Inl. 1. Rodinlion
Oncology Hol.
Phys.,
1977, Vol. 2, pp. 571-577.
Pergamon
Pres.
Printed
in the U.S.A.
??Technical Innovations and Notes SUPERVOLTAGE
BEAM FILMW
MAHMOUD M. HAMMOUDAH, D.R.P., M.Sc. Department of Radiology, Henrick Memorial Hospital, Abilene, TX 79601, U.S.A.
and ULRICH K. HENSCHKE, M.D., Ph.D. Department of Radiotherapy, Howard University Hospital, College of Medicine, Washington, DC 20060, U.S.A. In order to determine the optimal technic for obtahring supervoltage beams films, electron emission measurements, phantom films and gradation studies were carried out with various films, metal foils and fluorescent screens. The best beam iihus were obtained with a combination of a regular diagnostic X-ray film sandwiched between two lead foils in a cassette, which provides good contact between film and lead foils. The back lead foil huproves the contrast, while the front lead foil improves the sharpness of the beam film. The optimal foil thickness for the back lead foil is 0.1 mm for uCo, 0.2 mm for 4 MV and 0.5 mm for LOMV X-rays. The front lead foil should be twice as thick. The best films are obtahted if the film is in contact with the body. Beam films taken with our combination required only about l/lO of the radiation exposure of the commercially available “Localizatiou film” and were far superior in contrast and sharpness. Beam films, Localization
films, Supervoltage
INTRODUCTION
Beam films, also called port films or localization film, are important for precise radiotherapy and documentation. Even if a good simulator is available, a final film check of the set-up should be made with the beam of the therapy machine, since the simulator and the therapy machine may differ either because of technical problems or human error. It is desirable to document al1 treatment portals with beam films and save them for future reference. Only a few studies have been published on the choice of films and foils for supervoltage radiography: Tuddenham, Hale and Pendergrass” and Tuddenham et aL9 used Kodak industrial X-ray film between lead foils 0.005 in. thick for films taken with a Genera1 Electric 1 MV machine. McDonnell, Berman and LodmelY used Kodak No-Screen film encased between lead screens 0.005-0.02 in. in place of conventional intensifying screens. They also mentioned that “the backscatter tStudy supported in part by the Howard University Cancer Core Grant No. 1933of the National
films, Lead foil cassettes.
from the back screen plays a more important role in image definition than that from the front foil” and that “nothing is gained by increasing the thickness of the back 0.005 in.” For @‘CoPerryman,
screen
beyond
McAllister and Burwel16 used radiation cassettes from which the usual intensifying screens had been removed and replaced by lead foils 0.01 in. in thickness. Three types of industrial films were tried, namely, Kodak type AA, Ansco type Superay B and Dupont type 506. Springer et al.’ reported that the use of high speed intensifying light emitting fluorescent screens in conjunction with lead screens served to increase the overall contrast ás wel1 as to reduce exposure time with 13’Cs radiographs. Swain and Stecke18 recommended the insensitive Dupont Adlux film without lead foils and’ without intensifying screens for exposure during the whole treatment. A similar film now is distributed under the name of “Verification film” RP/V2 by Kodak. Faerber’ used standard X-ray films between 1 Cancer Institute. 571
572
Radiation Oncology 0 Biology 0 Physics
or 2 mm lead foils in a cardboard holder for cobalt X-rays. Galkin, Wu and Suntharaligam2 recommended the use of light emitting ultradetail intensifying screens for use with 6oCo teletherapy machines, 4MV linear accelerators and 45 MeV Betatrons. They found the highest contrast with front and back light emitting ultradetail screens, and recommended these for beam films. As this short survey of the literature shows, lead foils have been used by many authors to improve the quality of supervoltage films. However, little is known about how they work and what their optimal thickness is. It also is uncertain, whether light emitting fluorescent screens improve the quality of beam films more than lead foils. There is much confusion about which film is best for beam films: Some departments stil1 prefer industrial films such as Kodak type M or AA, although industrial film cannot be developed in the automatie developers commonly used in hospitals. Most departments appear now to use the special Kodak “Localization film” type RPITL2. This is the same film as marketed in smaller sizes for mammography (Kodak type RP/M2). Its main advantage is that it can be developed in automatic developers and that it can be obtained in a “ready pack” light tight envelope, which eleminates the chore of loading the film into a cassette. In this study, however, we determined that this film produces much poorer radiographs than the optimal film-foil cassette combination.3.4 x -RAYS -
BACK FOIL MEASUREMENT
May-June
1977, Volume 2, No. 5 and No. 6
METHODS
AND MATERIALS
Al1 metal foils readily available to US were tested for their value in improving the quality of beam films. The effects of the front foil and of the back foil were investigated separately. The experimental arrangement is pictured in Fig. 1. On the left is shown the set-up for the measurement of the electron emission of the front foil with a thin walled for chamber. The set-up ionization measurement of the electron emission from the back foil is shown on the right. The electron emission also was measured with X-ray films, which were read on a densitometer. The differente between these two measurement technics was within the error of the photometric measurements. Therefore, we have shown only the more reliable values measured with our ionization chamber in the curves of Figs. 2-4. For the determination of the electron emission of different metal foils, each foil was fastened to a lucite sheet. One measurement was made with the lucite toward the chamber and another with the metal foil toward the chamber. Fluorescent screens were tested in the same way as metal foils, but films had to be used for these measurements, since most of the blackening of the film in this case results from the light emitted from the screens. The measurements shown schematically in Fig. 1 determine the intensifying effect of the various foils and screens. This can be taken as an indicator of the improvement of the X-RAYS
FRONT FOIL MEASUREMENT
CHAMBER
Fig. 1. Experimental arrangement for the determination of the electron emission from the front foil and from the back foil. The measurements were done with thin-walled chambers and with films.
Supervoltage beam films 0 M. M.
contrast. High contrast is essential for good supervoltage films, which generally lack contrast. Contrast, however, is only one factor for the quality of radiographs. The other important factor is sharpness. Unfortunately, this is dificult to measure objectively in a way which is meaningful for the practica1 quality of supervoltage films. After several attempts with “edge” photometry and other tests suggested for objective measurements of the “sharpness,” we resorted to judging the sharpness subjectively with films of the thorax and of the head of our AldersonRando phantom. Even minor differences in sharpness can be detected clearly in this way. The distinction of contrast and sharpness is important, and both must be considered for obtaining optimal beam films. For instance, light emitting intensifying screens improve the contrast, but decrease the sharpness so much, that they gave poorer films than lead foils in al1 of our studies. Beside the measurements of the intensifying effect of foils and screens and the sharpness assessment with a phantom, we also determined gradation curves. As usual, these gradation curves were plotted in density versus dose after deduction for the basefog. As radiation source for the supervoltage radiation, we used principally the 4 MV photon beam of our Varian Clinac 4 linear accelerator unless otherwise indicated. For the measurements of the optimal thickness of the foils, we used in addition the 10 MV photon beam of our Varian Clinac 18 MeV linear accelerator and the beam of our cobalt machine. RESULTS Figure 2 shows the results of our measurement of foils of different elements. The X axis shows the atomic number of the foil. The Y axis gives the electron emission from the foil relative to the electron emission from lucite. Al1 foils were at least 1.0 mm thick. As the curves show, the electron emission from the back foil increases steadily with the atomic number. In contrast, the electron emission from al1 front foils is lower than from lucite. It has been wel1 known for more than 40 years that a minimum exists for
573
HAMMOUDAH and U. K. HENSCHKE
1.6.
FROM BACKFOIL
1.4. 1.2
v: 1.0.
Ta
Pb
U
: WO.6. ? 2 10.6-
FROM
FRONT
FOIL
: 0.4-
0.2. 10
20
30
40
50
ATOMIC
60
70
60
90
100
NUMBER
Fig. 2. Electron emission from the front and from the back of foils from different metals with 4 MV
X-rays. Essentially the same results were obtained with film measurements. metals of medium atomic number. Of the metals studied by US, silver has the lowest electron emission. The electron emission increases again for higher atomic number. However, even for lead foil, it is stil1 about
18% lower than for lucite. Thus, these curves show, that compared with lucite only the back foil acts like an intensifyer. Figure 3 gives our measurements with various thicknesses of the back lead foil. As the curves for the three machines in our department show, a greater thickness is necessary for higher energies to obtain maximum electron emission. We need a back foil of about 0.1 mm for the cobalt machine, of 0.2 mm for the 4 MV Varian accelerator and of 0.5 mm for the 10 MV photon beam of the 18 MeV Varian accelerator.
60c0 4 MV
THICKNESS
OFBACK
LEAD
FOIL
IN MILLIMETER
Fig. 3. Electron emission from back lead foils of different thickness for X-rays from 6oCo, from 4 MV and 10 MV accelerators.
Radiation Oncology 0 Biology 0 Physics
574
Figure 4 shows the corresponding measurements for the front lead foil. These curves represent build-up curves and are similar in shape to the familiar depth dose curves. The thickness of the lead foil to achieve complete build-up is about 0.2 mm lead for the cobalt radiation, 0.4 mm lead for the 4 MV photons and 1.0 mm lead for the 10 MV photons of our 18 MeV accelerator.
May-June 1977, Volume 2, No. 5 and No. 6
RP/TL2 and the right curve for the Kodak “ready pack ” “Verification film” type RP/V2. It is apparent, that our combination of regular diagnostic film between lead foils is at least as good in contrast (indicated by the steepness of the curve) and that it is about 10 times more sensitive than the Kodak Localization film type RE/TL2. We tested many films, foils and intensifying screens with
xIQ+L t
60~0
I
I
1
05
10
THICKNESS
OF
FRONT
x 1
e
l
F
4 MV
I
1.5
LEAD
FOIL
2.0
IN MILLIMETER
Fig. 4. Build-up curves for front lead foils of different thickness 4 MV and 10 MV photons. Figure 5 shows gradation curves in which, as usual, the dose is plotted on the X axis in logarithmic scale and the density on the Y axis in linear scale. The curve on the left is for our combination of a regular diagnostic film with lead foils in front and in back of the film. The middle curve is for the “ready pack” “Localization film” type Kodak 30
0
0
DOSE
IN RAD
Fig. 5. Gradation curves with 4MV X-rays for diagnostic film-lead foil combination, for Kodak Localization film type RP/TL2 and for Kodak Verification film RP/V2.
for X-rays from @Co,
gradation studies as wel1 as with films of the Rando phantom but found that the combination of regular diagnostic films with lead foils in back and front of the specified thickness is preferable for supervoltage beam films. Figure 6 shows a chest phantom film obtained by using our film-foil combination in a cassette in comparison with a “ready pack” Kodak Localization film type RP/TL2. The Kodak Localization film RP/TL2 on the left was exposed with a monitor setting of 20 on the control panel of the accelerator, while our combination on the right required only a monitor setting of 2 for optimal exposure. This monitor setting corresponds in our case to a dose of about 0.2 rad at the film. On our film on the right the bronchial tree in the mediastinum is shown clearly while it can hardly be recognized on the Kodak Localization film RPITL2 on the left. For chest localization film our combination is obviously superior.
Supervoltage
Fig. 6. Supervoltage
Localization
beam films 0
chest
film (RP/TL2).
M. M. HAMMOUDAH and U. K. HENSCHKE
X-ray of phantom taken Right. Own combination
515
with 4 MV X-rays. Left. Kodak (diagnostic X-ray film between 2
lead foils of optimal thickness). Figure 7 shows that our combination on the right also is much better for head and neck films than the Kodak “ready pack” “Localization film” RPITL2. Again the monitor setting for the Kodak Localization film RP/TL2 on the left was 20, while it was only 2 for our combination on the right. Again the superiority of our film-foil combination is evident.
Fig. 7. Supervoltage head Localization film (RPITL2).
Our phantom studies with the 10 MV X-ray beam of our 18 MeV Varian linear accelerator showed, that contrary to a common misconception the beam films were as good as with our 4 MV accelerator. The lead foils must, however, be 2.5 times as thick for the 10 MV photon beam. We also carried out phantom studies with different distances between film and phan-
X-ray of phantom taken with 4 MV X-rays. Left. Kodak Right. Own combination (diagnostic X-ray film between 2 lead foils of optimal thickness).
576
Radiation Oncology 0 Biology 0 Physics
tom. The sharpest films were obtained if the film or the cassette respectively was in contact with the phantom. This was more pronounced for chest films than for head films. It was present for al1 films and al1 film foil combinations which we studied. Therefore, whenever possible, the cassette should be placed directly behind the part from which a beam film is taken. An airgap is in the supervoltage range, detrimental to the quality of the beam films.
DISCUSSION According to our data, only the back foil acts as intensifyer compared with lucite and other tissue equivalent materials. This intensifying effect is important, because it increases the steepness of the gradation curve and therefore the contrast. Al1 supervoltage films look “flat” or “gray” to use the terms of the diagnostic radiologist, because the absorption differences for the supervoltage radiation are so small. The contrast simply cannot be high enough for supervoltage. The effect of the front foil is different. Without a front foil, as it is the case in “ready pack films,” scattered radiation from the patient reaches the film and produces a blurred image. One might think of using a plastic plate in front of the film which would give 18% more electron emission than lead foil as shown in Fig. 2. A plastic plate in front of the Kodak Localization film RPITL2 indeed improves the image, but it is not as good as the image with our diagnostic film-lead foil combination. The reason may be that part of the radiation which blackens the film comes from a distance which is about 10 times greater than if a lead foil is used. This would produce a similar unsharpness as a screen with poor contact in diagnostic radiography. The front lead foil thus brings the build-up material closer to the film and in this way improves the sharpness. The higher the voltage, the greater is the importante of this effect. Without a thick front lead foil, 1OMV beam films look blurred, while with the proper thickness of the lead foils, they are as good as @‘Coor 4 MV beam films. From the gradation curves, it is apparent
May-June 1977, Volume 2, No. 5 and No. 6
that our combination of regular diagnostic film and lead foils is at least as good in contrast (indicated by the steepness of the curve) and that it is about 10 times more sensitive than the Kodak Localization film RP/TL2. We tested many other films, foils and intensifying screens with gradation studies as wel1 as with films of our phantoms. The quality of the Kodak Localization film RP/TL2 and Kodak Verification film RP/V2 were improved by putting them into our lead foil cassette. This increased the sensitivity as wel1 as the contrast of both films, but they were neither as sensitive nor quite as conFlourescent trasty as our combination. screens gave a higher sensitivity than our combination, especially if used with additional lead foils, but the loss in sharpness was so great, that they clearly were inferior to our combination. Their sensitivity also was so high, that they were overexposed with the lowest monitor setting. Thus, under al1 circumstances, our combination of regular diagnostic films with lead foils in back and front of the specified thickness gave optimal beam films. Diagnostic film also has the advantage that it is less expensive than special films and always is available in hospitals. If several diagnostic films are available in one hospital, the one with the highest contrast should be used. In clinical practice, a useful criteria for satisfactory beam film quality is a beam film of the pelvis. In a good beam film such as possible with our film-foil-cassette combination, the pelvic bones are outlined clearly in spite of the smal1 absorption differences of bone and soft tissue. In a poor beam film, the bones are not visible in the pelvis. The better supervoltage beam films achieved with our lead-foil diagnostic filmcassette combination have improved greatly the accuracy of localization in our practice of radiotherapy. They even have been of diagnostic value in many instances. In particular, for the mediastinum they often have been as useful as tomograms. The excellent quality of our beam films also has made it possible to use our accelerator as a simulator, as wil1 be reported in the following paper.
Supervoltage beam filmsOM. M.
HAMMOUDAH
and
U. K. HENSCHKE
577
REFERENCES 1. Faerber, G.O.: Portal radiography in cobalt 6. Perryman, C.R., McAllister, J.D., Burwell, teletherapy, J. Am. Med. Assoc. 63: 239-246, J.A.: “Co radiography. Am. J. Roentgenol. 83: 1969. 525-532, 1960. 2. Galkin, B.M., Wu, R.K., Suntharalingram, N.: 7. Springer, E.B., Paper, L., Elsner, F., Jacobs, Methods for improving contrast in therapy M.L.: High energy radiography (“Co and 13’Cs) for tumor localization and treatment localization radiographs obtained with 6oCo, 4 MV and 45 MV photons. Scientific Exhibit planning. Radiology 78: 260-262, 1962. RSNA, 61 Scientific Assembly. Chicago, Il8. Swain, R.W., Steckel, R.J.: Beam localization linois, 30 Nov.-4 Dec. 1975. in cobalt and megavoltage therapy during 3. Hammoudah, M., Henschke, U.: Film-foil treatment. Radiology 87: 529-535, 1966. 9. Tuddenham, W.J., Gibbons, J.F., Hale, J., combinations for therapy localization films. Pendergrass, E.P.: Supervoltage and multiple 4th Inter. Conf. Med. Phys. Ottawa, Canada, simultaneous roentgenography-new tech1976, p. 4.3. niques for roentgen examination of chest. 4. Henschke, U., Kumar, P., Goldson, A., Radiology 63: 184-190, 1954. Hammoudah, M., Nibhanupudy, J.: Ac10. Tuddenham, W.J., Hale, J., Pendergrass, E.P.: cessories for linear accelerator. Scientific Exhibit RSNA, 61 Scientific Assembly. Chicago, Supervoltage diagnostic roentgenography; preliminary report. Am. J. Roentgenol. Radium Illinois, 30 Nov.-4 Dec. 1975. Ther. Nucl. Med. 70: 759-763, 19.53. 5. McDonnell, G.M., Berman, H.L., Lodmell, E.A.: Supervoltage roentgenography. Am. J. Roentgenol. Radium Ther. Nucl. Med. 79: 306320, 1958.
Oncology Hol.
Phys.,
1977, Vol. 2, pp. 571-577.
Pergamon
Pres.
Printed
in the U.S.A.
??Technical Innovations and Notes SUPERVOLTAGE
BEAM FILMW
MAHMOUD M. HAMMOUDAH, D.R.P., M.Sc. Department of Radiology, Henrick Memorial Hospital, Abilene, TX 79601, U.S.A.
and ULRICH K. HENSCHKE, M.D., Ph.D. Department of Radiotherapy, Howard University Hospital, College of Medicine, Washington, DC 20060, U.S.A. In order to determine the optimal technic for obtahring supervoltage beams films, electron emission measurements, phantom films and gradation studies were carried out with various films, metal foils and fluorescent screens. The best beam iihus were obtained with a combination of a regular diagnostic X-ray film sandwiched between two lead foils in a cassette, which provides good contact between film and lead foils. The back lead foil huproves the contrast, while the front lead foil improves the sharpness of the beam film. The optimal foil thickness for the back lead foil is 0.1 mm for uCo, 0.2 mm for 4 MV and 0.5 mm for LOMV X-rays. The front lead foil should be twice as thick. The best films are obtahted if the film is in contact with the body. Beam films taken with our combination required only about l/lO of the radiation exposure of the commercially available “Localizatiou film” and were far superior in contrast and sharpness. Beam films, Localization
films, Supervoltage
INTRODUCTION
Beam films, also called port films or localization film, are important for precise radiotherapy and documentation. Even if a good simulator is available, a final film check of the set-up should be made with the beam of the therapy machine, since the simulator and the therapy machine may differ either because of technical problems or human error. It is desirable to document al1 treatment portals with beam films and save them for future reference. Only a few studies have been published on the choice of films and foils for supervoltage radiography: Tuddenham, Hale and Pendergrass” and Tuddenham et aL9 used Kodak industrial X-ray film between lead foils 0.005 in. thick for films taken with a Genera1 Electric 1 MV machine. McDonnell, Berman and LodmelY used Kodak No-Screen film encased between lead screens 0.005-0.02 in. in place of conventional intensifying screens. They also mentioned that “the backscatter tStudy supported in part by the Howard University Cancer Core Grant No. 1933of the National
films, Lead foil cassettes.
from the back screen plays a more important role in image definition than that from the front foil” and that “nothing is gained by increasing the thickness of the back 0.005 in.” For @‘CoPerryman,
screen
beyond
McAllister and Burwel16 used radiation cassettes from which the usual intensifying screens had been removed and replaced by lead foils 0.01 in. in thickness. Three types of industrial films were tried, namely, Kodak type AA, Ansco type Superay B and Dupont type 506. Springer et al.’ reported that the use of high speed intensifying light emitting fluorescent screens in conjunction with lead screens served to increase the overall contrast ás wel1 as to reduce exposure time with 13’Cs radiographs. Swain and Stecke18 recommended the insensitive Dupont Adlux film without lead foils and’ without intensifying screens for exposure during the whole treatment. A similar film now is distributed under the name of “Verification film” RP/V2 by Kodak. Faerber’ used standard X-ray films between 1 Cancer Institute. 571
572
Radiation Oncology 0 Biology 0 Physics
or 2 mm lead foils in a cardboard holder for cobalt X-rays. Galkin, Wu and Suntharaligam2 recommended the use of light emitting ultradetail intensifying screens for use with 6oCo teletherapy machines, 4MV linear accelerators and 45 MeV Betatrons. They found the highest contrast with front and back light emitting ultradetail screens, and recommended these for beam films. As this short survey of the literature shows, lead foils have been used by many authors to improve the quality of supervoltage films. However, little is known about how they work and what their optimal thickness is. It also is uncertain, whether light emitting fluorescent screens improve the quality of beam films more than lead foils. There is much confusion about which film is best for beam films: Some departments stil1 prefer industrial films such as Kodak type M or AA, although industrial film cannot be developed in the automatie developers commonly used in hospitals. Most departments appear now to use the special Kodak “Localization film” type RPITL2. This is the same film as marketed in smaller sizes for mammography (Kodak type RP/M2). Its main advantage is that it can be developed in automatic developers and that it can be obtained in a “ready pack” light tight envelope, which eleminates the chore of loading the film into a cassette. In this study, however, we determined that this film produces much poorer radiographs than the optimal film-foil cassette combination.3.4 x -RAYS -
BACK FOIL MEASUREMENT
May-June
1977, Volume 2, No. 5 and No. 6
METHODS
AND MATERIALS
Al1 metal foils readily available to US were tested for their value in improving the quality of beam films. The effects of the front foil and of the back foil were investigated separately. The experimental arrangement is pictured in Fig. 1. On the left is shown the set-up for the measurement of the electron emission of the front foil with a thin walled for chamber. The set-up ionization measurement of the electron emission from the back foil is shown on the right. The electron emission also was measured with X-ray films, which were read on a densitometer. The differente between these two measurement technics was within the error of the photometric measurements. Therefore, we have shown only the more reliable values measured with our ionization chamber in the curves of Figs. 2-4. For the determination of the electron emission of different metal foils, each foil was fastened to a lucite sheet. One measurement was made with the lucite toward the chamber and another with the metal foil toward the chamber. Fluorescent screens were tested in the same way as metal foils, but films had to be used for these measurements, since most of the blackening of the film in this case results from the light emitted from the screens. The measurements shown schematically in Fig. 1 determine the intensifying effect of the various foils and screens. This can be taken as an indicator of the improvement of the X-RAYS
FRONT FOIL MEASUREMENT
CHAMBER
Fig. 1. Experimental arrangement for the determination of the electron emission from the front foil and from the back foil. The measurements were done with thin-walled chambers and with films.
Supervoltage beam films 0 M. M.
contrast. High contrast is essential for good supervoltage films, which generally lack contrast. Contrast, however, is only one factor for the quality of radiographs. The other important factor is sharpness. Unfortunately, this is dificult to measure objectively in a way which is meaningful for the practica1 quality of supervoltage films. After several attempts with “edge” photometry and other tests suggested for objective measurements of the “sharpness,” we resorted to judging the sharpness subjectively with films of the thorax and of the head of our AldersonRando phantom. Even minor differences in sharpness can be detected clearly in this way. The distinction of contrast and sharpness is important, and both must be considered for obtaining optimal beam films. For instance, light emitting intensifying screens improve the contrast, but decrease the sharpness so much, that they gave poorer films than lead foils in al1 of our studies. Beside the measurements of the intensifying effect of foils and screens and the sharpness assessment with a phantom, we also determined gradation curves. As usual, these gradation curves were plotted in density versus dose after deduction for the basefog. As radiation source for the supervoltage radiation, we used principally the 4 MV photon beam of our Varian Clinac 4 linear accelerator unless otherwise indicated. For the measurements of the optimal thickness of the foils, we used in addition the 10 MV photon beam of our Varian Clinac 18 MeV linear accelerator and the beam of our cobalt machine. RESULTS Figure 2 shows the results of our measurement of foils of different elements. The X axis shows the atomic number of the foil. The Y axis gives the electron emission from the foil relative to the electron emission from lucite. Al1 foils were at least 1.0 mm thick. As the curves show, the electron emission from the back foil increases steadily with the atomic number. In contrast, the electron emission from al1 front foils is lower than from lucite. It has been wel1 known for more than 40 years that a minimum exists for
573
HAMMOUDAH and U. K. HENSCHKE
1.6.
FROM BACKFOIL
1.4. 1.2
v: 1.0.
Ta
Pb
U
: WO.6. ? 2 10.6-
FROM
FRONT
FOIL
: 0.4-
0.2. 10
20
30
40
50
ATOMIC
60
70
60
90
100
NUMBER
Fig. 2. Electron emission from the front and from the back of foils from different metals with 4 MV
X-rays. Essentially the same results were obtained with film measurements. metals of medium atomic number. Of the metals studied by US, silver has the lowest electron emission. The electron emission increases again for higher atomic number. However, even for lead foil, it is stil1 about
18% lower than for lucite. Thus, these curves show, that compared with lucite only the back foil acts like an intensifyer. Figure 3 gives our measurements with various thicknesses of the back lead foil. As the curves for the three machines in our department show, a greater thickness is necessary for higher energies to obtain maximum electron emission. We need a back foil of about 0.1 mm for the cobalt machine, of 0.2 mm for the 4 MV Varian accelerator and of 0.5 mm for the 10 MV photon beam of the 18 MeV Varian accelerator.
60c0 4 MV
THICKNESS
OFBACK
LEAD
FOIL
IN MILLIMETER
Fig. 3. Electron emission from back lead foils of different thickness for X-rays from 6oCo, from 4 MV and 10 MV accelerators.
Radiation Oncology 0 Biology 0 Physics
574
Figure 4 shows the corresponding measurements for the front lead foil. These curves represent build-up curves and are similar in shape to the familiar depth dose curves. The thickness of the lead foil to achieve complete build-up is about 0.2 mm lead for the cobalt radiation, 0.4 mm lead for the 4 MV photons and 1.0 mm lead for the 10 MV photons of our 18 MeV accelerator.
May-June 1977, Volume 2, No. 5 and No. 6
RP/TL2 and the right curve for the Kodak “ready pack ” “Verification film” type RP/V2. It is apparent, that our combination of regular diagnostic film between lead foils is at least as good in contrast (indicated by the steepness of the curve) and that it is about 10 times more sensitive than the Kodak Localization film type RE/TL2. We tested many films, foils and intensifying screens with
xIQ+L t
60~0
I
I
1
05
10
THICKNESS
OF
FRONT
x 1
e
l
F
4 MV
I
1.5
LEAD
FOIL
2.0
IN MILLIMETER
Fig. 4. Build-up curves for front lead foils of different thickness 4 MV and 10 MV photons. Figure 5 shows gradation curves in which, as usual, the dose is plotted on the X axis in logarithmic scale and the density on the Y axis in linear scale. The curve on the left is for our combination of a regular diagnostic film with lead foils in front and in back of the film. The middle curve is for the “ready pack” “Localization film” type Kodak 30
0
0
DOSE
IN RAD
Fig. 5. Gradation curves with 4MV X-rays for diagnostic film-lead foil combination, for Kodak Localization film type RP/TL2 and for Kodak Verification film RP/V2.
for X-rays from @Co,
gradation studies as wel1 as with films of the Rando phantom but found that the combination of regular diagnostic films with lead foils in back and front of the specified thickness is preferable for supervoltage beam films. Figure 6 shows a chest phantom film obtained by using our film-foil combination in a cassette in comparison with a “ready pack” Kodak Localization film type RP/TL2. The Kodak Localization film RP/TL2 on the left was exposed with a monitor setting of 20 on the control panel of the accelerator, while our combination on the right required only a monitor setting of 2 for optimal exposure. This monitor setting corresponds in our case to a dose of about 0.2 rad at the film. On our film on the right the bronchial tree in the mediastinum is shown clearly while it can hardly be recognized on the Kodak Localization film RPITL2 on the left. For chest localization film our combination is obviously superior.
Supervoltage
Fig. 6. Supervoltage
Localization
beam films 0
chest
film (RP/TL2).
M. M. HAMMOUDAH and U. K. HENSCHKE
X-ray of phantom taken Right. Own combination
515
with 4 MV X-rays. Left. Kodak (diagnostic X-ray film between 2
lead foils of optimal thickness). Figure 7 shows that our combination on the right also is much better for head and neck films than the Kodak “ready pack” “Localization film” RPITL2. Again the monitor setting for the Kodak Localization film RP/TL2 on the left was 20, while it was only 2 for our combination on the right. Again the superiority of our film-foil combination is evident.
Fig. 7. Supervoltage head Localization film (RPITL2).
Our phantom studies with the 10 MV X-ray beam of our 18 MeV Varian linear accelerator showed, that contrary to a common misconception the beam films were as good as with our 4 MV accelerator. The lead foils must, however, be 2.5 times as thick for the 10 MV photon beam. We also carried out phantom studies with different distances between film and phan-
X-ray of phantom taken with 4 MV X-rays. Left. Kodak Right. Own combination (diagnostic X-ray film between 2 lead foils of optimal thickness).
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tom. The sharpest films were obtained if the film or the cassette respectively was in contact with the phantom. This was more pronounced for chest films than for head films. It was present for al1 films and al1 film foil combinations which we studied. Therefore, whenever possible, the cassette should be placed directly behind the part from which a beam film is taken. An airgap is in the supervoltage range, detrimental to the quality of the beam films.
DISCUSSION According to our data, only the back foil acts as intensifyer compared with lucite and other tissue equivalent materials. This intensifying effect is important, because it increases the steepness of the gradation curve and therefore the contrast. Al1 supervoltage films look “flat” or “gray” to use the terms of the diagnostic radiologist, because the absorption differences for the supervoltage radiation are so small. The contrast simply cannot be high enough for supervoltage. The effect of the front foil is different. Without a front foil, as it is the case in “ready pack films,” scattered radiation from the patient reaches the film and produces a blurred image. One might think of using a plastic plate in front of the film which would give 18% more electron emission than lead foil as shown in Fig. 2. A plastic plate in front of the Kodak Localization film RPITL2 indeed improves the image, but it is not as good as the image with our diagnostic film-lead foil combination. The reason may be that part of the radiation which blackens the film comes from a distance which is about 10 times greater than if a lead foil is used. This would produce a similar unsharpness as a screen with poor contact in diagnostic radiography. The front lead foil thus brings the build-up material closer to the film and in this way improves the sharpness. The higher the voltage, the greater is the importante of this effect. Without a thick front lead foil, 1OMV beam films look blurred, while with the proper thickness of the lead foils, they are as good as @‘Coor 4 MV beam films. From the gradation curves, it is apparent
May-June 1977, Volume 2, No. 5 and No. 6
that our combination of regular diagnostic film and lead foils is at least as good in contrast (indicated by the steepness of the curve) and that it is about 10 times more sensitive than the Kodak Localization film RP/TL2. We tested many other films, foils and intensifying screens with gradation studies as wel1 as with films of our phantoms. The quality of the Kodak Localization film RP/TL2 and Kodak Verification film RP/V2 were improved by putting them into our lead foil cassette. This increased the sensitivity as wel1 as the contrast of both films, but they were neither as sensitive nor quite as conFlourescent trasty as our combination. screens gave a higher sensitivity than our combination, especially if used with additional lead foils, but the loss in sharpness was so great, that they clearly were inferior to our combination. Their sensitivity also was so high, that they were overexposed with the lowest monitor setting. Thus, under al1 circumstances, our combination of regular diagnostic films with lead foils in back and front of the specified thickness gave optimal beam films. Diagnostic film also has the advantage that it is less expensive than special films and always is available in hospitals. If several diagnostic films are available in one hospital, the one with the highest contrast should be used. In clinical practice, a useful criteria for satisfactory beam film quality is a beam film of the pelvis. In a good beam film such as possible with our film-foil-cassette combination, the pelvic bones are outlined clearly in spite of the smal1 absorption differences of bone and soft tissue. In a poor beam film, the bones are not visible in the pelvis. The better supervoltage beam films achieved with our lead-foil diagnostic filmcassette combination have improved greatly the accuracy of localization in our practice of radiotherapy. They even have been of diagnostic value in many instances. In particular, for the mediastinum they often have been as useful as tomograms. The excellent quality of our beam films also has made it possible to use our accelerator as a simulator, as wil1 be reported in the following paper.
Supervoltage beam filmsOM. M.
HAMMOUDAH
and
U. K. HENSCHKE
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