the dose received
by a patient. Howe~7er, they often take hours to read and consequently pose
further discomfort to an akeady uncomfortable situation.
S There also exists a need in, for example, radiosurge~y for a dosimeter
which has high spatial resolution.
Summaly of the Invention
This invention seeks to provide packaging for a MOS~ET, whic~ is
relatively easy to manufacture and which also exhibits a high degree of spatial
resolution, while at the same time being extremely flexible.
In accordance with tbis invention there is provided a flexible radiation
probe comprising:
a pair of insulated gate field effect transistors integrated into the same
semiconductor substrate each having a gate, source and dra~n; and
lS an elongate fle~ble member for supporting the transistors at a first endthereof, and for electrically connecting the gate, source and drains of each of
the transistors to a second end remote to the first end, the second end being
adapted for connection to external circuitry.
A f~rther embodiment provides for a radiation dosimeter comprising:
~ 20 fle~ble member having a plurality of electrical connection tracks
- extending bet~veen first and second ends thereof;
a semiconductor radiation sensor supported on the first end of the flexible
member, the sensor having a pair of insulated gate field effect transistors
integrated into the same substrate each having a gate, source and drain, and
2S the electrical connection tracks for connecting to a respective one of the source, drain and gate; and
- means coDnectable to the second end of the flexible member fordiflerentially biasing the transistors so that one of the transistors is more
sensitive to ionising radiation than the other of the transistors during
exposure of the traDsistors to radiation.
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A further feature provides for a method for monitonng of radiation
applied to a body comprising the steps of:
(i) inserting into the body a flexible radiation probe having a flexible
member including a plurality of electrical connection tracks extending
between first and second ends thereof and a radiation sensor supported at a
first end of the flexible member, the sensor having a pair of insulated gate
field e~fect transistors integrated into the same substrate each having a gate,
source and drain, the electrical cormection tracks comlecting the source, drain
and gate to the second end;
(ii) periodically biasing the transistors differentially and reading out
the differential threshold voltage between the transistor through the flexible
member.
~:. Bfier Descfiption of the Drawings
These and other features of the invention will become apparent, by way
of example, from the following description in which reference is made to the
appended drawings wherein:
I;IGURES l(a) and (b) show a top and side view, respectively, of a
flexible sensor according to the present invention;
~;I&URE l(c) shows a magnified view of part of the flexible sensor as
showninFigures l(a) alld (b);
~;IGURE 2 is an electrical schematic diagram showing electrical
connections for a pair of IGFETs according to the present invention;
~IGURE 3(a) is a top view of a semiconductor die which is mouIlted
according to the present invention;
~` 2S FIGURE 3o) is a schematic drawing showing bonding of leads to an
IG~ET, according to the present invention;
~;IGURE 4(a) is a cross-sectional view of a packaging for a
serniconductor; ~:
~;IGURE 4~b) is a further embodiment of a semiconductor packa~g;
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FIGURE 5(a) is an electrical schematic drawing showing the connection
of a pair of IGP~Ts for a passiYe dosimeter;
FIGURE 5(b) is a schematic drawing of a passive dosimeter using a
plurality of flexible sensors according to the present invention;
S FIGURE 6(a) is a schematic top view of an extremity dosimeter;
`~ FIGURE 6(!~) is a schematic view showing a use of the dosimeter of
Figure 6(a);
~iIGURE 7 is a schematic drawing of a direct reiading dosimeter
according to ian embodiment of the present invention; and
` 10 ~iIGURE 8 is a graph showing the correlation between a dose measured
by a direct dosimeter as a function of exposure time for a dosimeter
~i according to the present invention.
Descniption of a Prer~md Embodiment
Referring to Figures 1(a) and ~b) a flexible radiation probe is shown
F-, 15 generally by numeral 2. The probe has a radiation sensor 4, which is
supported on a first end of an elongated flexible member or circuit boa~d C.
The radiation se~or 4 is comprised of a pair of IGFET 10 transistors sh~
schematically in Figure 2. These transistors are preferably of the same type
and are fabricated in the siame semiconductor die in order that they may have
the sa1ne temperature vi~iation chia~acteristics, the same substra~e resistivityand the same slow surfi3ce states prior to radiation. Each of the IGF~Ts 10
have a gate, source and drain terminals and are labelled G1, S1, D1 and G2,
~ S2, a~d D2, respectively. The common substrate is connected internally in
; j the IG~Ts to their sou~ces S1 and S2.
ReferIing back to FiglL~es 1(a), ~b) and more particularly Figure 1(c), ~he
flexible circuit boa¢d 6 is composed of a multilayer sandwich structure of
alternately superposed polyimide film 14 and layers of copper connection
tracks 16. Each layer of the sandwich structure is held together by epo~y 18.
` Polyimide film is used in the preferred embodiment, however any other
suitable material which may be bent a number of times without damage may
be used. A suitable polyimide film is kapton~. Each of the conductive
.
212~7~ ~
copper tracks 16 are each bonded to a respective one of the IGFET terminals
as shown in Figure 3(b). The conductive copper tracks 16 are gold plated at
least on a~ upper surface in the vicinity of the first end to form bonding pads
24 so that wires 28 may be bonded from these tracks 16 to aluminum
metallization pads 30 on the top surface of the IGFET die. The bonding wire
28 is generally 25,um diameter, gold or aluminum. The flexible circuit 6 is
enlarged at a second end 8 remote to the first end. In order that the probe
may be connected to suitable external circuitry, a plurality of connector pads
20 are provided thereat. Each pad is S mm long by 1 mm wide and connects
to a respective one of the copper tracks. The above described arrangement
is also shown schematically in Figure 2.
For the probe to be used in catheters or the like the flexible circuit must
be made as small as possible. Thus the width w2 of i~exible circuit board 6
is preferably not ~nuch larger than the width of the radiation sensor 4 which
lS itself is less than 2.2 mm o~ edge. A pre~erred length L of the flexible circuit
6 may be vaned depending on where it is to be placed aIId how the radiation
probe 2 is to be used. If, for example, the ilexible circuit 6 is to be placed in
the catheter (not shown) for use in brachy~erapy, the fle7~ble circuit may be
10 to 15 cm in length. It is preferred that the thickness w of ile~able circui~
C be veIy small, and at least less than .3 mm.
In the embodiment shown in Figures 1(a), (b) and (c) the dimensions for
the ~exible radiation probe 2 are:
L = 24.4 cm
`- w = 0.03 cm
2S h=0.1cm
Ll = 4.4 cm
Wl = 0.69 cm
W2 - Q22 cm
The sensor 4 is generally covered with epoxy, which inc~eases the height
h of the sensor 4 to appro~imately 0.1 cm. In the preferred embodiment the
copper conducting tracks 16 are composed of 0.002 - 0.01 cm electrodeposited
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g
or rolledfannealed copper. I~e IGFET semiconductor chip die 4 is epo~ed
- to a 0.025 cm2 gold pad 22 which in turn is bonded to a rigid circuit board
material. Five gold bonding pads 24, each 0.003 crn2 in area, llLm in
thickness, surround the IG~ET as shown in Fijgure 3(a). Once the
semiconductor die 10 has been epoxied into place onto the gold pad 22, wire
bond 28 connections are made from the semiconductor metallization pads 30
- ~ to the respective bonding pads 24 and the semiconductor is covered with an
;~ epoxy 32, such as ~ysol~ as shown cross-section in Figure 4(a).
The epoxy co~7ered sensor of Figure 4(a) is adequate for receiving x-rays
and gamma rays, however in some applications such as for exarnple in
measuring (beta particle~ radiation to the skin of a human body this
~- paclcaging is inadequate.
r .
A suitable packaging for beta particle measurement is shown in Figure
- 4(b). One key requirement for beta particle measurement is that the sensor
must measure the dose behind a shield which has the equivalent thickness of
;i (70 I~m of tissue) which is equivilant to 7 mg/cm2. The average thickness of
.` the epidermis (outer layer of human skin) is 70 ,um.` ~ This is generally achieved by placing the IGFETs 10 in a package having
a foil lid 34. .9s described earlier, the serniconductor die is bonded to the
end of the flexible material aIld wire bond connections are made as before.
In addition thereto an annular wall 36 is constructed arou~d the
semiconductor die to forrn a "well" by bonding insulating mateAal such as
printed circuit board to the fle~ible circuit as shown in Figure 4(b) and Figure3(a). The foil lid 34 generally comprising alurninum or metallized Mylarn' of
appropriate thickness is bonded to the top of the wall, in order to protect the
~` chip and provide a means for beta particles to enter the package.
The IGFETs are known to respond to beta particles in the same way as
other types of radiation and over a wide range of energies. This is because
of the extremely thin active region of the device ~the silicon dioxide is < l,umthick) thus beta particles down to 20 keV can be detected with unshielded
dosimeters. Thick dosimeters such as TLDs have poorer performance with
. . ~.. ~ , .
. . - ~ :
.. . .
. ,. .. . - . .. -
~25374
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low energy beta particles. The thinnest T~ crystals commercially available
for extrernity dosimetry are about 25~m thick.
The extremity dosimeter previously described can be used to monitor skin
dose from X-rays and gamma rays, but can also be simplified for these
applications. The packaging as shown in Figure 4(a) is less expensive to
make than using a 70,um window of Figure 4(b) and may be used in
applicadons where it is kI;own that only X-rays or gamma rays exist. ln
nuclear medicine, for example, over 90% of the radiation exposure is due to
the use of Tc99m, which emits X-rays at 140 keV. A simple epo~ covered
sensor with or without a '~vell" is perfec~y adequate for this single-use
application.
'rhe typical dimensions for a sensor as shown in Figure 3(a) are:
xl = 0.274"
x2 = 0.174"
--~ 15 Yl = 0.185"
Y2 = 0.085
hl = 0.05"
In use, the I(;~ETs are exposed to radiation while one transistor is biased
relative to the other transistor and it has been found that more charge will
accumulate upon radiation under the gate of the biased transistor compared
to that of the unbiased transistor, thereby shifting its threshold voltage by a
; greater amount. By measuling the difference in threshold voltages it is
possible to determine the radiation received by the IG~;ETs 10. In prac~ice,
an external circuit 11, as shown in Figure 7, provides the bias for the IG~ETs
2S 10. This circuit 11 operates the IGFETs in a bias mode and a read mode.
A circuit and method for performing the read and bias mode operations is
described in the inventor's United States Patent No. 5,117,113.
The IGFETs 10 are in bias mode 97% of the time, mal~ng the sensor
sensitive to radiation. The other 3% of the time the sensor is placed into a
read mode. In this manner, a direct reading can be taken a~d the radiation
dose can be monitored in real time. This would be of particular interest
` 2125374
.:
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when a radiotherapy machine is to be checked for beam flatness. The
medical physicist often has to shape the beam using collimators. This is often
- time consuming using ionisation chambers or thermolurninescence devices
(TLDs). It is em~isioned that the flexible dosimeter connected eo direct
reading circuitry will replace these TI~s and increase the efficiency of the
beam shaping process. An array of flexible probes 2 m~y be used to monitor
- ~ the beam in real time and irradiation iterations could be performed without
terIuptions.
There are several advantages to using a direct reading circuit. The most
apparent is decreased time spene reading absorbed dose either to patients or
pha~torns. Traditionally TLDs are used however they take several hours to
be read. With the flexible dosimeter cormected to direct reading circuitry
more patients canbe treated thus increasing the efficiency of the radiotherapy
clinic. Another advantage is that the direct read;ng circuit could also be
interfaced with a data recorder or a computer and absorbed dose could be
monitored and recorded. This would be useful when a patient has been
- implanted with either radioactive beads or wires. Often the implant may last
- for a nurnber of days with doses of several thousand of rads. Currently the
inventor is not aware of any direct-reading method to measure absorbed dose
for these patients. Post radiation stability may also be er~anced by using a
test/bias c~rcuit.
The read/bias circuitIy requires a large number of wires (SLl~ leading
` from the flexible probe 2 to the read/bias circuitry. There are times however
when it is more convenient to have few wires leading from the flexible
radiation probe 2 to the read/bias circuitry. For ins~ance, in radiotherapy, it
is desirable to keep the material on the patient to a mir~imum. Figures 5(a)
~` and (b) show a configuration for this application. The IGFETs 10 are placed
in their most serlsitive mode by placing a srnall bias voltage on the gate Gl
and G2, respectively of each IGFET 10. A bias source such as a hearing aid
battery B1 and B2 are connected in series with the positive terminal of a
battery B2 connected to the gate G2 of a IGF~T 10 and with the other
` 212~374
~ 12
`~ terminal of the battery B1 connected to the gate G1 of the other IGFET 10.
The sources S1 and S2 and drains D1 and D2 are connected together as
- shown schematically in Figure 5(a3. With the IGF~Ts 10 in their bias mode
there is no current to the gate oxide thus the lifetime of the batteries B1 and
B2 is essentially their shelf life. After irradiation, the fle~ble radiation probe
2 is disconnected from the bias batteries B1 and B2 and is connected to a
; readout circuit (not shown). The read out circuit may provide the user with
the dose received by the IGF ETs 10 and its past radiation history (example
:~ cumulative dose). ~ order to increase the sensitivity of the pair to measure
; 10 lower doses, the difEerential bias is increased by mRans of more batteries
-~ from typically 6V/3V to 15V/3V. This increases the sensor differential
sensitivity from 1 mV/rem to appro~mately 4 mV/rem thus increasing the
.
signal to the reader.
There are applications which require multiple passive ilexible radiation
probes. In this instance as shown in Figure S(b) a passive circuit bias box 40
is used to provide the similar bias levels of the batteries B1 and B2 of Figure
S(a). Each of the multiple flexible radiation probes 2 may be connected via
a suitable cable 26 to the passive circuit bias box 40.
In the case of an extremity dosimet~r for personal use, the bias box may
also include a senuconductor memory device (not shown) such as an
EEPROM. T~is device only requires power when it is being read from or
writt~n to. The memory may be used to permanently store data such as:
(i) Identification of the dosimeter and, if required, the user. Where
dosimeters are not shared by different workers, the wearer's employee
number or other unique identifier is written into this memory.
(ii) The dose history of the dosimeter, so that this information may be
read by any other reader and is not reader specific.
(iii) Calibration information for the dosimeter, such as the exact
calibration factor (mV/rem).
(iv) Other data which may be required to ~alculate dose or identify ~e
wearer or location of the dosimeter.
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This configuration of the flexible radiation probe 2 is known as a passive
flexible radiation probe.
In the case of an extren~ty dosimeter as shown in Figures 6(a) and 6(b), ~ -
a relatively small passive bias box 40 is attached to the second end 60 of the
!
ilexible member ~. The bias box is typically attached to the wlist for hand
extrernities, but may also be attached to other parts of the body for other
extremities (e.g. leg, head or feet). The sensor end 61 is positioned on the
extremity using the tabs C2 to fasten around a finger as shown in Figure 6(b).
,~ The position of the sensor at the end of the ilexible member can be
determined by the user e.g. finger tip, knuckle joint etc. In many cases, a
protective glove (not shown) will also be worn on top of the dosimeter and
this helps to keep the dosimeter in place.
In t~is configuration the dosimeter is used as follows. The dosirneter and
its bias box 40 are electrically connected by a connector G4 on the Was box
to the reader (not sho~n) which measures the differential threshold voltage
of the MOSFET pair as described previously. The dosimeter is then worn for
a period of tirne, ~pically a work period of one day, and the device is read
again. The difference in the differential threshold voltages is proportional to
` the radiation dose received by the dosimeter. A dosimeter user can carry out
a reading in less than 1 minute in the field, thus requiring only one dosimeter
per worker.
The reader for this application is difEerent from that for radiotherapy in
that it must be able to read smaller dif~ereneial MOSF~T voltages than is
required for radiotherapy since the doses in extremity dosimetry are much
smaller than in therapy. This means that, with 4 mV/rem, we require to
measure 0.8 mV (i.e. 800 uV~ to correspond to 200 mrems. In practice, an
electronic reading instrument of the ~pe described for the radiotherapy
- system with increased voltage arnplification can read to S uV, which is more
than adequate as this corresponds to appro~amately 1 mrem.
The reader must also be capable of measuling the bias on the MOSF~Ts
and indicating if the batteries require ehanging. It must read and write the
~ ` 2~2~37~
- 14-
information described above in the EEPROM memory device and print this
- out to give a hardcopy and/or transfer it electronically to another device such
as a computer.
A simplified version of the reading circuitry (not shown~ may also be
,
included in the bias box 40 so that the wearer may get a direct reading of the
~ radiation dose. This may have advantages in potentially dangerous radiation
- ` fields where an alarm may be required. This would mean that the reading
c~rcuitry would require only a differential threshold measurement and alalm
function. The main reader would still be used for all other functions. This
approach would not be used in all applications as the normal mode is to
leave the dosimeter and glove on until the end of a work period.
-- Beam Qualib Checks
Cobalt-60 and linear accelerators are of ~en used in radiotherapy.
- Traditionally, ionization chambers, Thermoluminescence Dosimeters and/or
1~ films are most often used to measure the beam proiile. Good spatial
resolution of absorbed dose measurement is needed, and in the measurement
of high absorbed dose gradien~s it is essential. It is iD t~liS area that the
IGFET dosimeter has its greatest advantages over the above mentioned
technologies. The IG~;ET sensor 4 area can be made less than 1mm by lmm.
-~ 20 It is envisioned that an array of flexible circuits can be positioned to measure
the prohle of a radiation beam in real time. T~is would greatly enhance the
efficiency of the beam profiling process.
- Phantom Dodmetly
In radiotherapy, the spec~fication of the complete absorbed dose
2S distribution within the radiation beam in a phantom is a prerequisite for
calculating the pres~ ribed absorbed dose to the target volume in the patient.
It is em~isioned that the direct reading flexible radiation probe 2 can be
implanted at various locations in a phantom to measure absorbed dose. With
this method the phantom does not have to be disassembled. The IGFETs
can be read while still in the phantom and further beam modifications can
take place.
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,~
- In Vho
In vivo measurements can be divided into lïve classes.
,
(i) Entrance absorbed dose.
These are mainly to check the machine output, the absorbed dose
- S distnbution across the patient and the positioning of shielding in relation to
the position of the patient. The excellent spatial resolution of the flexible
IGFET sensor may be exploited in this instance and greatly reduce set up
time and enhance beam time efficiency.
(ii) lntra-cavity absorbed dose measurements
The absorbed dose within a body cavity (e.g. the mouth, nasopharynx,
vagina9 rectum, etc) can be measured using the flexible radiation probe 2.
The flexible probe 2 may, if required for sterility reasons, first be placed
within a catheter (not shown~ and then placed within the body cavity (not
show~). The flexible probes 2 are useful in that they are temperature
independent, as desc~ibed in U.S. Patent 5,117,113. In this manner the
ilexible dosimeter can be read in real time or placed as a passiYe dosimeter
and read periodically.
(iii) Individual spared organ absorbed dose measurements.
The absorbed dose to spared (shielded) organs can be measured using the
flexible probe 2. The flexible probe 2 can be placed in and around the shield
to measure the absorbed dose. Once again the sensor 4 can be read in real
time or as a passive beam check.
(iv) Radiosurgery.
Radiosurgery is a teehnique that employs accurate stereotactic
localization of intracranial targets and allows accurate deposition of a single
high dose of radiation to the target volume, while minimizing dose to the
surrouIlding brain structure. Very small volumes are treated with
radiosurgery ranging from less than 1 cc to a few cc. This ability of precise
tumour and treatment localization and the use of small beams creates steep
dose gradients. It is possible to measure absorbed dose using a flex~ble probe
2 and circuit. It is very important that tissue just a few millimetres from the
212~37~
- 16-
targeted volume receives insignificant dose. The inventor is not aware of
other direct-reading dosimeters on the market at this time that are capable
of the spatial resolution that is needed for radiosurgery. To date patient's
planned treatments are calculated through computer simulations.
;~ S (v) Conductive intersti~ial hyperthermia.
The goal for hyper~hermia is to r~use tumour temperature above a
~ytoto~c~c threshold for an extended period of time without ha~mful elevation
of surrounding normal tissue temperatures. The flexible probe 2 can be used
to measure absorbed dose when the patient is undergoing hyperthernua
treabnents. The response of the flexible probe 2 is independent of
temperature, and is therefore ideal for this procedure.
The flexible probe 2 may be used in the same manner as standard
packaged MOS~ET dosimeters with the following perceived advantages:
(i~ The spatial resolution of the flexible probe dosimeter has been
substantially improved. The sensor 2 area (MOS dosimeter plus surrounding
package) is 2.2mm x 2mm. The flexible circuit is 1mm thick while its present
length is 24.4 cm. The size of the flexible dosimeter makes dosimetry for
radiosu~gery possible.
(i~) The flexible probe dosimeter is capable of being inserted into a
catheter. The very nature of the ilexible circuit allows it to be bent into
maIIy coni igurations. It is therefore possible to place the sensitive area of the
ilexible dosimeter wi~in areas of the body that were heretofore difficult to
reach.
(iii) The sensitive a~ea of the flexible dosimeter can also be placed
2~ within a water tank without fear of destroying the MOS dosimeter. Since the
flexible dosimeter has been designed to be placed within the body for in vivo
measurements its advantage over other types of dosimetry is its hun~idi~
independence.
(iv) The fle~ible probe can be configured as a passive dosimeter or as
a direct reading dosimeter. The advantage of the passive dosimeter is that
there a~e a reduced number of cables connecting it to a readout circuit, thus
~:`
~ 212537~
~,~
minimizing the mateAal that is placedl within or near the radiation beam. In
` direct reading mode the radiation be~m can be monitored in real time. This
has the advaIltage of decreasing the set up time for patient planning.
Absorbed dose can be mor~itored at a specific point within a phantom, and
-~ 5 the beam can be modified and subsequent measurements can be made
without disassembling the phantom.
,~
(v~ Beam quality checks can be made with the flexible circuit placed
in an array. The configuration of the array is left to the user.
It has been found that the radiation performance of the flexible probes
.
2 with respect to sensitivity, linearity, temperature dependence, and dose rate
are similar to that of other packaged MOS devices and more particularly as
described in U.S. Patent No. 5,117,113. Referring to ~igure 8, results of a
direct reading flexible probe are shown where the radiation data was
.~
recorded in a data recorder 30 as shown in Figure 7.
ile the invention has been described in connection with a specific
embodiment thereof and in a specific use, various modifications thereof will
occur to those skilled in the a~t without departing from the spirit and scope
of the invention as set forth in the appended claims.
The terms and expressioDs which have been employed in the spedfication
` ~ 20 are used as te~ms of description and not of limitations, and there is no
inten~on in the use of such terms and expressions to exclude aIly equivalents
of the features shown and described or portions thereof, but it is recognized
~: that various modifications are possible within the scope of the claims to the
invention. -
.`
