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Effect of lead shielding cutouts on dose to overlying tissue for 7 MV electrons
0360-3016/80!121751~3S02.00/0 Copyrkghr Q 1980 Pergamon Press Ltd.
In!. J. Radiation Oncolo~ Biol. Phyr.. Vol. 6. pp 1751-1753 Printed in the U.S.A. All rights reserved.
0 Technical Innovations and Notes EFFECT OF LEAD SHIELDING CUTOUTS ON DOSE TO OVERLYING TISSUE FOR 7 MV ELECTRONS ARDEN
Department
of Radiology,
MARTIN Department
of Radiological
E. DOCKTER, MS.
South Bay Hospital,
Sciences,
W.
514 North HERMAN,
Prospect
Ave., Redondo
Beach, CA 90277
PH.D.
LAC Harbor-UCLA Medical Center and L’CLA School of Medicine, West Carson St., Torrance, CA 90505
1000
Treatment of the helix of the ear with a lead shield placed behind the ear du:ing electron beam therapy significantly increases the radiation close to the skin above the block. With film densitometry, the increased radiation dose is 10 8 higher than the dose at the dose maximum. It is believed that the increase is a result of bremssfra~lung and scattered electrons originating in the lead. The increased dose should be taken into account when calculating the dose to those structures. Electron beam therapy, Patient shielding, Bremsstrahlung
contamination.
used in the top layer was to simulate the thickness of the ear. This phantom was also exposed to 300 MU of 7 MV electrons. The film densitometer was calibrated to indicate 100% at the maximum density of the calibration film. The traces for the other films were obtained at the same densitometer settings.
INTRODUCTION
patient with a basal cell carcinoma of the helix of the left ear was seen. As the tumor extended through the cartilage, it was decided to treat the lesion with 7 MV electrons. A I/ 16th inch lead shield was fabricated to fit behind and around the ear to prevent exposure of other structures. (Fig. 1). The question arose as to the radiation dose to the ear directly above the lead shield. A series of experiments were performed to determine if there was a significant change in exposure. A
bIETHODS
AND
RESULTS
The calibration scan in Fig. 4a was compared with the tracing from above the lead shield in Fig. 4b and it was seen that there was a 10% increase in the radiation exposure directly above the lead shield. However, the dose above the unshielded portion of the field at the same level was decreased by 2%.
MATERIALS
cm methacryiate* phantom was covered with a film pack? which in turn was covered with a 1 cm block of methacrylate (Fig. 2). The film was exposed to 300 monitor units (MU) of 7 MV electrons. After processing in an automatic processor for 2 minutes, the film was scanned with a film densit0meter.t This was the calibration scan to which our results were compared and represents the dose at dmax without the lead shield. To simulate the acutal treatment conditions, a 1/16th inch lead sheet and 1/16th methacrylate were placed upon the film pack as in the first experiment. A second film was placed upon this layer and covered with 0.64 cm methacrylate (Fig. 3). The reason that less material was A
30
Accepted *Lucite.
x
30
x
15
for publication
Fig. I. Schematic
?Kodak KV-2. $Artonix Model 3301.
23 July 1980.
1751
of treatment
set up.
1752
’
Radiation
Oncology 0 Biology 0
Physics
December
I?
1980, Volume 6, Number
cm Methacrylote
0.625 cm Methacrylote /
I
Film_
/ +
I
Film
,d,
I
I5 cm Thick Methacrylate
Methactylate
Fig. 3. Phantom for determining
the effect of the lead shield.
Fig. 2. Phantom for obtaining calibration scan.
The lead shield dramatically reduced the radiation dose to tissue directly below it with approximately 20% of the calibration dose present (Fig. 4~). The area adjacent to the lead shield showed an increased dose of 3%. The radiation dose was increased for a distance of 1.5 cm from the edge of the shield. DISCUSSION It should be noted that the radiation dose to a thin layer of tissue overlying a lead .shield is significantly
I.1
increased during electron therapy. The increase in radiation above the lead shield is probably a result of bremsstruhlung and scattered electrons originating in the lead. The radiation dose to tissues below the lead shield is most probably because of bremsstrahlung and a small percentage of primary electrons that are produced in the lead. The exact cause of the increased radiation above and below the lead shield was not determined. A number of authors’.’ have discussed the effect of external shielding on the dose from electrons at the edge or below the edge of the block. None of these authors to
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DISTANCE
Fig. 4a
(cm 1
-I
I
DISTANCE
Fig.
4b
3 (cm)
Lead shielding cutouts 0 A. E. DOCKTERAND
” IO 0.9
knowledge have discussed the effect of external shields on overlying tissues during electron therapy. The increased radiation should be taken into consideration when calculating dosages to those structures.
1
REFERENCES Choi, M.C., Purdy, J.A., Gerbi, B.. Abrath. F.G.. Glasscow, G.P.: Variation in output factor caused b) secondary blocking for 7-16 MEV electron beams. ,Ued. ,Phys. 6: 137-139, 1979. Mok. E., Cheng-bin, A.,Cheo, J.H.: “vlodification of isodose curves of high energy electrons b) field shaping materials and its clinical applications. ,Med. Phps. 4: 349, 1977.
:
07
I
5
I
is 2 0.5 W L G _I W u
$
1753
our
c
0.6
g
M. W. HERMAN
0.4
0.3
0.6 i 0.2 0. I
-3
-I
0
I
3
DISTANCE (cm) Fig 4. Densitometer scans. a. Calibration scan. b. Scan of film above the lead shield and adjacent area. c. Scan directly below the lead shield and adjacent area.
In!. J. Radiation Oncolo~ Biol. Phyr.. Vol. 6. pp 1751-1753 Printed in the U.S.A. All rights reserved.
0 Technical Innovations and Notes EFFECT OF LEAD SHIELDING CUTOUTS ON DOSE TO OVERLYING TISSUE FOR 7 MV ELECTRONS ARDEN
Department
of Radiology,
MARTIN Department
of Radiological
E. DOCKTER, MS.
South Bay Hospital,
Sciences,
W.
514 North HERMAN,
Prospect
Ave., Redondo
Beach, CA 90277
PH.D.
LAC Harbor-UCLA Medical Center and L’CLA School of Medicine, West Carson St., Torrance, CA 90505
1000
Treatment of the helix of the ear with a lead shield placed behind the ear du:ing electron beam therapy significantly increases the radiation close to the skin above the block. With film densitometry, the increased radiation dose is 10 8 higher than the dose at the dose maximum. It is believed that the increase is a result of bremssfra~lung and scattered electrons originating in the lead. The increased dose should be taken into account when calculating the dose to those structures. Electron beam therapy, Patient shielding, Bremsstrahlung
contamination.
used in the top layer was to simulate the thickness of the ear. This phantom was also exposed to 300 MU of 7 MV electrons. The film densitometer was calibrated to indicate 100% at the maximum density of the calibration film. The traces for the other films were obtained at the same densitometer settings.
INTRODUCTION
patient with a basal cell carcinoma of the helix of the left ear was seen. As the tumor extended through the cartilage, it was decided to treat the lesion with 7 MV electrons. A I/ 16th inch lead shield was fabricated to fit behind and around the ear to prevent exposure of other structures. (Fig. 1). The question arose as to the radiation dose to the ear directly above the lead shield. A series of experiments were performed to determine if there was a significant change in exposure. A
bIETHODS
AND
RESULTS
The calibration scan in Fig. 4a was compared with the tracing from above the lead shield in Fig. 4b and it was seen that there was a 10% increase in the radiation exposure directly above the lead shield. However, the dose above the unshielded portion of the field at the same level was decreased by 2%.
MATERIALS
cm methacryiate* phantom was covered with a film pack? which in turn was covered with a 1 cm block of methacrylate (Fig. 2). The film was exposed to 300 monitor units (MU) of 7 MV electrons. After processing in an automatic processor for 2 minutes, the film was scanned with a film densit0meter.t This was the calibration scan to which our results were compared and represents the dose at dmax without the lead shield. To simulate the acutal treatment conditions, a 1/16th inch lead sheet and 1/16th methacrylate were placed upon the film pack as in the first experiment. A second film was placed upon this layer and covered with 0.64 cm methacrylate (Fig. 3). The reason that less material was A
30
Accepted *Lucite.
x
30
x
15
for publication
Fig. I. Schematic
?Kodak KV-2. $Artonix Model 3301.
23 July 1980.
1751
of treatment
set up.
1752
’
Radiation
Oncology 0 Biology 0
Physics
December
I?
1980, Volume 6, Number
cm Methacrylote
0.625 cm Methacrylote /
I
Film_
/ +
I
Film
,d,
I
I5 cm Thick Methacrylate
Methactylate
Fig. 3. Phantom for determining
the effect of the lead shield.
Fig. 2. Phantom for obtaining calibration scan.
The lead shield dramatically reduced the radiation dose to tissue directly below it with approximately 20% of the calibration dose present (Fig. 4~). The area adjacent to the lead shield showed an increased dose of 3%. The radiation dose was increased for a distance of 1.5 cm from the edge of the shield. DISCUSSION It should be noted that the radiation dose to a thin layer of tissue overlying a lead .shield is significantly
I.1
increased during electron therapy. The increase in radiation above the lead shield is probably a result of bremsstruhlung and scattered electrons originating in the lead. The radiation dose to tissues below the lead shield is most probably because of bremsstrahlung and a small percentage of primary electrons that are produced in the lead. The exact cause of the increased radiation above and below the lead shield was not determined. A number of authors’.’ have discussed the effect of external shielding on the dose from electrons at the edge or below the edge of the block. None of these authors to
-I!
I.1
P
B
1.0
/
1.0 0.9
0.9
0.6
0.8
w
f2 P
0.7
zi 5
I g
0.7
0.6
B 5
0.6
0.5
% a
0.5
B 2 W
9
I. t
0.4
5 i
0.3
0.4 0.3
0.2
0.2
0. I
0. I
4;; -3
DISTANCE
Fig. 4a
(cm 1
-I
I
DISTANCE
Fig.
4b
3 (cm)
Lead shielding cutouts 0 A. E. DOCKTERAND
” IO 0.9
knowledge have discussed the effect of external shields on overlying tissues during electron therapy. The increased radiation should be taken into consideration when calculating dosages to those structures.
1
REFERENCES Choi, M.C., Purdy, J.A., Gerbi, B.. Abrath. F.G.. Glasscow, G.P.: Variation in output factor caused b) secondary blocking for 7-16 MEV electron beams. ,Ued. ,Phys. 6: 137-139, 1979. Mok. E., Cheng-bin, A.,Cheo, J.H.: “vlodification of isodose curves of high energy electrons b) field shaping materials and its clinical applications. ,Med. Phps. 4: 349, 1977.
:
07
I
5
I
is 2 0.5 W L G _I W u
$
1753
our
c
0.6
g
M. W. HERMAN
0.4
0.3
0.6 i 0.2 0. I
-3
-I
0
I
3
DISTANCE (cm) Fig 4. Densitometer scans. a. Calibration scan. b. Scan of film above the lead shield and adjacent area. c. Scan directly below the lead shield and adjacent area.