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A proposed proton therapy facility at the SSC
NIOMI B
Nuclear Instruments and Methods in Physics Research B79 (1993) 895497
North-Holland
Beam Intaractions with Materials&Atoms
A proposed proton therapy facility at the SSC * B.A. Prichard, Jr. Superconducting Super Collider Laboratory, Dallas, Texas, USA
A conceptual design study of a proton therapy facility at the SSC has been performed. Participants included Lawrence Berkeley Laboratory, the Particle Accelerator Corporation, Aguirre Associates Inc., and the SSCL. The effort was conducted in two phases. The first phase involved a series of trade studies where three alternative approaches to beam formation were examined. They were: 1) using the SW linac beam directly with intensity reduction techniques; 2) using a pulse stretcher ring attached to the linac; and 3) using a small synchrotron attached to the linac. The relative costs of the three approaches were evaluated and the program flexibility of each was examined. Such factors as implications on nozzle design, dose rate adjustability, perturbation on the linac operation, potential radioisotope production, possibilities for B-N capture therapy, and proton radiography were weighed. Additionally, such features as gantries, vertical and horizontal beams, and variable collimators were reviewed. The second phase of the study focused in detail on using the linac beam directly, one fixed beam room with intersecting vertical and horizontal beams, one gantry room, and a radioisotope production facility. The results of these studies are presented.
1. Introduction
Southwestern Medical Center has requested an investigation into the feasibility and cost of establishing a proton therapy facility at the Superconducting Super Collider (SSC). The availability of proton beams of suitable energy and more than adequate intensity at the end of the SSC linear accelerator (linac) offers an attractive opportunity for the construction of a medical facility for radiation therapy. The therapy facility would parasitically use the proton beam accelerated by the linac of the SSC. Their request is motivated by the superior dose localization of protons [1] over that realized in conventional radiation therapy.
2. Trade studies
The studies focused on three options, all of which used the SSC linac as a source of protons. The system options examined were: 1) drift 70-250 MeV beam out of the full length of the linac and inject into a storage/pulse stretcher ring; 2) drift 70-250 MeV beam out of the full length of the linac and use intensity reduction techniques; and 3) drift 70 MeV beam out of the full length of the linac and inject into a small synchrotron. Each of these approaches offered differing benefits and drawbacks. Similarly, each of the three accelerator
* Study supported by The University of Texas Southwestern Medical Center at Dallas. 0168-583X/93/$06.00
system options implied different treatment delivery options. Therefore a close coordination of accelerator concepts and treatment delivery hardware was necessary. After examining a variety of potential solutions, it was decided that the most beneficial approach was to treat patients with the linac beam directly, after passing the beam through a series of devices to appropriately reduce its intensity. This approach provides a treatment time of less than 2 min and dose controllability a factor of ten below the desired dose uniformity. Dose control requires a system time response of only 0.1 s, which is easily achievable. The study concluded that all three alternatives were technically feasible and that all three could be made safe against accidental overdose to patients. It was also concluded that all three alternatives could provide beam to any treatment arrangement desired by the physicians. The deciding factor was that, as far as the hardware was concerned, alternative 2) was by far the least costly (see table 1).
3. Description of the design The therapy facility features two treatment rooms with associated diagnostic rooms, a radioisotope production room and positron emission tomography (PET) scanning capabilities. One treatment room houses a gantry which can deliver treatment at any angle between + 185” and - 185”. The second treatment room employs a vertical and a horizontal beam which intersect at a point in space called the isocenter. Included
0 1993 - Elsevier Science Publishers B.V. All rights reserved
XVII. RADIATION THERAPY
896
EA. Prichard, Jr. / Proton therapy facility at the SSC
Table 1 Trade study summary
Accelerator system costs Beam scattering Beam scanning Dose rate adjustability Perturbation on linac operation Radioisotope production B-N capture therapy
Proton radiography
Stretcher ring
Linac
Synchrotron
%12.6M yes yes awkward
$3.8M yes no facile
$12.7M yes yes awkward
moderate low yield
more
less
low yield low yield marginal marginal marginal yes yes yes
are all elements of the treatment facility that are needed to allow full stand-alone operation, including in addition to beam- and patient-support functions, ancillary needs such as treatment planning systems, shops for fabrication of collimators, immobilization devices, and control areas. During normal operation, the linac accelerates a beam of pulse length variable from 2 to 35 ps to an energy of 600 MeV at lo-Hz repetition rate. It has been shown, using particle-dynamics computer programs, that it is possible to drift a lower-energy beam through higher-energy modules in the coupled-cavity section of the linac, which are not excited with rf power during the passage of the beam. However, it is necessary to lower the excitation of the focusing quad~poles between the modules for the lower-energy beam if the beam is to remain well-enough focused to pass through the small beam apertures of the modules without loss. It is presently planned to construct the Iinac quadrupole magnets of laminated steel so that they can be rapidly pulsed to lower values (in 0.1 s) to accommodate lower-energy beams. This plan will allow the discrete intermediate energies of 70, 110, 157, 210, 267.5 and 329 MeV to be accelerated and drifted to the high-energy end of the Iinac. Even though 250 MeV is expected to be the maximum energy needed for therapy, the treatmentroom system is designed for 300 MeV for possible use in proton radiography. A 70-MeV beam, used directly for isotope production, was included in the study, This feature may be deleted at a later date. Bringing the proton beam into the patient is the primary mission of the facility. The design of the beam ports or “nozzles” as they are sometimes called, has evolved [2,3] over many years at many facilities around the world. The length of the beam port (the distance from the last bending magnet to the patient) is a critical parameter, and has a strong impact on the size of the treatment room and the gantry. At present, the optimum length for a proton beam port is 3 m. One would like longer ports for producing larger fields, but
the impact on increased gantry size drives one to minimize the port length. The 3 m length is obviously a compromise, but is a distance over which a 20 x 20 cm field can be produced with a well-understood compound scattering system. This is also the port length employed at Loma Linda [4,5], which provides a good design example. ISvo main constituents of these beam ports are the field-forming systems, producing the large, uniform fields required for therapy, and the dosimetry and safety systems verifying that the proper dose is given to the patient in an accurate, error-free manner. It is in this area that the impact of beam characteristics from the accelerator are most strongly felt, constraining the options available for system designs. Measurement f6] of the dose to the patient, verification that proper field uniformity is being delivered, and cutoff of the beam when the full dose has been delivered are crucial elements of the treatment delivery systems. The devices required for this include beam monitoring instrumentation, proper readout devices, and a highly sophisticated control system. A system to provide the necessary beam attenuation is placed between the Ql,Q2 doublet and the Q3,Q4 doublet in the linac-LEB transfer line. A bump magnet bends the H- beam 5.59” away from the transfer line beam axis to provide enough transverse space for a second bump magnet and longitudinal space for a device to neutralize about 10% of the beam. A cw laser is sufficient, and it has the great advantage that its power can be adjusted to regulate the fraction of beam neutralized. It is also fail-safe. The second bump magnet has a large enough aperture to contain both the deflected and undeflected beams. It bends the ‘Hbeam back toward the transfer line axis, while the Ho beam passes through the magnet undeflected. A third bump magnet bends the H- back onto the transfer line axiS.
A stripping foil is placed in the Ho beam beyond the second bump magnet to provide an H+ beam. This foif can be thick enough to stand the heat generated in it by the beam, because it need have no function of beam attenuation and can strip the entire beam. The foil stripper is also a mirror that reflects the laser beam in order to generate head-on collisions of the beam ions with the photons. The H’ beam is bent through 24.41” toward the treatment facility by a bending magnet, then focused or defocused onto adjustable horizontal and vertical collimators by a quadrupole doublet. The quadrnpoles and collimators are beyond the transfer line tunnel wall, so they are accessible during normal SSC operation. They provide a passive method of achieving the remaining factor of 10 attenuation. The laser stripper is designed to minimize the laser power, and create simple, tunable light paths. The
BA. prichard, Jr. / Proton therapy
simplest light paths are to collide the photon and Hbeams at either 0” (same direction) or 180” (head on). In the head-on geometry, the 1.16-eV fundamental of a Nd:YAG laser transforms to 1.7 eV in the rest frame of the beam, an energy that is still close to the maximum of the stripping cross section (a = 4 X 10-l’ cm’). Then for an interaction regiqn 2 m long, the required photon flux to strip 10% of the H- beam is Zp= 5.0 x lo= photons/s, corresponding to an instantaneous power of 93 kW. The average laser power is approximately 2 W. Thus the laser power is well within the limits of existing technology.
4. Summary The availability of proton beams of suitable energy and more than adequate intensity at the end of the SSC linac offers an attractive opportunity for the construction of medical facilities for radiation therapy. The study concludes that such a project is certainly merited, and although the most advanced beam deliv-
facilityat the SSC
897
ery concepts cannot be implemented because of the pulsed structure of the linac beam, treatment capabilities on a par with the best presently available in the world are possible. References 111 R.R. Wilson, Radiology 47 (19461487. [Z] W.T. Chu, S.B. Curtis, J. Lfacer, T.R. Renner and R.W. Sorenson, Proc. 1985 IEEE Particle Accelerator Conf., Vancouver, BC, Canada, 1985, IEEE Trans. Nucl. Sci. NS-32, part II (1985) 3321. [3] T.R. Renner, W. Chu, B. Ludewigt, J. Halliwell, M. Nyman, R.P. Singh, G.D. Stover and R. Stradtner, Proc. 1989 IEEE Particle Accelerator Conf., Chicago, IL, vol. 1, (19891 p. 672. 141J.M. Slater, D.W. Miller and 3.W. Slater, Proc. 1991 IEEE Particle Accelerator Conf., San Francisco, CA, vol. l(1991) p. 532. 151 B.A. Prichard, Jr., Proc. IISSC, New Orleans, Louisiana, 1989, p. 13. 161W. Chu, M. McEnvoy, M. Nyman, T. Renner, B. Gonzales, R.P. Singh and R. Stradner, Proc. 1985 IEEE Particle Accelerator, IEEE Trans. Nucl. Sci. NS-32, part II (1985) 3324.
XVII. RADIATION THERAPY
Nuclear Instruments and Methods in Physics Research B79 (1993) 895497
North-Holland
Beam Intaractions with Materials&Atoms
A proposed proton therapy facility at the SSC * B.A. Prichard, Jr. Superconducting Super Collider Laboratory, Dallas, Texas, USA
A conceptual design study of a proton therapy facility at the SSC has been performed. Participants included Lawrence Berkeley Laboratory, the Particle Accelerator Corporation, Aguirre Associates Inc., and the SSCL. The effort was conducted in two phases. The first phase involved a series of trade studies where three alternative approaches to beam formation were examined. They were: 1) using the SW linac beam directly with intensity reduction techniques; 2) using a pulse stretcher ring attached to the linac; and 3) using a small synchrotron attached to the linac. The relative costs of the three approaches were evaluated and the program flexibility of each was examined. Such factors as implications on nozzle design, dose rate adjustability, perturbation on the linac operation, potential radioisotope production, possibilities for B-N capture therapy, and proton radiography were weighed. Additionally, such features as gantries, vertical and horizontal beams, and variable collimators were reviewed. The second phase of the study focused in detail on using the linac beam directly, one fixed beam room with intersecting vertical and horizontal beams, one gantry room, and a radioisotope production facility. The results of these studies are presented.
1. Introduction
Southwestern Medical Center has requested an investigation into the feasibility and cost of establishing a proton therapy facility at the Superconducting Super Collider (SSC). The availability of proton beams of suitable energy and more than adequate intensity at the end of the SSC linear accelerator (linac) offers an attractive opportunity for the construction of a medical facility for radiation therapy. The therapy facility would parasitically use the proton beam accelerated by the linac of the SSC. Their request is motivated by the superior dose localization of protons [1] over that realized in conventional radiation therapy.
2. Trade studies
The studies focused on three options, all of which used the SSC linac as a source of protons. The system options examined were: 1) drift 70-250 MeV beam out of the full length of the linac and inject into a storage/pulse stretcher ring; 2) drift 70-250 MeV beam out of the full length of the linac and use intensity reduction techniques; and 3) drift 70 MeV beam out of the full length of the linac and inject into a small synchrotron. Each of these approaches offered differing benefits and drawbacks. Similarly, each of the three accelerator
* Study supported by The University of Texas Southwestern Medical Center at Dallas. 0168-583X/93/$06.00
system options implied different treatment delivery options. Therefore a close coordination of accelerator concepts and treatment delivery hardware was necessary. After examining a variety of potential solutions, it was decided that the most beneficial approach was to treat patients with the linac beam directly, after passing the beam through a series of devices to appropriately reduce its intensity. This approach provides a treatment time of less than 2 min and dose controllability a factor of ten below the desired dose uniformity. Dose control requires a system time response of only 0.1 s, which is easily achievable. The study concluded that all three alternatives were technically feasible and that all three could be made safe against accidental overdose to patients. It was also concluded that all three alternatives could provide beam to any treatment arrangement desired by the physicians. The deciding factor was that, as far as the hardware was concerned, alternative 2) was by far the least costly (see table 1).
3. Description of the design The therapy facility features two treatment rooms with associated diagnostic rooms, a radioisotope production room and positron emission tomography (PET) scanning capabilities. One treatment room houses a gantry which can deliver treatment at any angle between + 185” and - 185”. The second treatment room employs a vertical and a horizontal beam which intersect at a point in space called the isocenter. Included
0 1993 - Elsevier Science Publishers B.V. All rights reserved
XVII. RADIATION THERAPY
896
EA. Prichard, Jr. / Proton therapy facility at the SSC
Table 1 Trade study summary
Accelerator system costs Beam scattering Beam scanning Dose rate adjustability Perturbation on linac operation Radioisotope production B-N capture therapy
Proton radiography
Stretcher ring
Linac
Synchrotron
%12.6M yes yes awkward
$3.8M yes no facile
$12.7M yes yes awkward
moderate low yield
more
less
low yield low yield marginal marginal marginal yes yes yes
are all elements of the treatment facility that are needed to allow full stand-alone operation, including in addition to beam- and patient-support functions, ancillary needs such as treatment planning systems, shops for fabrication of collimators, immobilization devices, and control areas. During normal operation, the linac accelerates a beam of pulse length variable from 2 to 35 ps to an energy of 600 MeV at lo-Hz repetition rate. It has been shown, using particle-dynamics computer programs, that it is possible to drift a lower-energy beam through higher-energy modules in the coupled-cavity section of the linac, which are not excited with rf power during the passage of the beam. However, it is necessary to lower the excitation of the focusing quad~poles between the modules for the lower-energy beam if the beam is to remain well-enough focused to pass through the small beam apertures of the modules without loss. It is presently planned to construct the Iinac quadrupole magnets of laminated steel so that they can be rapidly pulsed to lower values (in 0.1 s) to accommodate lower-energy beams. This plan will allow the discrete intermediate energies of 70, 110, 157, 210, 267.5 and 329 MeV to be accelerated and drifted to the high-energy end of the Iinac. Even though 250 MeV is expected to be the maximum energy needed for therapy, the treatmentroom system is designed for 300 MeV for possible use in proton radiography. A 70-MeV beam, used directly for isotope production, was included in the study, This feature may be deleted at a later date. Bringing the proton beam into the patient is the primary mission of the facility. The design of the beam ports or “nozzles” as they are sometimes called, has evolved [2,3] over many years at many facilities around the world. The length of the beam port (the distance from the last bending magnet to the patient) is a critical parameter, and has a strong impact on the size of the treatment room and the gantry. At present, the optimum length for a proton beam port is 3 m. One would like longer ports for producing larger fields, but
the impact on increased gantry size drives one to minimize the port length. The 3 m length is obviously a compromise, but is a distance over which a 20 x 20 cm field can be produced with a well-understood compound scattering system. This is also the port length employed at Loma Linda [4,5], which provides a good design example. ISvo main constituents of these beam ports are the field-forming systems, producing the large, uniform fields required for therapy, and the dosimetry and safety systems verifying that the proper dose is given to the patient in an accurate, error-free manner. It is in this area that the impact of beam characteristics from the accelerator are most strongly felt, constraining the options available for system designs. Measurement f6] of the dose to the patient, verification that proper field uniformity is being delivered, and cutoff of the beam when the full dose has been delivered are crucial elements of the treatment delivery systems. The devices required for this include beam monitoring instrumentation, proper readout devices, and a highly sophisticated control system. A system to provide the necessary beam attenuation is placed between the Ql,Q2 doublet and the Q3,Q4 doublet in the linac-LEB transfer line. A bump magnet bends the H- beam 5.59” away from the transfer line beam axis to provide enough transverse space for a second bump magnet and longitudinal space for a device to neutralize about 10% of the beam. A cw laser is sufficient, and it has the great advantage that its power can be adjusted to regulate the fraction of beam neutralized. It is also fail-safe. The second bump magnet has a large enough aperture to contain both the deflected and undeflected beams. It bends the ‘Hbeam back toward the transfer line axis, while the Ho beam passes through the magnet undeflected. A third bump magnet bends the H- back onto the transfer line axiS.
A stripping foil is placed in the Ho beam beyond the second bump magnet to provide an H+ beam. This foif can be thick enough to stand the heat generated in it by the beam, because it need have no function of beam attenuation and can strip the entire beam. The foil stripper is also a mirror that reflects the laser beam in order to generate head-on collisions of the beam ions with the photons. The H’ beam is bent through 24.41” toward the treatment facility by a bending magnet, then focused or defocused onto adjustable horizontal and vertical collimators by a quadrupole doublet. The quadrnpoles and collimators are beyond the transfer line tunnel wall, so they are accessible during normal SSC operation. They provide a passive method of achieving the remaining factor of 10 attenuation. The laser stripper is designed to minimize the laser power, and create simple, tunable light paths. The
BA. prichard, Jr. / Proton therapy
simplest light paths are to collide the photon and Hbeams at either 0” (same direction) or 180” (head on). In the head-on geometry, the 1.16-eV fundamental of a Nd:YAG laser transforms to 1.7 eV in the rest frame of the beam, an energy that is still close to the maximum of the stripping cross section (a = 4 X 10-l’ cm’). Then for an interaction regiqn 2 m long, the required photon flux to strip 10% of the H- beam is Zp= 5.0 x lo= photons/s, corresponding to an instantaneous power of 93 kW. The average laser power is approximately 2 W. Thus the laser power is well within the limits of existing technology.
4. Summary The availability of proton beams of suitable energy and more than adequate intensity at the end of the SSC linac offers an attractive opportunity for the construction of medical facilities for radiation therapy. The study concludes that such a project is certainly merited, and although the most advanced beam deliv-
facilityat the SSC
897
ery concepts cannot be implemented because of the pulsed structure of the linac beam, treatment capabilities on a par with the best presently available in the world are possible. References 111 R.R. Wilson, Radiology 47 (19461487. [Z] W.T. Chu, S.B. Curtis, J. Lfacer, T.R. Renner and R.W. Sorenson, Proc. 1985 IEEE Particle Accelerator Conf., Vancouver, BC, Canada, 1985, IEEE Trans. Nucl. Sci. NS-32, part II (1985) 3321. [3] T.R. Renner, W. Chu, B. Ludewigt, J. Halliwell, M. Nyman, R.P. Singh, G.D. Stover and R. Stradtner, Proc. 1989 IEEE Particle Accelerator Conf., Chicago, IL, vol. 1, (19891 p. 672. 141J.M. Slater, D.W. Miller and 3.W. Slater, Proc. 1991 IEEE Particle Accelerator Conf., San Francisco, CA, vol. l(1991) p. 532. 151 B.A. Prichard, Jr., Proc. IISSC, New Orleans, Louisiana, 1989, p. 13. 161W. Chu, M. McEnvoy, M. Nyman, T. Renner, B. Gonzales, R.P. Singh and R. Stradner, Proc. 1985 IEEE Particle Accelerator, IEEE Trans. Nucl. Sci. NS-32, part II (1985) 3324.
XVII. RADIATION THERAPY