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A non-docking intraoperative electron beam applicator system
M. J Raduuion Oncology Lb/. Phys., Vol. 17, pp. 41 I-417 Printed in the U.S.A. All rights reserved.
Copyright
0360-3017/89 0 1989 Maxwell Pergamon
$3.00 + .a0 Macmillan plc
0 Technical Innovations and Notes A NON-DOCKING JATINDER Department
INTRAOPERATIVE R. PALTA, PH.D.
ELECTRON
AND NAGALINGAM
BEAM
APPLICATOR
SUNTHARALINGAM,
SYSTEM
PH.D.
of Radiation Oncology and Nuclear Medicine, Bodine Center for Cancer Treatment, Thomas Jefferson University Hospital, Philadelphia, PA 19 107
A non-docking intraoperative radiation therapy electron beam applicator system for a linear accelerator has been designed to minimize the mechanical, electrical, and tumor visualization problems associated with a docking system. A number of technical innovations have been used in the design of this system. These include: (a) a new intraoperative radiation therapy cone design that gives a better dose uniformity in the treatment volume at all depths; (b) a collimation system which reduces the leakage radiation dose to tissues outside the intraoperative radiation therapy cone; (c) a nondocking system with a translational accuracy of 2 mm and a rotational accuracy of 0.5’; and (d) a
rigid clamping system for the cones. A comprehensive set of dosimetric characteristics of the intraoperative radiation therapy applicator system is presented. Intraoperative radiation therapy, Non-docking cones, Electron dosimetry. design principles are illustrated and the description and the dosimetric characteristics of the IORT applicators are presented.
INTRODUCTION Intraoperative radiation therapy (IORT) is a multi-disciplinary procedure which combines two conventional methods of cancer treatment, namely, surgery and radiation therapy. The purpose is to deliver a large single dose of radiation to the tumor or tumor bed, while minimizing to a much higher degree the dose to normal structures. Each institution involved in IORT has adapted their existing radiation therapy equipment to provide satisfactory IORT beams (l-3,6). The concept of a “dedicated” linear accelerator, placed in an operating room setting, for intraoperative electron beam therapy is also receiving increased interest (4). The initiation of an IORT program at an institution involves the design and fabrication of many devices. This includes the applicator system and a viewing system for correct positioning of an applicator. There are several commercial IORT applicator systems available for various types of linear accelerators. Most of these systems are such that the applicator placed in the patient is attached to the collimator head of the accelerator using a rigid “hard docking” mechanism (l-3,6). This is essential because a precise alignment and proper docking is very important to reproduce, in the treatment mode, the measured dosimetric characteristics of the intraoperative applicator system. At Thomas Jefferson University Hospital (TJUH), a non-docking applicator system was designed to minimize the mechanical, electrical, and tumor visualization concerns associated with a docking system. In this paper, the
METHODS
A linear accelerator* has been designated for IORT at TJUH. This accelerator is placed in a dedicated intraoperative suite which includes an operating room (OR) connected by a second maze arrangement to the treatment room. After the surgical procedure, the patient is moved from the OR to the treatment room and again brought back to the OR for completion of the surgery. This procedure minimizes the time for which the treatment room is used for IORT. This accelerator* produces nine electron beams in the energy range from 4 MeV to 22 MeV. It uses a dual scattering foil arrangement for each energy to produce broad electron beams. The primary scattering foil, made of thin tantalum, produces a broad beam profile which is then flattened by a thin aluminum secondary flattening foil. The latter selectively scatters and attenuates the beam to a high degree of uniformity. The applicator defines the field size, in a “picture frame” fashion, without the use of any wall scatter. Therefore, all electron beams have good flatness and symmetry even without an applicator. This characteristic of electron beam has simplified the design of the non-docking IORT applicators with little modification in the beam collimation and cost.
Reprint requests to: Jatinder R. P&a, Ph.D. Accepted for publication
AND MATERIALS
* SL25, Philips Medical Systems, Crawley, England.
15 February 1989. 411
412
I. J. Radiation Oncology 0 Biology0 Physics
August 1989, Volume 17, Number 2
Schematic of Collimation for Intraoperatlve Electron Treatments
Fig. 2. The profiles of electron fluence incident on top of a 10 cm inside diameter treatment cone. Radial profiles for 4 MeV, 10 MeV, and 22 MeV electron beams are shown. The innermost profile is for the 4 MeV electron beam.
Fig. 1. Schematic of the intraoperative radiation therapy applicator system for the linear accelerator.
Collimation design The primary collimation for the IORT applicators consists of a shortened standard 14 cm X 14 cm electron applicator with brass end frames. These end frames have different size circular apertures, one for each size of IORT cones. The collimation for intraoperative electron therapy is schematically illustrated in Figure 1. The photon collimator jaw setting is automatically selected with the energy. The collimator jaw settings are optimized for each energy to provide a uniformly flat field for each standard electron applicator. The 14 cm square cutout in the truncated applicator provides the first collimation for the electron beam defined by the photon jaws opening. This cutout is made of lead and is a part of the original standard electron applicator. The lower downstream collimator circular aperture is made of brass because of its durability and medium atomic number. The bremsstrahlung production in brass is significantly less than in lead. Measurements show that 15.9 mm (i”) brass is sufficiently thick to collimate the highest energy electron beam (22 MeV). The brass aperture has an opening that is 1 cm larger than the inside diameter of the IORT treatment cone. This assures that the electron beam fluence incident on the cone is uniform even when there are slight translational errors in alignment.
The clinical beam is further defined by a treatment cone not directly attached to the radiation head, but aligned with the central axis of the beam using a mechanical jig. The treatment cones are made of lucite. A brass annulus is placed on top of the cone to minimize leakage radiation outside the treatment area caused by electron scatter in air between the brass aperture and treatment cone. The lateral dimensions of this annulus is such that the leakage radiation from electron scatter is less than 3% for all energies of electron beams. The lowest energy electron beam (4 MeV), dictates the lateral dimensions because of its maximum angular spread. The fluence profile of the beam incident on top of a 10 cm diameter treatment cone for 4 MeV, 10 MeV, and 22 MeV electron beams are shown in Figure 2. This shows Two Piece
T
11.5cm
1
i’ 12.0cm
lntraoperative
Cone
I .
I ,,,Steel Ring I .
‘SScm
T
I .
a,
I .
I
.I
1
j1.c.. lnterchangable Bottom Section (Flat, 15O or 30°)
- IFig. 3. Schematic of the two-piece intraoperative cone. The steel ring is removable.
413
Intraoperative electron beam applicators 0 J. R. PALTA AND N. SUNTHARALINGAM
that there is uniform beam intensity coming out of the downstream brass aperture for all energies. It also demonstrates that the magnitude of bremsstrahlung radiation produced in the brass aperture is less than 3% for the highest energy electron beam (22 MeV). IOR T electron cones All lucite intraop cones have wall thickness of 0.635 cm (t”). The cones are designed and built with circular cross sections. The available sizes are 5 cm, 5.7 cm, 7 cm, 7.6 cm, 8.8 cm, and 10.1 cm inner diameter. The length of these cones is 2 1.5 cm. The bottom end of the cone is intended to be positioned at the isocenter of the treatment machine, 100 cm SAD. The cones are made up of two pieces as shown in Figure 3. The two-piece cone has an advantage because the bottom section, which goes into the surgical opening of the patient, can be selected to have a flat edge or beveled edges of 15’ or 30”. An important improvement to the design of the IORT cones is the use of a thin steel ring in the top section of the cone. This improves the dose uniformity inside the treatment field. The steel ring is removable if needed. Non-docking alignment In the non-docking arrangement, the electron treatment cone is rigidly held in the patient by a clampt which in turn is attached to the horizontal post of the assemb1y.S The clamp? provides a free motion through a ball joint for quick setup. The treatment cone is aligned with the central axis of the beam via longitudinal, lateral, vertical, and tilt motions of the surgical table. Standard surgical tables generally have no provision for longitudinal and lateral motion. Therefore, a standard table9 has been adapted for intraoperative radiation therapy by making the necessary modifications. These changes are similar to those described by Fraas et al. (3). The modified IORT table has the following features: 1. Precision lateral and longitudinal drives for fine positioning. 2. Slow vertical motion for safe and precise alignment. 3. Larger castors for ease in transportation of the patient from operating room to treatment room. Alignment verijication The verification of the central axis of beam and cone alignment is done by using a special alignment jig, as shown in Figure 4. It is a simple device that has a set of cross wires and a cross mark, which are placed at the two ends of a cylindrical tube. This jig is inserted in the treatment cone and the accelerator beam light is switched on. The cross wires in the machine and the cross wires in the t Mick clamp, Mick Radio Nuclear Instruments, Inc., NY. $ Bookwalter, Codman Instruments, Inc., Randolph, MA. 0 Model 2080RC, American Sterilizer Company, Erie, PA.
Top Crosswire
Projections wlth Midlgnment
Bottom Cross Mark
I I
Alignment
I
Jig
Fig. 4. Cone alignment jig. Cross-wire the jig, are also shown.
Projection wlth Acceptable Alignment
projections,
as visible on
jig both cast a shadow on the bottom plate on which the cross mark is etched. If there is a misalignment of the axis of the cone and the beam central axis, then the two crosswire images will be separate and will not superimpose on the cross mark on the bottom plate. The final alignment is achieved by using the fine movements of the table. The alignment is acceptable if only one shadow is visible and it falls on top of the cross mark on the plate. The alignment jig is removed before irradiation. The table is raised back to achieve the correct distance for treatment. If the treatment cone is not vertical, the table can be tilted or the gantry can be rotated such that the top of the cone and the face of the gantry are parallel. The set-up procedure for non-vertical gantry position is a little more involved because it requires both the vertical and translational movements of the table for final alignment and the correct source-axis distance. Note that the precision of this alignment procedure is only as good as the alignment of the cross wires and the radiation axis of the linear accelerator. Therefore, the use of this type of alignment device requires a regular quality assurance check of the accelerator. Evaluation criteria and measurement technique The dosimetric quantities of interest in intraoperative electron beam radiation therapy are: (a) beam uniformity, (b) depth dose coverage and surface dose, (c) lateral therapeutic coverage (of 90% isodose line), and (d) cone leakage radiation to the surrounding tissues. The new design cones were evaluated for the above characteristics. The dosimetry data have been obtained from measurements in water using a 3dimensional beam scanner.* Two shonka plastic 0.147 cm3 volume ionization chambers, one as a reference detector and the other as scanning detector, were used in the depth dose and cross beam profile measurements. The central axis depth ionization data were converted into dose using the TG21 protocol. * Wellhofer WP 609, Medical Roseland, NJ.
Physics Instrumentation
Inc.,
414
I. J. Radiation Oncology 0 Biology 0 Physics
The isodose distributions were generated from the measured 2dimensional profiles. Treatment cone leakage measurements were made, both in water and air, perpendicular to the 10 cm diameter cone for a 22 MeV electron beam, where leakage was most significant. All leakage measurement data were normalized to the maximum dose on the central axis in water. Surface dose measurements were made in a lucite phantom using a parallel plate chamber.? The chamber was positioned within a 30 cm X 30 cm lucite slab so that the entrance window was flush with the lucite phantom surface. Measurements were made with both positive and negative polarity and the average value of these readings was taken for relative dose calculations.
August 1989, Volume 17, Number 2 22 MoV ?? loctron berm depth = 1 cm
i Y 4
-
wlth stool ring
-
wlthout stool ring
i
1
i”f
i
RESULTS
Beam uniformity Measurements of beam profiles for all IORT cones have shown areas of increased dose just inside the edges of the cone. This is caused by the streaming of scattered electrons from the lucite wall. The magnitude of this high dose region is independent of the photon jaws opening as long as the incident electron fluence profile on top of the applicator does not change. It has been suggested that the high dose areas can be reduced by: (a) optimizing the photon jaws opening to adjust the electron fluence striking the walls of the cone, ( 1, 3) and (b) adding a small lucite ring on the inside of the cone to intercept the streaming electrons (4, 5). In the present cone design, a steel ring which is 15 mm wide in the beam direction and has a radial thickness of 1 mm is used. The advantage of a steel ring is that it difF&s the streaming electrons through large angles thus giving a very uniform and flat beam profile. Various positions of the steel ring along the beam axis were experimented. For this collimation design, positioning the ring in the middle of the cone was optimal for all energies. Figure 5 illustrates the change in beam uniformity with the addition of the steel ring for 22 MeV electron beam. The addition of the ring removes the high dose areas without compromising the beam flatness at field edges for all energies of electron beams. Depth dose The depth doses were calculated from the measured ionization data for all cones and all energies. The depth dose data for flat cone was measured along the central axis. The data for the beveled edges ( 15’ and 30”) were measured perpendicular to the water surface and in the ‘center of the field. The end of the cone was flush with the water surface at nominal 100 cm SSD. The gantry was rotated such that the central axis of the beam was coincident with the central axis of the cone. All depth doses t F’TWModel 30-330, Nuclear Enterprises America, Fairfield, NJ.
(cm)
Fig. 5. The beam profile for a 7 cm inside diameter intraoperative flat end cone. The electron beam energy is 22 MeV and the depth of measurement is 1 cm (dmax/2).
are normalized to the maximum dose on the measurement axis. The results of the measured data are shown in Figure 6. The central axis depth dose data for the flat end cone is almost identical to the data for the manufacturer supplied treatment applicators with inserts of same field size. The central axis depth dose change significantly for small cone size and energies greater than 15 MeV. The depth dose coverage with bevel ended cones is less than that with flat ended cones. The measured surface doses increase from 82-96% with increase in electron energy from 4 MeV-22 MeV. The surface dose does not change significantly with the bevel end. Adding a brass mesh (.28 mm diameter wire; 5 squares/8 mm) at the end of the IORT cone increases the surface dose from 82% to 93% for 4 MeV. However, the penetration is altered slightly. The depth dose coverage at 90% dose level decreased by 2-3 mm. The presence of the brass mesh has little effect at higher energies. At TJUH, we have tabulated the 90%, 80%, 50%, and 10% depth coverage and surface dose for each cone at all energies. We have found these values to be very useful in arriving at the appropriate electron beam energy for the specific treatment as soon as the target depth is known. Lateral therapeutic coverage The design of an applicator system influences the shape of isodose curves. Normally, for a lucite treatment cone there are high dose areas laterally at shallow depths. The
415
Intraoperative electron beam applicators 0 J. R. PALTA AND N. SUNTHARALINGAM (3
7 cm dlamotor cona ?? ndl
7
cm dlamotof
COM (1 so bowl and)
10 depth (cm)
depth (cm)
7 om dhmotor
eons
Fig. 6. Percent depth dose for all electron beams for the 7 cm inside diameter intraoperative cone: (a) Flat end; (b) 15’ Bevel end; (c) 30” Bevel end.
improvement in the lateral coverage is always associated with high dose areas at the periphery. The magnitude of the high dose at the periphery can be as high as 20% for 22 MeV electron beam through a 7 cm diameter cone. A more meaningful evaluation of lateral coverage of intraoperative beams is to compare the isodose curves obtained with standard electron beams of same field sizes; such a comparison is shown in Figure 8. The lateral coverage of isodose lines in both cases is very similar. The lateral coverage with IORT cones is typically within 1 cm of the inside diameter of the cone with the usual rounding of the isodose curves at the periphery. However, if a clinical situation warrants an increased lateral coverage, the steel ring can be removed.
dorm (cm)
increased fluence of electrons at the periphery of the cone produces much flatter isodose curves, thus improving lateral coverage (the distance between 90% or 80% points) at therapeutic depths. This is obvious from a comparison of the therapeutic isodose lines (Fig. 7), with and without the steel ring, in the designed applicators. However, this
Cone leakage The dose outside the treatment cone is clinically important in intraoperative radiation therapy because a large dose is delivered to the tumor in a single fraction. It is not only important to determine leakage dose at the end of the cones, but also along the wall of the cones. Invariably, normal tissues extend up to some height along the walls of the cone. Figure 9 shows the radiation leakage measured, in air and in water, at different depths perpendicular to the 10 cm diameter cone wall for 22 MeV electron beam, which is representative of the worst leakage situation. The leakage radiation in the region surrounding the bottom half of the cone is less than lo%, but can be
416
I. J. Radiation Oncology 0 Biology 0 Physics
August 1989, Volume 17, Number 2 0 MeV
22 MeV L
______ 7___ I__1
^B...___,_I_
‘-1
0
7
-*-c-c
with steel ring without
steel ring
Fig. 7. Comparison of therapeutic isodose curves (90% and 80%) for 22 MeV and 8 MeV electron beams for the 7 cm diameter intraoperative cone.
as high as 22% in regions close to the wall at the upper part of the cone. The radiation leakage through the brass collimator, which is primarily due to bremmstrahlung photons, is always less than 3% for all cones. At TJUH, much effort is taken to retract normal tissues away from the treatment cone to decrease the radiation dose to the normal tissue. SUMMARY
Though the concept of intraoperative radiation therapy is relatively simple, the technical complexity of this procedure requires a careful evaluation and appropriate modification of the conventional dosimetric methods used in electron beam therapy. A special cone arrangement is
22 MeV electron
needed to direct radiation into the surgical opening that exposes the tumor or tumor bed. The cone serves the primary purpose of collimation of the electron beam and additionally directs the beam to a well defined target. Sometimes, it also helps in retracting normal tissues away from the treatment field. The dosimetric characteristics of an IORT applicator system depend largely on the characteristics of the electron beams generated in the treatment head of a particular linear accelerator. Therefore, similar applicator systems used with different accelerators will have different beam characteristics. The design, shape, and size of each intraoperative cone affects the following dosimetric characteristics: (a) depth dose and surface dose; (b) X ray contamination; (c) dose output per monitor unit; (d) flatness and
beam
crossplane Q
8 MeV electron beam km1
(I
Fig. 8. Comparison of isodose curves for a 7 cm diameter intraoperative cm square applicator. The broken lines are for 7 cm cutout.
crosaplene
Icrl
cone and a 7 cm diameter cutout in 14
Intraoperative electron beam applicators 0 J. R. PALTA AND N. SUNTHARALINGAM
dlcl5nc5
dl5tcnco
(cm)
417
(cm)
Fig. 9. Leakage radiation through the walls of the 10 cm inside diameter cone for 22 MeV electron beam. Data are normalized to the maximum dose in water on the central axis of the cone at 100 cm SSD. The depths are the distances from the top of the cone where the dose scans are measured: (a) Dose scans measured in air; (b) Dose
scans measured in water. The water level is up to 4 cm from the top of the cone.
symmetry; (e) leakage radiation through the cone walls; and (f) shape of the isodose distribution. Therefore, all intraoperative cones must be evaluated for these characteristics. McCulluough and Biggs (7) have given a good overview of the dosimetric considerations for electron beam intraoperative radiation therapy. The IORT applicators designed for use at TJUH provide optimal beam characteristics with our linear acceleratoTs.* This system has the following features: (a) a nondocking system which minimizes the concerns of mechanical and electrical accidents. It also provides a clear visualization of the irradiation area; (b) the high dose areas in the periphery of the fields are removed using a steel ring without compromising the lateral coverage. The constriction of higher isodose lines as compared to the conventional electron fields is similar; (c)the surface dose for low energy beams is improved marginally because of large angle scattering of electrons from the steel ring; and (d) the alignment bf the cones with the gantry is quick and
precise with the modified OR table. The design of the collimation system for the intraoperative cones is such that applicators of different cross sectional shapes, namely, squares, rectangulars, and “squircles” (3) can be conveniently constructed with optimized beam characteristics. The only modification will be in the downstream removable brass collimation piece. The collimator aperture will have the same shape as the intraoperative cone. Its size will be 1 cm larger on all sides to ensure that a uniform intensity of electrons is incident on the cone. The intraoperative radiation therapy program at TJUH with the non-docking applicator has been successful. The set-up of the patient for treatment is precise and quick. The intraoperative radiation therapy team of radiation oncologists, physicists, surgeons, anesthesiologists, and nursing staff has expressed satisfaction with the whole procedure. Further work is continuing to design applicators with different cross-sectional shapes for various clinical sites.
REFERENCES
Bagne, F. R.; Samsani, N.; Dobelbower, R. R. Radiation contamination and leakage assessment of intraoperative electron applicators. Med. Phys. 15:530-537; 1988. Biggs,P. J.; Epp, E. R.; Ling, C. C.; Novack, D. H.; Michaels, H. B. Dosimetry, field shaping and other considerations for Intra-operative electron therapy. Int. J. Radiat. Oncol. Biol. Phys. 7:875-884; 1981. Fraas, B. A.; Miller, R. W.; Kinsella, T. J.; Sindelar, W. F.; Harrington, F. S.; Yeakel, K.; Van de Geijn, J.; Glatstein, E. Intraoperative radiation therapy at the National Cancer Institute: technical innovations and dosimetry. Int. J. Radiat. Oncol. Biol. Phys. 11:1299-1311; 1985.
Hogstrom, K. R.; Boyer, A. L.; Kirsner, S. M.; Shui, A. S. Design of metallic electron beam cones for an intraoperative therapy Iinac (Abstract). Med. Phys. 14:484; 1987. Kao, M.; Lanzl, L.; Rosenfeld, M.; Pagnamenta, A. Dose uniformity of the intraoperative radiation therapy cone (Abstract). Med. Phys. 13:605; 1986. McCullough, E. C.; Anderson, J. A. The dosimetric prop-
erties of an applicator system for intraoperative therapy utilizing a Clinac 18 accelerator. Med. Phys. 9: 161- 168; 1982. McCullough, E. C.; Biggs,P. J. Intraoperative electron beam radiation therapy. AAPM Monograph 15, American Institute of Physics; 1986:333-347.
Copyright
0360-3017/89 0 1989 Maxwell Pergamon
$3.00 + .a0 Macmillan plc
0 Technical Innovations and Notes A NON-DOCKING JATINDER Department
INTRAOPERATIVE R. PALTA, PH.D.
ELECTRON
AND NAGALINGAM
BEAM
APPLICATOR
SUNTHARALINGAM,
SYSTEM
PH.D.
of Radiation Oncology and Nuclear Medicine, Bodine Center for Cancer Treatment, Thomas Jefferson University Hospital, Philadelphia, PA 19 107
A non-docking intraoperative radiation therapy electron beam applicator system for a linear accelerator has been designed to minimize the mechanical, electrical, and tumor visualization problems associated with a docking system. A number of technical innovations have been used in the design of this system. These include: (a) a new intraoperative radiation therapy cone design that gives a better dose uniformity in the treatment volume at all depths; (b) a collimation system which reduces the leakage radiation dose to tissues outside the intraoperative radiation therapy cone; (c) a nondocking system with a translational accuracy of 2 mm and a rotational accuracy of 0.5’; and (d) a
rigid clamping system for the cones. A comprehensive set of dosimetric characteristics of the intraoperative radiation therapy applicator system is presented. Intraoperative radiation therapy, Non-docking cones, Electron dosimetry. design principles are illustrated and the description and the dosimetric characteristics of the IORT applicators are presented.
INTRODUCTION Intraoperative radiation therapy (IORT) is a multi-disciplinary procedure which combines two conventional methods of cancer treatment, namely, surgery and radiation therapy. The purpose is to deliver a large single dose of radiation to the tumor or tumor bed, while minimizing to a much higher degree the dose to normal structures. Each institution involved in IORT has adapted their existing radiation therapy equipment to provide satisfactory IORT beams (l-3,6). The concept of a “dedicated” linear accelerator, placed in an operating room setting, for intraoperative electron beam therapy is also receiving increased interest (4). The initiation of an IORT program at an institution involves the design and fabrication of many devices. This includes the applicator system and a viewing system for correct positioning of an applicator. There are several commercial IORT applicator systems available for various types of linear accelerators. Most of these systems are such that the applicator placed in the patient is attached to the collimator head of the accelerator using a rigid “hard docking” mechanism (l-3,6). This is essential because a precise alignment and proper docking is very important to reproduce, in the treatment mode, the measured dosimetric characteristics of the intraoperative applicator system. At Thomas Jefferson University Hospital (TJUH), a non-docking applicator system was designed to minimize the mechanical, electrical, and tumor visualization concerns associated with a docking system. In this paper, the
METHODS
A linear accelerator* has been designated for IORT at TJUH. This accelerator is placed in a dedicated intraoperative suite which includes an operating room (OR) connected by a second maze arrangement to the treatment room. After the surgical procedure, the patient is moved from the OR to the treatment room and again brought back to the OR for completion of the surgery. This procedure minimizes the time for which the treatment room is used for IORT. This accelerator* produces nine electron beams in the energy range from 4 MeV to 22 MeV. It uses a dual scattering foil arrangement for each energy to produce broad electron beams. The primary scattering foil, made of thin tantalum, produces a broad beam profile which is then flattened by a thin aluminum secondary flattening foil. The latter selectively scatters and attenuates the beam to a high degree of uniformity. The applicator defines the field size, in a “picture frame” fashion, without the use of any wall scatter. Therefore, all electron beams have good flatness and symmetry even without an applicator. This characteristic of electron beam has simplified the design of the non-docking IORT applicators with little modification in the beam collimation and cost.
Reprint requests to: Jatinder R. P&a, Ph.D. Accepted for publication
AND MATERIALS
* SL25, Philips Medical Systems, Crawley, England.
15 February 1989. 411
412
I. J. Radiation Oncology 0 Biology0 Physics
August 1989, Volume 17, Number 2
Schematic of Collimation for Intraoperatlve Electron Treatments
Fig. 2. The profiles of electron fluence incident on top of a 10 cm inside diameter treatment cone. Radial profiles for 4 MeV, 10 MeV, and 22 MeV electron beams are shown. The innermost profile is for the 4 MeV electron beam.
Fig. 1. Schematic of the intraoperative radiation therapy applicator system for the linear accelerator.
Collimation design The primary collimation for the IORT applicators consists of a shortened standard 14 cm X 14 cm electron applicator with brass end frames. These end frames have different size circular apertures, one for each size of IORT cones. The collimation for intraoperative electron therapy is schematically illustrated in Figure 1. The photon collimator jaw setting is automatically selected with the energy. The collimator jaw settings are optimized for each energy to provide a uniformly flat field for each standard electron applicator. The 14 cm square cutout in the truncated applicator provides the first collimation for the electron beam defined by the photon jaws opening. This cutout is made of lead and is a part of the original standard electron applicator. The lower downstream collimator circular aperture is made of brass because of its durability and medium atomic number. The bremsstrahlung production in brass is significantly less than in lead. Measurements show that 15.9 mm (i”) brass is sufficiently thick to collimate the highest energy electron beam (22 MeV). The brass aperture has an opening that is 1 cm larger than the inside diameter of the IORT treatment cone. This assures that the electron beam fluence incident on the cone is uniform even when there are slight translational errors in alignment.
The clinical beam is further defined by a treatment cone not directly attached to the radiation head, but aligned with the central axis of the beam using a mechanical jig. The treatment cones are made of lucite. A brass annulus is placed on top of the cone to minimize leakage radiation outside the treatment area caused by electron scatter in air between the brass aperture and treatment cone. The lateral dimensions of this annulus is such that the leakage radiation from electron scatter is less than 3% for all energies of electron beams. The lowest energy electron beam (4 MeV), dictates the lateral dimensions because of its maximum angular spread. The fluence profile of the beam incident on top of a 10 cm diameter treatment cone for 4 MeV, 10 MeV, and 22 MeV electron beams are shown in Figure 2. This shows Two Piece
T
11.5cm
1
i’ 12.0cm
lntraoperative
Cone
I .
I ,,,Steel Ring I .
‘SScm
T
I .
a,
I .
I
.I
1
j1.c.. lnterchangable Bottom Section (Flat, 15O or 30°)
- IFig. 3. Schematic of the two-piece intraoperative cone. The steel ring is removable.
413
Intraoperative electron beam applicators 0 J. R. PALTA AND N. SUNTHARALINGAM
that there is uniform beam intensity coming out of the downstream brass aperture for all energies. It also demonstrates that the magnitude of bremsstrahlung radiation produced in the brass aperture is less than 3% for the highest energy electron beam (22 MeV). IOR T electron cones All lucite intraop cones have wall thickness of 0.635 cm (t”). The cones are designed and built with circular cross sections. The available sizes are 5 cm, 5.7 cm, 7 cm, 7.6 cm, 8.8 cm, and 10.1 cm inner diameter. The length of these cones is 2 1.5 cm. The bottom end of the cone is intended to be positioned at the isocenter of the treatment machine, 100 cm SAD. The cones are made up of two pieces as shown in Figure 3. The two-piece cone has an advantage because the bottom section, which goes into the surgical opening of the patient, can be selected to have a flat edge or beveled edges of 15’ or 30”. An important improvement to the design of the IORT cones is the use of a thin steel ring in the top section of the cone. This improves the dose uniformity inside the treatment field. The steel ring is removable if needed. Non-docking alignment In the non-docking arrangement, the electron treatment cone is rigidly held in the patient by a clampt which in turn is attached to the horizontal post of the assemb1y.S The clamp? provides a free motion through a ball joint for quick setup. The treatment cone is aligned with the central axis of the beam via longitudinal, lateral, vertical, and tilt motions of the surgical table. Standard surgical tables generally have no provision for longitudinal and lateral motion. Therefore, a standard table9 has been adapted for intraoperative radiation therapy by making the necessary modifications. These changes are similar to those described by Fraas et al. (3). The modified IORT table has the following features: 1. Precision lateral and longitudinal drives for fine positioning. 2. Slow vertical motion for safe and precise alignment. 3. Larger castors for ease in transportation of the patient from operating room to treatment room. Alignment verijication The verification of the central axis of beam and cone alignment is done by using a special alignment jig, as shown in Figure 4. It is a simple device that has a set of cross wires and a cross mark, which are placed at the two ends of a cylindrical tube. This jig is inserted in the treatment cone and the accelerator beam light is switched on. The cross wires in the machine and the cross wires in the t Mick clamp, Mick Radio Nuclear Instruments, Inc., NY. $ Bookwalter, Codman Instruments, Inc., Randolph, MA. 0 Model 2080RC, American Sterilizer Company, Erie, PA.
Top Crosswire
Projections wlth Midlgnment
Bottom Cross Mark
I I
Alignment
I
Jig
Fig. 4. Cone alignment jig. Cross-wire the jig, are also shown.
Projection wlth Acceptable Alignment
projections,
as visible on
jig both cast a shadow on the bottom plate on which the cross mark is etched. If there is a misalignment of the axis of the cone and the beam central axis, then the two crosswire images will be separate and will not superimpose on the cross mark on the bottom plate. The final alignment is achieved by using the fine movements of the table. The alignment is acceptable if only one shadow is visible and it falls on top of the cross mark on the plate. The alignment jig is removed before irradiation. The table is raised back to achieve the correct distance for treatment. If the treatment cone is not vertical, the table can be tilted or the gantry can be rotated such that the top of the cone and the face of the gantry are parallel. The set-up procedure for non-vertical gantry position is a little more involved because it requires both the vertical and translational movements of the table for final alignment and the correct source-axis distance. Note that the precision of this alignment procedure is only as good as the alignment of the cross wires and the radiation axis of the linear accelerator. Therefore, the use of this type of alignment device requires a regular quality assurance check of the accelerator. Evaluation criteria and measurement technique The dosimetric quantities of interest in intraoperative electron beam radiation therapy are: (a) beam uniformity, (b) depth dose coverage and surface dose, (c) lateral therapeutic coverage (of 90% isodose line), and (d) cone leakage radiation to the surrounding tissues. The new design cones were evaluated for the above characteristics. The dosimetry data have been obtained from measurements in water using a 3dimensional beam scanner.* Two shonka plastic 0.147 cm3 volume ionization chambers, one as a reference detector and the other as scanning detector, were used in the depth dose and cross beam profile measurements. The central axis depth ionization data were converted into dose using the TG21 protocol. * Wellhofer WP 609, Medical Roseland, NJ.
Physics Instrumentation
Inc.,
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The isodose distributions were generated from the measured 2dimensional profiles. Treatment cone leakage measurements were made, both in water and air, perpendicular to the 10 cm diameter cone for a 22 MeV electron beam, where leakage was most significant. All leakage measurement data were normalized to the maximum dose on the central axis in water. Surface dose measurements were made in a lucite phantom using a parallel plate chamber.? The chamber was positioned within a 30 cm X 30 cm lucite slab so that the entrance window was flush with the lucite phantom surface. Measurements were made with both positive and negative polarity and the average value of these readings was taken for relative dose calculations.
August 1989, Volume 17, Number 2 22 MoV ?? loctron berm depth = 1 cm
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RESULTS
Beam uniformity Measurements of beam profiles for all IORT cones have shown areas of increased dose just inside the edges of the cone. This is caused by the streaming of scattered electrons from the lucite wall. The magnitude of this high dose region is independent of the photon jaws opening as long as the incident electron fluence profile on top of the applicator does not change. It has been suggested that the high dose areas can be reduced by: (a) optimizing the photon jaws opening to adjust the electron fluence striking the walls of the cone, ( 1, 3) and (b) adding a small lucite ring on the inside of the cone to intercept the streaming electrons (4, 5). In the present cone design, a steel ring which is 15 mm wide in the beam direction and has a radial thickness of 1 mm is used. The advantage of a steel ring is that it difF&s the streaming electrons through large angles thus giving a very uniform and flat beam profile. Various positions of the steel ring along the beam axis were experimented. For this collimation design, positioning the ring in the middle of the cone was optimal for all energies. Figure 5 illustrates the change in beam uniformity with the addition of the steel ring for 22 MeV electron beam. The addition of the ring removes the high dose areas without compromising the beam flatness at field edges for all energies of electron beams. Depth dose The depth doses were calculated from the measured ionization data for all cones and all energies. The depth dose data for flat cone was measured along the central axis. The data for the beveled edges ( 15’ and 30”) were measured perpendicular to the water surface and in the ‘center of the field. The end of the cone was flush with the water surface at nominal 100 cm SSD. The gantry was rotated such that the central axis of the beam was coincident with the central axis of the cone. All depth doses t F’TWModel 30-330, Nuclear Enterprises America, Fairfield, NJ.
(cm)
Fig. 5. The beam profile for a 7 cm inside diameter intraoperative flat end cone. The electron beam energy is 22 MeV and the depth of measurement is 1 cm (dmax/2).
are normalized to the maximum dose on the measurement axis. The results of the measured data are shown in Figure 6. The central axis depth dose data for the flat end cone is almost identical to the data for the manufacturer supplied treatment applicators with inserts of same field size. The central axis depth dose change significantly for small cone size and energies greater than 15 MeV. The depth dose coverage with bevel ended cones is less than that with flat ended cones. The measured surface doses increase from 82-96% with increase in electron energy from 4 MeV-22 MeV. The surface dose does not change significantly with the bevel end. Adding a brass mesh (.28 mm diameter wire; 5 squares/8 mm) at the end of the IORT cone increases the surface dose from 82% to 93% for 4 MeV. However, the penetration is altered slightly. The depth dose coverage at 90% dose level decreased by 2-3 mm. The presence of the brass mesh has little effect at higher energies. At TJUH, we have tabulated the 90%, 80%, 50%, and 10% depth coverage and surface dose for each cone at all energies. We have found these values to be very useful in arriving at the appropriate electron beam energy for the specific treatment as soon as the target depth is known. Lateral therapeutic coverage The design of an applicator system influences the shape of isodose curves. Normally, for a lucite treatment cone there are high dose areas laterally at shallow depths. The
415
Intraoperative electron beam applicators 0 J. R. PALTA AND N. SUNTHARALINGAM (3
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Fig. 6. Percent depth dose for all electron beams for the 7 cm inside diameter intraoperative cone: (a) Flat end; (b) 15’ Bevel end; (c) 30” Bevel end.
improvement in the lateral coverage is always associated with high dose areas at the periphery. The magnitude of the high dose at the periphery can be as high as 20% for 22 MeV electron beam through a 7 cm diameter cone. A more meaningful evaluation of lateral coverage of intraoperative beams is to compare the isodose curves obtained with standard electron beams of same field sizes; such a comparison is shown in Figure 8. The lateral coverage of isodose lines in both cases is very similar. The lateral coverage with IORT cones is typically within 1 cm of the inside diameter of the cone with the usual rounding of the isodose curves at the periphery. However, if a clinical situation warrants an increased lateral coverage, the steel ring can be removed.
dorm (cm)
increased fluence of electrons at the periphery of the cone produces much flatter isodose curves, thus improving lateral coverage (the distance between 90% or 80% points) at therapeutic depths. This is obvious from a comparison of the therapeutic isodose lines (Fig. 7), with and without the steel ring, in the designed applicators. However, this
Cone leakage The dose outside the treatment cone is clinically important in intraoperative radiation therapy because a large dose is delivered to the tumor in a single fraction. It is not only important to determine leakage dose at the end of the cones, but also along the wall of the cones. Invariably, normal tissues extend up to some height along the walls of the cone. Figure 9 shows the radiation leakage measured, in air and in water, at different depths perpendicular to the 10 cm diameter cone wall for 22 MeV electron beam, which is representative of the worst leakage situation. The leakage radiation in the region surrounding the bottom half of the cone is less than lo%, but can be
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I. J. Radiation Oncology 0 Biology 0 Physics
August 1989, Volume 17, Number 2 0 MeV
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Fig. 7. Comparison of therapeutic isodose curves (90% and 80%) for 22 MeV and 8 MeV electron beams for the 7 cm diameter intraoperative cone.
as high as 22% in regions close to the wall at the upper part of the cone. The radiation leakage through the brass collimator, which is primarily due to bremmstrahlung photons, is always less than 3% for all cones. At TJUH, much effort is taken to retract normal tissues away from the treatment cone to decrease the radiation dose to the normal tissue. SUMMARY
Though the concept of intraoperative radiation therapy is relatively simple, the technical complexity of this procedure requires a careful evaluation and appropriate modification of the conventional dosimetric methods used in electron beam therapy. A special cone arrangement is
22 MeV electron
needed to direct radiation into the surgical opening that exposes the tumor or tumor bed. The cone serves the primary purpose of collimation of the electron beam and additionally directs the beam to a well defined target. Sometimes, it also helps in retracting normal tissues away from the treatment field. The dosimetric characteristics of an IORT applicator system depend largely on the characteristics of the electron beams generated in the treatment head of a particular linear accelerator. Therefore, similar applicator systems used with different accelerators will have different beam characteristics. The design, shape, and size of each intraoperative cone affects the following dosimetric characteristics: (a) depth dose and surface dose; (b) X ray contamination; (c) dose output per monitor unit; (d) flatness and
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Fig. 8. Comparison of isodose curves for a 7 cm diameter intraoperative cm square applicator. The broken lines are for 7 cm cutout.
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Intraoperative electron beam applicators 0 J. R. PALTA AND N. SUNTHARALINGAM
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Fig. 9. Leakage radiation through the walls of the 10 cm inside diameter cone for 22 MeV electron beam. Data are normalized to the maximum dose in water on the central axis of the cone at 100 cm SSD. The depths are the distances from the top of the cone where the dose scans are measured: (a) Dose scans measured in air; (b) Dose
scans measured in water. The water level is up to 4 cm from the top of the cone.
symmetry; (e) leakage radiation through the cone walls; and (f) shape of the isodose distribution. Therefore, all intraoperative cones must be evaluated for these characteristics. McCulluough and Biggs (7) have given a good overview of the dosimetric considerations for electron beam intraoperative radiation therapy. The IORT applicators designed for use at TJUH provide optimal beam characteristics with our linear acceleratoTs.* This system has the following features: (a) a nondocking system which minimizes the concerns of mechanical and electrical accidents. It also provides a clear visualization of the irradiation area; (b) the high dose areas in the periphery of the fields are removed using a steel ring without compromising the lateral coverage. The constriction of higher isodose lines as compared to the conventional electron fields is similar; (c)the surface dose for low energy beams is improved marginally because of large angle scattering of electrons from the steel ring; and (d) the alignment bf the cones with the gantry is quick and
precise with the modified OR table. The design of the collimation system for the intraoperative cones is such that applicators of different cross sectional shapes, namely, squares, rectangulars, and “squircles” (3) can be conveniently constructed with optimized beam characteristics. The only modification will be in the downstream removable brass collimation piece. The collimator aperture will have the same shape as the intraoperative cone. Its size will be 1 cm larger on all sides to ensure that a uniform intensity of electrons is incident on the cone. The intraoperative radiation therapy program at TJUH with the non-docking applicator has been successful. The set-up of the patient for treatment is precise and quick. The intraoperative radiation therapy team of radiation oncologists, physicists, surgeons, anesthesiologists, and nursing staff has expressed satisfaction with the whole procedure. Further work is continuing to design applicators with different cross-sectional shapes for various clinical sites.
REFERENCES
Bagne, F. R.; Samsani, N.; Dobelbower, R. R. Radiation contamination and leakage assessment of intraoperative electron applicators. Med. Phys. 15:530-537; 1988. Biggs,P. J.; Epp, E. R.; Ling, C. C.; Novack, D. H.; Michaels, H. B. Dosimetry, field shaping and other considerations for Intra-operative electron therapy. Int. J. Radiat. Oncol. Biol. Phys. 7:875-884; 1981. Fraas, B. A.; Miller, R. W.; Kinsella, T. J.; Sindelar, W. F.; Harrington, F. S.; Yeakel, K.; Van de Geijn, J.; Glatstein, E. Intraoperative radiation therapy at the National Cancer Institute: technical innovations and dosimetry. Int. J. Radiat. Oncol. Biol. Phys. 11:1299-1311; 1985.
Hogstrom, K. R.; Boyer, A. L.; Kirsner, S. M.; Shui, A. S. Design of metallic electron beam cones for an intraoperative therapy Iinac (Abstract). Med. Phys. 14:484; 1987. Kao, M.; Lanzl, L.; Rosenfeld, M.; Pagnamenta, A. Dose uniformity of the intraoperative radiation therapy cone (Abstract). Med. Phys. 13:605; 1986. McCullough, E. C.; Anderson, J. A. The dosimetric prop-
erties of an applicator system for intraoperative therapy utilizing a Clinac 18 accelerator. Med. Phys. 9: 161- 168; 1982. McCullough, E. C.; Biggs,P. J. Intraoperative electron beam radiation therapy. AAPM Monograph 15, American Institute of Physics; 1986:333-347.