- Email: [email protected]
Variation of electron beam uniformity with beam angulation and scatterer position for total skin irradiation with the stanford technique
Int. J. Radiation
Oncology
Biol.
Phys., Vol. 33, No. 2, pp. 469-474, 1995 Copyright 8 1995 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/95 $9.50 + .lw
0360-3016(95)00112-3
l
Technical Innovations and Notes VARIATION OF ELECTRON BEAM UNIFORMITY WITH BEAM ANGULATION AND SCATTERER POSITION FOR TOTAL SKIN IRRADIATION WITH THE STANFORD TECHNIQUE ELLEN EL-KHATIB, MIF~OSLAV NIKOLIC,
PH.D., F.C.C.P.M.,” SHERALI HUSSEIN, PH.D., F.C.C.P.M.,* PH.D., M.C.C.P.M.,* NICHOLAS J. S. Voss, M.D., F.R.C.P.C.’ AND CHRISTINA PARSONS, M.D., F.R.C.P.C.+
*Division of Medical Physics and +Division of Radiation Oncology, British Columbia Cancer Agency, Vancouver, B.C. Canada VSZ 4E6 Ptupo~: The intluence of different scatterer-degraders and beam angulations on beam uniformity for total skm electron irradiation using the six dual beam Stanford technique is investigated. Methods and Materials: The 6 MeV high dose rate total skin electron irradiitlon mode on a linear accelerator was used. Beam pro&s and percentage depth doses in the patient plane for single, dual, and six dual beams were measured for different dual beam angulations and acrylic scatterer-degraders of diierent thicknesses mounted on the treatment head or in front of the patient in the treatment plane. Results: It ls demonstrated that, with the same electron nominal energy, total skin irradiation techniques witnerent beam penetrations can be obtained by inserting various beam scatterer-degraders into the beam, either mounted on the accelerator head or close to the patient. For our patient treatment, a beam penetration was selected so that the 80% dose lay at 8-9 mm and the 50% dose at 15-16 mm depth. This was achieved by mountlng a 0.32-cm thick acrylic beam scatterer-degrader on the accelerator head. A uniform verticai protlle was obtained for gantry angulations of 221”. Conclusions: To implement a total skin electron irradiation technique using the Stanford method, the uired depth of penetration needs to be selected. Based on this, the appropriate combination of scatterer?mders and dual beam angulations to produce a uniform beam in the treatment plane needs to be determined. Different techniques with diierent beam penetrations can be developed using the same high dose rate mode on the linear accelerator by a proper choice of scatterer-degraders and beam anguhttions. Total skin electron irradiition,
Stanford technique, Beam scatterer-degraders.
INTRODUCTION
the central 160 X 60 cm’ area of the treatment plane, an
horizontal uniformity is 24%. However, even with good beam uniformity in the treatment plane, dose uniformity at the patient’s surface cannot be better than 215% because of variable skin distance, self-shielding, and differences in curvature (13). In addition, areas such as the top of the head, shoulders, perineum, and soles of feet will get much lower doses and may need a boost treatment. The skin radiation dose, penetration, and dose distribution for a patient depend primarily upon the electron energy distribution and the angle of incidence of the electrons on the shin of the patient. The cylindrical uniformity around the patient is achieved by using multiple or rotational beams. The dose uniformity is best for rotational
achievable goal for vertical uniformity
techniques,
Total body skin irradiation with low energy electrons (TSEI) has been used for the treatment of mycosis fungoides since the 1950s (14). Several different techniques have been developed at various centers and these are described in a report published by the American Association of Physicists in Medicine (2). The main objective of the treatment is to treat the entire surface of the body uniformly to a limited depth. To achieve this, the field size of the electron beam at the treatment plane should be a minimum of 200 cm in height and 80 cm in width to encompass large patients. Within is 28% and for
Presented at the Fortieth Annual Meeting of the Canadian Organization of Medical Physicists and the Canadian College of Physicists in Medicine, Toronto, Canada, 16- 18 September 1994.
but it is not satisfactory
for a two- or four-
Reprint requests to: Ellen El-Khatib, Ph.D., F.C.C.P.M., Division of Medical Physics, British Columbia Cancer Agency, 600 West 10th Avenue, Vancouver, B.C. Canada VSZ 4E6. Accepted for publication 10 March 1995. 469
ml
I. J. Radiation
Oncology
0 Biology
0 Physics
field technique (16). A widely used technique using six dual fields has been developed at Stanford University (5, 10-12, 15) and many institutions use variations of this technique (7). To achieve the required beam uniformity and dose penetration in the patient, various combinations of angled beams and beam scatterers or degraders are used (2, 5, 6). In particular, the percentage depth dose, surface dose, and bremsstrahlung contamination of the beam will be different for multiple beams compared to single beam irradiation (2, 9). Although the x-ray contamination for electron beams of energies lower than 6 MeV is typically < 1% and of no clinical significance, the x-ray dose can become significant for multiple beams (8), and an x-ray dose of 4% averaged over the whole body is considered unsatisfactory by many physicians. Certain types of accelerators have less x-ray contamination than others. For example, it is lower for scanned electron beams than those having scattering foils, although some electron beams produced by scattering foils achieve low photon contamination that is almost comparable to that of scanned beams (8). A method to reduce the photon contamination in the patient plane was to use angled beams, as in the Stanford technique, where the peak of the bremsstrahlung, which is along the central axis of the beam, is directed above and below the patient. Beam scatterers or beam degraders are often used either to flatten the beam or reduce penetration. Brahme (4) has examined the effect of placing a given scatterer near the exit window or close to the phantom surface. The energy distribution of the two beams was almost identical. However, the angular distribution of the electrons that reaches the phantom surface was totally different. The scatterer placed at the exit window produced a narrower angular spread than those produced by a scatterer placed near the patient surface. The wider angular spread resulted in a higher surface dose and shallower depth dose (2-4). Penetration depths such that the 50% isodose line lies between 5- 15 mm are considered adequate to treat most lesions (2). The Stanford technique was recently modified to allow selection of beams with different penetration (5). Because patients need treatment at extended distances and the set-up is long, a high dose rate that shortens the treatment time is desirable. A high dose rate (888 MU/ min) electron irradiation mode (HDTSEI) is now available on some commercial linear accelerators.’ In the present work, we describe the development of a TSEI technique and the choice of beam angulation and scatterer that produce adequate beam uniformity and pen-
’ Clinac 2100 C, Varian Assoc., Palo Alto, CA. * Victoreen, Cleveland, OH. 3 Nuclear Assoc., Carle Place, NY. 4 Eastman Kodak Co., Rochester, NY.
Volume
33, Number
2. 1995
160 ,- Head
Feet
120 ‘;; L 8 100 : .z m 2
60 60 0 f20° 7 +rs” Pfl6o 0 ,,4O
40
01
’
’
’
’ ’
’
’
’
’
’
’
’
-105 -90 -75 -60 45 -30 -15 0 15 30 45 60 Distance From Horizontal lsocenter
’
’
J
75 90 105
Fig. 1. The vertical beam profiles at a depth of 1 cm in polystyrene 340 cm from the exit window are shown for dual beams with angulations of + 14”, +16”, +18”, +20”, and ~22” from the horizontal isocenter.
etration. More than one technique with different penetration can be used with the same HDTSEI mode on the linear accelerator by using different beam degraders. METHODS
AND MATERIALS
A linear accelerator’ with a 6-MeV high dose rate total skin electron irradiation mode was used. Acrylic scattererdegraders of thicknesses 0.32 cm and 0.56 cm were mounted on the head of the accelerator 49.2 cm from the exit window. A large 0.65-cm acrylic degrader (213 x 122 cm’) to be placed close to the patient was also constructed. The penetration of the beam was measured for a single horizontal beam and for the dual angulated beams with the Holt parallel plate chambe? and Markus chamber’ in a polystyrene phantom, and with film4 in a solid water phantom cassette.5 The Holt chamber forms an integral part of a polystyrene phantom and has a collection volume of 1.0 cm” and a wall thickness of 4 mm. The Markus chamber has a collection volume of 0.055 cm3 and a wall thickness of 2.3 mg/cm”. Both chambers were used with the same electrometer.6 The Holt chamber, because of its large collection volume, is suitable for measuring ionization in large electron beams. However, the Markus chamber was needed to measure ionization within the first 4 mm of the surface. The beam penetration for the six angulated dual beams was measured in a humanoid phantom7 with film sandwiched between several slabs of the phantom. The film
5 Radiation
Measurements
Inc., Middleton,
WI.
6 Keithley Radiation Measurements Division, Cleveland, OH. 7 The Rando Phantom, Alderson Stamford, CT.
Research Laboratory,
Inc.,
Electron
Head
0’
beam uniformity
Feet
’ ’ ’ ’ ’ ’ ’ ’ ’ 30’ 45’ 60’ 75’ 90’ 105’
-105 -90 -75 -60 -45 -30 -15 0 15 Distance From Horizontal
lsocenter
Fig. 2. The vertical and horizontal beamprofiles for dual open beamsangledat + 18” from the horizontal arecomparedto dual beamswith a 0.32 cm acrylic scattereron the treatmenthead and angledat t21”, and dual beamswith a 0.65 cm acrylic scatterer40 cm in front of the treatmentplane angledat 220”.
dose was determined from a sensitometric curve measured for the beam at the extended distance. The films placed at several levels in the humanoid phantom also demonstrated the cylindrical beam uniformity. This was also confirmed by LiF TLD8 placed around the humanoid circumference. Determination of the factor relating dose at maximum for the six dual beam combination to one dual beam was done with both the TLDs and film. Beam profiles were measured at the point of maximum dose in polystyrene with both the Holt parallel plate chamber and with film for the various combinations of beam angulation and scatterer-degrader configurations. Beam calibration was done by means of a Holt chamber, which was calibrated against our dosimetry standard in a 6 MeV 10 X 10 cm2 field.
RESULTS AND DISCUSSION
in TSEI
0 E. EL-KHATIB
et
471
al.
choice of angulation is critical. For the open beams, an angulation of + 18” was optimal. With beam scatterers in the beam, the beam, as expected, no longer had optimal uniformity at 2 18”, and the beam angulation needed to be determined. For a 0.32cm acrylic scatterer mounted on the treatment head 49.2 cm from the exit window, the vertical profiles measured for beam angulations of t 15 to 221” indicated that beam flatness was no longer optimal for the + 18” beam angulation, but was good for the 221” gantry angulation. Both the vertical and horizontal beam profiles for the open beams angled at ? 18” are shown in Fig. 2, and are compared to the beams scattered by 0.32 cm acrylic angled at 221”. The lateral profiles were measured at several distances up to 90 cm above the central direction and did not change. For a large 0.65cm scatterer placed 40 cm in front of the treatment plane, good beam uniformity was obtained for gantry angulations of +20” from the horizontal. These profiles are also shown in Fig. 2. Percentage depth dose
The beam penetration with the 0.65-cm large scatterer in the beam and single dual beams was found not to be penetrating enough with an electron range R, < 1.8 cm. Therefore, further dosimetry measurements were not done because this beam was unsuitable for our treatment. The percentage depth dose (PDD) obtained from the depth ionization curves (1) in polystyrene at 340 cm exit window-to-surface distance (SSD), with the open dual beams angled + 18” both on the horizontal center and 90 cm distant from the center, are shown in Fig. 3. The PDD measured on the horizontal center is more penetrating than at 90 cm distant from the center. This is attributed to the different angles of incidence of the electrons at these different positions. Also, because of the
Beam profdes
The beam profiles in the vertical direction were measured for dual angulated beams to determine the angle at which a dose profile is uniform to within ?8% over 160 cm of the treatment plane as recommended by Task Group 23 of the AAPM (2). The profiles were measured both with the Holt chamber at a depth of 1 cm and by film sandwiched between 0.7 cm acrylic and 0.5 cm pressed wood at 340 cm from the exit window for gantry angulations of 2 14”, + 16”, + 18”, +20”, and +22” from the horizontal. For this first set of measurements, there were no beam scatterers and/or degraders in the beam, although it was expected that these may alter the beam flatness. As illustrated in Fig. 1, the beam angulation has a dramatic effect on beam uniformity, and therefore, the ’ HarshawChemicalCo., Cleveland,OH.
no scatterer - degrader, 0 Central Axis 5
$ n
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18”
60 -
$ s 40 5 e a" 20 -
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.6 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Depth in Polystyrene (g/cm2)
Fig. 3. The percentagedepth dosesat horizontal ¢er and 90 cm above areshownfor dual beamsangled218” at 340 cm SSD.
472
I. J. Radiation
l Biology
Oncology
6 Dual -
beams,
No scatterer
- - - 0.32
cm Acrylic
--
cm Acrylic
0.56
0 Physics
Volume
33, Number
2, 1995
f 16” 6 Dual beams,
- degrader
0.32cm Acrylic,
f 21’
n 2 40 5 a 20 -
I 0 0.2 0.4 0.6 0.6 1.0 1.2 1.4 1.6 1.6 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Depth in Humanoid Phantom (cm)
Fig. 4. The percentage depth doses measured in a humanoid phantom for six dual beams are shown for open beams and those scattered by 0.32 cm and 0.56 cm acrylic mounted on the treatment head.
oblique incidence of electrons in the 12-field combination
(2), the PDD is expected to be shallower for six dual beams than for one dual beam. Therefore, the desired beam penetration for the six dual beams needs to be determined, and the appropriate beam scatterer selected, which will produce the required beam penetration. Our radiation oncologists wanted a beam penetration such that Rso - 7-8 mm and RsO - 14-15 mm. The PDDs measured for the open beam and for beams with 0.32 cm and 0.56 cm acrylic mounted on the treatment head are shown in Fig. 4 for beam angulations of + 18”. Figure 4 illustrates that 0.32 cm acrylic would give the required penetration. As discussed previously for this scatterer mounted on the treatment head, the beam flatness was no longer optimal in the vertical direction for +18” beam angulations. However, beam flatness was good for angulations of ?21”, as shown in Fig. 2.
o) 80 :: 0 .K z 60 2
I 0 0.2 0.4 0.6 0.6 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Depth in Humanoid Phantom (cm)
Fig. 6. The percentage depth doses for six dual beamsangled at 221” to the horizontal and with a 0.32 cm acrylic scatterer mountedon the treatment head are shown. Percentagedepth dosesweremeasuredat severalpointsaroundthe circumference of a humanoidphantom.All measuredPDD curveslay between the two shown.
The PDDs were then measuredfor both one dual beam and the six dual beams at gantry angulations of ?21”,
and are shown in Figs. 5 and 6. For one dual beam, the PDD was also measured with film on the horizontal center and was identical to that obtained from the ionization measurements (Fig. 5), except for the slightly higher dose measured for x-ray contamination. The bremsstrahlung dose measured with the ion chamber was 0.2% on the horizontal isocenter and 1% 90 cm above. The film measurement indicated 0.5% bremsstrahlung on the horizontal isocenter. The PDDs shown in Fig. 6 are for the six dual field combinations for various circular or oblong slices of the humanoid phantom. The beam penetration is such that the 80% depth dose lies between 8 and 9 mm and the 50% depth dose lies between 15 and 16 mm. The bremsstrahlung contamination has been measured with both TLD and film, and is about 2% to the total body. The cylindrical beam uniformity was measured with both
Dual beam, 0.32cm Acrylic, + 21’ 0
Central
0
90 cm from Central Central
axis, ion chamber axis, ion chamber
axis, Film
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Depth in Polystyrene (gkm2)
Fig. 5. The percentagedepth dosesat horizontal center and 90 cm above are shownfor dual beamswith a 0.32 cm acrylic scattereron the treatment head and angled +21”. The depth doseswere measuredboth with ion chamberand film. Thesemeasurements were identical.
Electron beam uniformity in TSEI 0
Anterior
Posterior
E. EL-KHATIB
et al.
473
output factor at depth of maximum dose (2-3 mm) for all six dual fields. This was done using both film and TLD and was found to be 0.0036 Gy/MU of each field (i.e., 1 MU to each of the 12 fields). Therefore, to deliver 30 Gy in eight fractions to the patient, a dose of 3.75 Gy is delivered
in one fraction
of six dual fields given on
two consecutive days. This corresponds to: 3.75 = 1042 MU per each of the 12 fields, 0.0036 and takes 1.2 mm per field. In-vivo dosimetry
Three sets of 40 TLD measurements were performed on each patient. A typical dose distribution is shown in Fig. 7. The areas showing low doses because of self-shielding were boosted with small fields. Feet and hands showing large doses were shielded for the final two fractions. Fig. 7. Radiation-absorbed doses measured using TLD at various positions on a patient treated with six dual beams angled 521” and modified by the 0.32 cm acrylic scatterer are shown. All doses are in cGy relative to 3.75 Gy, which represents one fraction of dose given to the patient.
CONCLUSIONS A Stanford six dual beam total skin electron irradiation technique was developed with a 6 MeV high dose rate mode of a linear accelerator. Different combinations of beam scatterer-degraders mounted either on the head of the accelerator or close to the patient produced beams of different penetrations
film and TLD in the humanoid phantom and it was found to be +8%. Beam calibration
The output factor for the dual beams at a depth of d = 1.06 cm, which is the point of maximum dose at extended distance SSD = 338-348 = 340 cm (50-60 cm from wall), has been determined from ion chamber dosimetry to be 0.00135 Gy/MU for each field (i.e., 1 MU each to both fields). This output factor needs to be related to the
and profiles. The thickness
of beam
scatterer-degrader of 0.32 cm acrylic mounted on the accelerator head produced the required penetration for the 6 dual beams. With this degrader in place, the vertical profiles are uniform within +-8” over 160 cm for gantry angulations of 221”. Beams with different penetrations can easily be obtained using the same 6 MeV high dose rate mode by simply changing the thickness of the beam scatterer-degrader. However, in this case, the new beam angulations required to provide the vertical beam uniformity will need to be determined.
REFERENCES 1. American Association of Physicists in Medicine Task Group 21. A protocol for the determination of absorbed dose from high energy photon and electron beams. Med. Phys. 10:741-771; 1983. 2. American Association of Physicists in Medicine Task Group 23. Total skin electron therapy: Technique and dosimetry. New York: American Institute of Physics; 1988. 3. B&me, A.; Svensson, H. Specification of electron beam quality from the central-axis depth absorbed-dose distribution. Med. Phys. 3:95-102; 1976. 4. Brahme, A. Physics of electron beam penetration: Fluence and absorbed dose. In: Proceedings of the Symposium on Electron Dosimetry and Arc Therapy, Madison, WI; September 1981, pp. 45-68. 5. Cox, R. S.; Heck, R. J.; Fessenden, P; Karzmark, C. J.; Rust, D. C. Development of total-skin electron therapy at two energies. Int. J. Radiat. Oncol. Biol. Phys. 18:659669; 1990.
6. Edelstein, G. R.; Clark, T.; Holt, J. G. Dosimetry for total body electron-beam therapy in the treatment of mycosis fungoides. Radiology 108:691-694; 1973. 7. El-Khatib, E.; Rymel, R.; Al-Mokhlef, J. A technique for total skin electron irradiation using six large flattened electron beams: Implementation on linear accelerators having different modes of electron beam production. Br. J. Radiol. 62:744-748; 1989. 8. El-Khatib, E.; Scrimger, J.; Murray, B. Reduction of the bremsstrahlung component of clinical electron beams: Implications for electron arc therapy and total skin electron irradiation. Phys. Med. Biol. 36: 111- 118; 199 1. 9. Holt, J. G.; Perry, D. J. Some physical considerations in whole skin electron beam therapy. Med. Phys. 9:769-776; 1982. 10. Hoppe, R. T.; Cox, R. S.; Fuks, Z.; Price, N. M.; Bagshaw, M. A.; Farber, E. M. Electron-beam therapy for mycosis
474
I. J. Radiation Oncology l Biology l Physics
The Stanford University experience. Cancer Treat. Rep. 63:691-700; 1979. 11. Karzmark, C. J.; Loevinger, R.; Steele,R. E.; Weissbluth, M. A techniquefor large-field,superficialelectrontherapy. Radiology 74:633-644; 1960. 12. Karzmark, C. J. Large-field superficial electron therapy with linear accelerators.Br. J. Radiol. 37:302-305; 1964. 13. Kumar, P. P.; Henschke,U. K.; Mandal, K. P.; Nibhanupudy, J. R.; Patel, I. S. Early experience in using an 18 MeV linear acceleratorfor mycosisfungoidesat Howard University Hospital. J. Natl. Med. Assoc. 69:223-227; 1977. fungoides:
Volume 33, Number 2, 1995
14. Lo, T. C. M.; Salzman,F. A.; Moschella, S. L.; Tolman, E. L.; Wright, K. A. Whole body surfaceelectron irradiation in the treatment of mycosis fungoides. Radiology 130:453-457; 1979. 15. Page,V.; Gardner,A.; Karzmark, C. J. Patient dosimetry in the treatmentof large superficiallesionswith electrons. Radiology 94:635-641; 1970. 16. Tetenes,P. J.; Goodwin,P. N. Comparativestudy of superficial whole-body radiotherapeutictechniquesusing a 4 MeV nonangulatedelectron beam. Radiology 122:219226; 1977.
Oncology
Biol.
Phys., Vol. 33, No. 2, pp. 469-474, 1995 Copyright 8 1995 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/95 $9.50 + .lw
0360-3016(95)00112-3
l
Technical Innovations and Notes VARIATION OF ELECTRON BEAM UNIFORMITY WITH BEAM ANGULATION AND SCATTERER POSITION FOR TOTAL SKIN IRRADIATION WITH THE STANFORD TECHNIQUE ELLEN EL-KHATIB, MIF~OSLAV NIKOLIC,
PH.D., F.C.C.P.M.,” SHERALI HUSSEIN, PH.D., F.C.C.P.M.,* PH.D., M.C.C.P.M.,* NICHOLAS J. S. Voss, M.D., F.R.C.P.C.’ AND CHRISTINA PARSONS, M.D., F.R.C.P.C.+
*Division of Medical Physics and +Division of Radiation Oncology, British Columbia Cancer Agency, Vancouver, B.C. Canada VSZ 4E6 Ptupo~: The intluence of different scatterer-degraders and beam angulations on beam uniformity for total skm electron irradiation using the six dual beam Stanford technique is investigated. Methods and Materials: The 6 MeV high dose rate total skin electron irradiitlon mode on a linear accelerator was used. Beam pro&s and percentage depth doses in the patient plane for single, dual, and six dual beams were measured for different dual beam angulations and acrylic scatterer-degraders of diierent thicknesses mounted on the treatment head or in front of the patient in the treatment plane. Results: It ls demonstrated that, with the same electron nominal energy, total skin irradiation techniques witnerent beam penetrations can be obtained by inserting various beam scatterer-degraders into the beam, either mounted on the accelerator head or close to the patient. For our patient treatment, a beam penetration was selected so that the 80% dose lay at 8-9 mm and the 50% dose at 15-16 mm depth. This was achieved by mountlng a 0.32-cm thick acrylic beam scatterer-degrader on the accelerator head. A uniform verticai protlle was obtained for gantry angulations of 221”. Conclusions: To implement a total skin electron irradiation technique using the Stanford method, the uired depth of penetration needs to be selected. Based on this, the appropriate combination of scatterer?mders and dual beam angulations to produce a uniform beam in the treatment plane needs to be determined. Different techniques with diierent beam penetrations can be developed using the same high dose rate mode on the linear accelerator by a proper choice of scatterer-degraders and beam anguhttions. Total skin electron irradiition,
Stanford technique, Beam scatterer-degraders.
INTRODUCTION
the central 160 X 60 cm’ area of the treatment plane, an
horizontal uniformity is 24%. However, even with good beam uniformity in the treatment plane, dose uniformity at the patient’s surface cannot be better than 215% because of variable skin distance, self-shielding, and differences in curvature (13). In addition, areas such as the top of the head, shoulders, perineum, and soles of feet will get much lower doses and may need a boost treatment. The skin radiation dose, penetration, and dose distribution for a patient depend primarily upon the electron energy distribution and the angle of incidence of the electrons on the shin of the patient. The cylindrical uniformity around the patient is achieved by using multiple or rotational beams. The dose uniformity is best for rotational
achievable goal for vertical uniformity
techniques,
Total body skin irradiation with low energy electrons (TSEI) has been used for the treatment of mycosis fungoides since the 1950s (14). Several different techniques have been developed at various centers and these are described in a report published by the American Association of Physicists in Medicine (2). The main objective of the treatment is to treat the entire surface of the body uniformly to a limited depth. To achieve this, the field size of the electron beam at the treatment plane should be a minimum of 200 cm in height and 80 cm in width to encompass large patients. Within is 28% and for
Presented at the Fortieth Annual Meeting of the Canadian Organization of Medical Physicists and the Canadian College of Physicists in Medicine, Toronto, Canada, 16- 18 September 1994.
but it is not satisfactory
for a two- or four-
Reprint requests to: Ellen El-Khatib, Ph.D., F.C.C.P.M., Division of Medical Physics, British Columbia Cancer Agency, 600 West 10th Avenue, Vancouver, B.C. Canada VSZ 4E6. Accepted for publication 10 March 1995. 469
ml
I. J. Radiation
Oncology
0 Biology
0 Physics
field technique (16). A widely used technique using six dual fields has been developed at Stanford University (5, 10-12, 15) and many institutions use variations of this technique (7). To achieve the required beam uniformity and dose penetration in the patient, various combinations of angled beams and beam scatterers or degraders are used (2, 5, 6). In particular, the percentage depth dose, surface dose, and bremsstrahlung contamination of the beam will be different for multiple beams compared to single beam irradiation (2, 9). Although the x-ray contamination for electron beams of energies lower than 6 MeV is typically < 1% and of no clinical significance, the x-ray dose can become significant for multiple beams (8), and an x-ray dose of 4% averaged over the whole body is considered unsatisfactory by many physicians. Certain types of accelerators have less x-ray contamination than others. For example, it is lower for scanned electron beams than those having scattering foils, although some electron beams produced by scattering foils achieve low photon contamination that is almost comparable to that of scanned beams (8). A method to reduce the photon contamination in the patient plane was to use angled beams, as in the Stanford technique, where the peak of the bremsstrahlung, which is along the central axis of the beam, is directed above and below the patient. Beam scatterers or beam degraders are often used either to flatten the beam or reduce penetration. Brahme (4) has examined the effect of placing a given scatterer near the exit window or close to the phantom surface. The energy distribution of the two beams was almost identical. However, the angular distribution of the electrons that reaches the phantom surface was totally different. The scatterer placed at the exit window produced a narrower angular spread than those produced by a scatterer placed near the patient surface. The wider angular spread resulted in a higher surface dose and shallower depth dose (2-4). Penetration depths such that the 50% isodose line lies between 5- 15 mm are considered adequate to treat most lesions (2). The Stanford technique was recently modified to allow selection of beams with different penetration (5). Because patients need treatment at extended distances and the set-up is long, a high dose rate that shortens the treatment time is desirable. A high dose rate (888 MU/ min) electron irradiation mode (HDTSEI) is now available on some commercial linear accelerators.’ In the present work, we describe the development of a TSEI technique and the choice of beam angulation and scatterer that produce adequate beam uniformity and pen-
’ Clinac 2100 C, Varian Assoc., Palo Alto, CA. * Victoreen, Cleveland, OH. 3 Nuclear Assoc., Carle Place, NY. 4 Eastman Kodak Co., Rochester, NY.
Volume
33, Number
2. 1995
160 ,- Head
Feet
120 ‘;; L 8 100 : .z m 2
60 60 0 f20° 7 +rs” Pfl6o 0 ,,4O
40
01
’
’
’
’ ’
’
’
’
’
’
’
’
-105 -90 -75 -60 45 -30 -15 0 15 30 45 60 Distance From Horizontal lsocenter
’
’
J
75 90 105
Fig. 1. The vertical beam profiles at a depth of 1 cm in polystyrene 340 cm from the exit window are shown for dual beams with angulations of + 14”, +16”, +18”, +20”, and ~22” from the horizontal isocenter.
etration. More than one technique with different penetration can be used with the same HDTSEI mode on the linear accelerator by using different beam degraders. METHODS
AND MATERIALS
A linear accelerator’ with a 6-MeV high dose rate total skin electron irradiation mode was used. Acrylic scattererdegraders of thicknesses 0.32 cm and 0.56 cm were mounted on the head of the accelerator 49.2 cm from the exit window. A large 0.65-cm acrylic degrader (213 x 122 cm’) to be placed close to the patient was also constructed. The penetration of the beam was measured for a single horizontal beam and for the dual angulated beams with the Holt parallel plate chambe? and Markus chamber’ in a polystyrene phantom, and with film4 in a solid water phantom cassette.5 The Holt chamber forms an integral part of a polystyrene phantom and has a collection volume of 1.0 cm” and a wall thickness of 4 mm. The Markus chamber has a collection volume of 0.055 cm3 and a wall thickness of 2.3 mg/cm”. Both chambers were used with the same electrometer.6 The Holt chamber, because of its large collection volume, is suitable for measuring ionization in large electron beams. However, the Markus chamber was needed to measure ionization within the first 4 mm of the surface. The beam penetration for the six angulated dual beams was measured in a humanoid phantom7 with film sandwiched between several slabs of the phantom. The film
5 Radiation
Measurements
Inc., Middleton,
WI.
6 Keithley Radiation Measurements Division, Cleveland, OH. 7 The Rando Phantom, Alderson Stamford, CT.
Research Laboratory,
Inc.,
Electron
Head
0’
beam uniformity
Feet
’ ’ ’ ’ ’ ’ ’ ’ ’ 30’ 45’ 60’ 75’ 90’ 105’
-105 -90 -75 -60 -45 -30 -15 0 15 Distance From Horizontal
lsocenter
Fig. 2. The vertical and horizontal beamprofiles for dual open beamsangledat + 18” from the horizontal arecomparedto dual beamswith a 0.32 cm acrylic scattereron the treatmenthead and angledat t21”, and dual beamswith a 0.65 cm acrylic scatterer40 cm in front of the treatmentplane angledat 220”.
dose was determined from a sensitometric curve measured for the beam at the extended distance. The films placed at several levels in the humanoid phantom also demonstrated the cylindrical beam uniformity. This was also confirmed by LiF TLD8 placed around the humanoid circumference. Determination of the factor relating dose at maximum for the six dual beam combination to one dual beam was done with both the TLDs and film. Beam profiles were measured at the point of maximum dose in polystyrene with both the Holt parallel plate chamber and with film for the various combinations of beam angulation and scatterer-degrader configurations. Beam calibration was done by means of a Holt chamber, which was calibrated against our dosimetry standard in a 6 MeV 10 X 10 cm2 field.
RESULTS AND DISCUSSION
in TSEI
0 E. EL-KHATIB
et
471
al.
choice of angulation is critical. For the open beams, an angulation of + 18” was optimal. With beam scatterers in the beam, the beam, as expected, no longer had optimal uniformity at 2 18”, and the beam angulation needed to be determined. For a 0.32cm acrylic scatterer mounted on the treatment head 49.2 cm from the exit window, the vertical profiles measured for beam angulations of t 15 to 221” indicated that beam flatness was no longer optimal for the + 18” beam angulation, but was good for the 221” gantry angulation. Both the vertical and horizontal beam profiles for the open beams angled at ? 18” are shown in Fig. 2, and are compared to the beams scattered by 0.32 cm acrylic angled at 221”. The lateral profiles were measured at several distances up to 90 cm above the central direction and did not change. For a large 0.65cm scatterer placed 40 cm in front of the treatment plane, good beam uniformity was obtained for gantry angulations of +20” from the horizontal. These profiles are also shown in Fig. 2. Percentage depth dose
The beam penetration with the 0.65-cm large scatterer in the beam and single dual beams was found not to be penetrating enough with an electron range R, < 1.8 cm. Therefore, further dosimetry measurements were not done because this beam was unsuitable for our treatment. The percentage depth dose (PDD) obtained from the depth ionization curves (1) in polystyrene at 340 cm exit window-to-surface distance (SSD), with the open dual beams angled + 18” both on the horizontal center and 90 cm distant from the center, are shown in Fig. 3. The PDD measured on the horizontal center is more penetrating than at 90 cm distant from the center. This is attributed to the different angles of incidence of the electrons at these different positions. Also, because of the
Beam profdes
The beam profiles in the vertical direction were measured for dual angulated beams to determine the angle at which a dose profile is uniform to within ?8% over 160 cm of the treatment plane as recommended by Task Group 23 of the AAPM (2). The profiles were measured both with the Holt chamber at a depth of 1 cm and by film sandwiched between 0.7 cm acrylic and 0.5 cm pressed wood at 340 cm from the exit window for gantry angulations of 2 14”, + 16”, + 18”, +20”, and +22” from the horizontal. For this first set of measurements, there were no beam scatterers and/or degraders in the beam, although it was expected that these may alter the beam flatness. As illustrated in Fig. 1, the beam angulation has a dramatic effect on beam uniformity, and therefore, the ’ HarshawChemicalCo., Cleveland,OH.
no scatterer - degrader, 0 Central Axis 5
$ n
l
18”
60 -
$ s 40 5 e a" 20 -
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.6 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Depth in Polystyrene (g/cm2)
Fig. 3. The percentagedepth dosesat horizontal ¢er and 90 cm above areshownfor dual beamsangled218” at 340 cm SSD.
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6 Dual -
beams,
No scatterer
- - - 0.32
cm Acrylic
--
cm Acrylic
0.56
0 Physics
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33, Number
2, 1995
f 16” 6 Dual beams,
- degrader
0.32cm Acrylic,
f 21’
n 2 40 5 a 20 -
I 0 0.2 0.4 0.6 0.6 1.0 1.2 1.4 1.6 1.6 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Depth in Humanoid Phantom (cm)
Fig. 4. The percentage depth doses measured in a humanoid phantom for six dual beams are shown for open beams and those scattered by 0.32 cm and 0.56 cm acrylic mounted on the treatment head.
oblique incidence of electrons in the 12-field combination
(2), the PDD is expected to be shallower for six dual beams than for one dual beam. Therefore, the desired beam penetration for the six dual beams needs to be determined, and the appropriate beam scatterer selected, which will produce the required beam penetration. Our radiation oncologists wanted a beam penetration such that Rso - 7-8 mm and RsO - 14-15 mm. The PDDs measured for the open beam and for beams with 0.32 cm and 0.56 cm acrylic mounted on the treatment head are shown in Fig. 4 for beam angulations of + 18”. Figure 4 illustrates that 0.32 cm acrylic would give the required penetration. As discussed previously for this scatterer mounted on the treatment head, the beam flatness was no longer optimal in the vertical direction for +18” beam angulations. However, beam flatness was good for angulations of ?21”, as shown in Fig. 2.
o) 80 :: 0 .K z 60 2
I 0 0.2 0.4 0.6 0.6 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Depth in Humanoid Phantom (cm)
Fig. 6. The percentage depth doses for six dual beamsangled at 221” to the horizontal and with a 0.32 cm acrylic scatterer mountedon the treatment head are shown. Percentagedepth dosesweremeasuredat severalpointsaroundthe circumference of a humanoidphantom.All measuredPDD curveslay between the two shown.
The PDDs were then measuredfor both one dual beam and the six dual beams at gantry angulations of ?21”,
and are shown in Figs. 5 and 6. For one dual beam, the PDD was also measured with film on the horizontal center and was identical to that obtained from the ionization measurements (Fig. 5), except for the slightly higher dose measured for x-ray contamination. The bremsstrahlung dose measured with the ion chamber was 0.2% on the horizontal isocenter and 1% 90 cm above. The film measurement indicated 0.5% bremsstrahlung on the horizontal isocenter. The PDDs shown in Fig. 6 are for the six dual field combinations for various circular or oblong slices of the humanoid phantom. The beam penetration is such that the 80% depth dose lies between 8 and 9 mm and the 50% depth dose lies between 15 and 16 mm. The bremsstrahlung contamination has been measured with both TLD and film, and is about 2% to the total body. The cylindrical beam uniformity was measured with both
Dual beam, 0.32cm Acrylic, + 21’ 0
Central
0
90 cm from Central Central
axis, ion chamber axis, ion chamber
axis, Film
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Depth in Polystyrene (gkm2)
Fig. 5. The percentagedepth dosesat horizontal center and 90 cm above are shownfor dual beamswith a 0.32 cm acrylic scattereron the treatment head and angled +21”. The depth doseswere measuredboth with ion chamberand film. Thesemeasurements were identical.
Electron beam uniformity in TSEI 0
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Posterior
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output factor at depth of maximum dose (2-3 mm) for all six dual fields. This was done using both film and TLD and was found to be 0.0036 Gy/MU of each field (i.e., 1 MU to each of the 12 fields). Therefore, to deliver 30 Gy in eight fractions to the patient, a dose of 3.75 Gy is delivered
in one fraction
of six dual fields given on
two consecutive days. This corresponds to: 3.75 = 1042 MU per each of the 12 fields, 0.0036 and takes 1.2 mm per field. In-vivo dosimetry
Three sets of 40 TLD measurements were performed on each patient. A typical dose distribution is shown in Fig. 7. The areas showing low doses because of self-shielding were boosted with small fields. Feet and hands showing large doses were shielded for the final two fractions. Fig. 7. Radiation-absorbed doses measured using TLD at various positions on a patient treated with six dual beams angled 521” and modified by the 0.32 cm acrylic scatterer are shown. All doses are in cGy relative to 3.75 Gy, which represents one fraction of dose given to the patient.
CONCLUSIONS A Stanford six dual beam total skin electron irradiation technique was developed with a 6 MeV high dose rate mode of a linear accelerator. Different combinations of beam scatterer-degraders mounted either on the head of the accelerator or close to the patient produced beams of different penetrations
film and TLD in the humanoid phantom and it was found to be +8%. Beam calibration
The output factor for the dual beams at a depth of d = 1.06 cm, which is the point of maximum dose at extended distance SSD = 338-348 = 340 cm (50-60 cm from wall), has been determined from ion chamber dosimetry to be 0.00135 Gy/MU for each field (i.e., 1 MU each to both fields). This output factor needs to be related to the
and profiles. The thickness
of beam
scatterer-degrader of 0.32 cm acrylic mounted on the accelerator head produced the required penetration for the 6 dual beams. With this degrader in place, the vertical profiles are uniform within +-8” over 160 cm for gantry angulations of 221”. Beams with different penetrations can easily be obtained using the same 6 MeV high dose rate mode by simply changing the thickness of the beam scatterer-degrader. However, in this case, the new beam angulations required to provide the vertical beam uniformity will need to be determined.
REFERENCES 1. American Association of Physicists in Medicine Task Group 21. A protocol for the determination of absorbed dose from high energy photon and electron beams. Med. Phys. 10:741-771; 1983. 2. American Association of Physicists in Medicine Task Group 23. Total skin electron therapy: Technique and dosimetry. New York: American Institute of Physics; 1988. 3. B&me, A.; Svensson, H. Specification of electron beam quality from the central-axis depth absorbed-dose distribution. Med. Phys. 3:95-102; 1976. 4. Brahme, A. Physics of electron beam penetration: Fluence and absorbed dose. In: Proceedings of the Symposium on Electron Dosimetry and Arc Therapy, Madison, WI; September 1981, pp. 45-68. 5. Cox, R. S.; Heck, R. J.; Fessenden, P; Karzmark, C. J.; Rust, D. C. Development of total-skin electron therapy at two energies. Int. J. Radiat. Oncol. Biol. Phys. 18:659669; 1990.
6. Edelstein, G. R.; Clark, T.; Holt, J. G. Dosimetry for total body electron-beam therapy in the treatment of mycosis fungoides. Radiology 108:691-694; 1973. 7. El-Khatib, E.; Rymel, R.; Al-Mokhlef, J. A technique for total skin electron irradiation using six large flattened electron beams: Implementation on linear accelerators having different modes of electron beam production. Br. J. Radiol. 62:744-748; 1989. 8. El-Khatib, E.; Scrimger, J.; Murray, B. Reduction of the bremsstrahlung component of clinical electron beams: Implications for electron arc therapy and total skin electron irradiation. Phys. Med. Biol. 36: 111- 118; 199 1. 9. Holt, J. G.; Perry, D. J. Some physical considerations in whole skin electron beam therapy. Med. Phys. 9:769-776; 1982. 10. Hoppe, R. T.; Cox, R. S.; Fuks, Z.; Price, N. M.; Bagshaw, M. A.; Farber, E. M. Electron-beam therapy for mycosis
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The Stanford University experience. Cancer Treat. Rep. 63:691-700; 1979. 11. Karzmark, C. J.; Loevinger, R.; Steele,R. E.; Weissbluth, M. A techniquefor large-field,superficialelectrontherapy. Radiology 74:633-644; 1960. 12. Karzmark, C. J. Large-field superficial electron therapy with linear accelerators.Br. J. Radiol. 37:302-305; 1964. 13. Kumar, P. P.; Henschke,U. K.; Mandal, K. P.; Nibhanupudy, J. R.; Patel, I. S. Early experience in using an 18 MeV linear acceleratorfor mycosisfungoidesat Howard University Hospital. J. Natl. Med. Assoc. 69:223-227; 1977. fungoides:
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14. Lo, T. C. M.; Salzman,F. A.; Moschella, S. L.; Tolman, E. L.; Wright, K. A. Whole body surfaceelectron irradiation in the treatment of mycosis fungoides. Radiology 130:453-457; 1979. 15. Page,V.; Gardner,A.; Karzmark, C. J. Patient dosimetry in the treatmentof large superficiallesionswith electrons. Radiology 94:635-641; 1970. 16. Tetenes,P. J.; Goodwin,P. N. Comparativestudy of superficial whole-body radiotherapeutictechniquesusing a 4 MeV nonangulatedelectron beam. Radiology 122:219226; 1977.