Radiotherapy and Oncology, 17 (1990) 17-20 Elsevier
17
RADION 00640
A horizontal proton beam line for the development of a scanning technique H. B l a t t m a n n and A. C o r a y Medical Project, Paul Scherrer Institute - PSI, formerly SIN CH-5234 Villigen, Switzerland
(Received 5 October 1988, revision received 6 April 1989, accepted 23 May 1989)
Key words: Proton therapy; Spot scan; Pencil beam scan; Conformationtherapy
Summary A project for the development of a proton spot scan technique for deep-seated tumours, based on the experience of 6 years pion conformation radiotherapy, developed at SIN, is discussed. A horizontal proton beam line for the development of techniques and for treatment is presented.
Introduction Protons of energies of approximately 200 MeV have favourable physical characteristics for radiotherapy of deep-seated tumours, as has been pointed out by Wilson [9] already in 1949. Due to their large mass (980 MeV) they have very little multiple scattering and range straggling and are therefore of special interest, where not only an excellent collimation is needed, but also where the distal fall-off is of equal importance. At a few places, mainly Boston and Moscow, high energy proton therapy has been in use for many years predominantly for small field irradiations close to critical structures, at the base of the skull and near to the spinal cord. Proton therapy could prove Address for correspondence: H. Blattmann, Ph.D., Medical
Project, Paul Scherrer Institute - PSI, CH-5232 Villigen, Switzerland.
beneficial for a wide spectrum of indications, especially if treatment facilities with isocentric gantries, producing protons having a large range would be available. Today a growing interest is obvious with the first project of an accelerator situated in a hospital [7].
Dynamic treatment with protons At SIN, other charged particles, pions, are used for treatment of patients for more than 6 years with dynamic, three dimensional spot scan technique [2,6], conforming the dose distribution to the target volume. Although the primary reason for the use ofpions was the favourable depth dose distribution combined with the increased relative biological efficiency in the stopping region due to the various charged particles emitted after pion capture in a target nucleus, the development of
0167-8140/90/$03.50 © 1990 Elsevier Science Publishers B.V. (BiomedicalDivision)
18 dose shaping was given a high priority. Due to this technique an advantageous ratio of integral normal tissue dose to target volume dose can be achieved. This is probably the most important factor why with the P I O T R O N it is possible to treat, for example, retroperitoneal masses of soft tissue and bone sarcoma to an adequate target absorbed dose without overdose to the normal tissues [3 ]. The limits of the treatment technique with the P I O T R O N is the spot size of 55 mm F W H M , related to the size of the waterbolus and hence the energy of the pions necessary to treat abdominal tumours. This spot size determines the fall-off of the dose distribution outside the target volume. A special problem in some cases is the lack of the possibility to select a sharper fall-off in any direction - at the expense of other directions - which is of importance if the target volume is close to critical organs. Proton treatment is, up to now, performed with large fields produced by passive scattering [5,8], with the exception of the 70 MeV proton project an NIRS [4] where dynamic treatment is used. With the large field technique, especially if only one or two beam angles are used, the protons stopping outside the target volume may lead to an unnecessary high dose in some areas of surrounding normal tissues. This can be avoided with a spot scanning technique, similar to the pion therapy technique developed at SIN, where the stopped particles are confined as much as possible to the target volume, and where the dose distribution is optimized with mathematical methods. This three dimensionally shaped dose distribution might be able to improve significantly the therapeutic ratio for specific cases. Calculations have been made with a modified version of the pion treatment planning software [6] for proton pencil beam scanning. The target volume is outlined on each of a series of CT-slices, and the dose distribution optimized to yield a homogeneous dose inside the target volume, falling outside according to the characteristics of the beam used. For proton therapy the dose distribution is optimized for each port individually; several ports can be overlayed for one target volume. In the example used
Fig. 1. Examples of dose distributions in a single plane of an excentrically positioned target volume, irradiated (a) in the piotron with a sector of 30 beams centred around a 45 ° angle from the right upper quadrant (b) by two hypotheticalproton beams, entering horizontally from the right and under 45 ° from the right upper quadrant. The isodose lines are 95, 85, 65, 45 and 25% of the target dose, respectively.
19 I
120
I
I
I
I
I
I
I
I
i
!
I
I
!
!
protons. The dose distribution in the transverse cross-section demonstrates, that even using only a sector of beams the dose gradient is only moderate in every direction for pions (Fig. la). In contrast, using two ports for protons, from similar directions as the pion beams, results in a sharp fall-off distally, and laterally, allowing for sparing adjacent normal structures (Fig. lb). A pronounced difference can be seen in the dose volume histograms for the two radiation sources for the spinal canal (Fig. 2). An additional advantage of the spot scanning technique is the likely reduced time for positioning of the patient, as no bolus material is necessary. This is of special importance when multiple beam techniques are used, and when it will be possible to switch from one beam to the next without repositioning. At the same time the errors due to misalignment of bolus and inhomogeneity are avoided. As the time for patient positioning is usually large compared to the time for irradiation, this is an important step in making proton therapy cost-efficient. An inherent problem of the spot scanning technique is the movement of the patient and the movement of the organs inside the patient during
100 80.-~
60-
-
E
> 40-
I
.
Protons
200
i
0
|
2b
4b'6b
8o
Ioo
12o
Dose [ ~ )
Fig. 2. Dose volume histograms for the spinal canal in the involved section for pion and proton spot scanning treatment, demonstratinga marked reductionof radiationburden for protons. for intercomparison, an excentric target volume has been chosen to illustrate the differences between spot scanning with pions in the geometry of the P I O T R O N and pencil beam scanning with
~% ~
IOTR
~ c _ - ~ - proton-therapy-PreJ ect "
"
experimental
area
1
5m
Fig. 3. Floor plan of the experimental hall housing the energy degraded protons beam line. Indicated is the experimental area for the set up of the proton scanning experiment. As for the PIOTRON, a beam of up to 20/zA split from the main proton beam will be used.
20 treatment. To avoid cold or hot regions, due to incorrect overlapping, the spot size would have to be adjusted in relation to the size of the treatment volume, the expected movements of the patient, organ or inhomogeneities anticipated during one fraction, depending on the dose homogeneity or dose fall-off to be yielded. A selection of different methods are available to increase the spot size as defocussing, increasing straggling and multiple scattering by chosing higher energy protons or by introducing a drift distance between range shifter and patient.
The proton project at PSI In the proton accelerator at PSI, protons are accelerated to 590 MeV. From the main beam of 150-250 #A up to 20 #A are split off, guided into a separate beam line serving a material science irradiation facility and a nucleon physics area (Fig. 3). Included in this beam line is an energy degrader to reduce the energy down to the medically interesting range of 160 to 250 MeV. The degraded beam which will be used for physics experiments in the future is used to test a fast pencil beam scanning on various phantoms. The spot scanning technique and wobbling in connection with a multileaf collimator will be explored. Radiosurgery, for irradiation of arteriovenous malformations and other small field treatments, as performed for many years in Boston [1 ], would be possible in a scattered beam.
Conclusion Common use of proton radiotherapy is certainly reserved for hospital-based accelerators with isocentric beam delivery systems to one or several
rooms. A medical division of a physics laboratory as the one of PSI can contribute to this development by evaluating and testing treatment techniques from physical, biological and oncological standpoints.
References 1 Austin-Seymour, M., Munzenrider, J.E., Goitein, M., Gentry, R., Gragoudas, E., Koehler, A. M., McNulty, P., Osborne, E., Ryugo, D. K., Seddon, J., Uric, M., Verhey, L. and Suit, H.D. Progress in low LET heavy particle therapy: intracranial and paracranial tumours and uveal melanomas. Radiat. Res. 104: S-219-S-226, 1985. 2 Blattmann, H., Pedroni, E., Crawford, J. and Salzmann, M. Treatment planning for dynamic therapy at SIN. In: Pion and Heavy Ion Radiotherapy, Pre-clinical and Clinical Studies, pp. 119-128, Editor: L.D. Skarsgard. Elsevier, New York, 1981. 3 Greiner, R.H., yon Essen, C.F., Blattmann, H.J., Pedroni, E., Studer, U. E., Thum, P. A. and Zimmermann, A. Pion therapy at Swiss Institute for Nuclear Research (SIN). Int. J. Radiat. Oncol. Biol. Phys. 12, Supp. 1: 98, 1986. 4 Kanai, T., Kawachi, K., Matsuzawa, H. and Inada, T. Three dimensional beam scanning for proton therapy. N.I.M. 214: 491-496, 1983. 5 Koehler, A.M., Schneider, R.J. and Sisterson, J.M. Flattening of proton close distributions for largefield radiotherapy. Med. Phys. 4: 297-301, 1977. 6 Pedroni, E., Blattmann, H., Salzmann, M., Walder, E., Crawford, J. F., Dietlicher, R., Cordt, J. Schaeppi, K., von Essen, C. F. and Perret, C. Treatment planning and dosimetry for the pi meson therapy facility at SIN. In: Proc. Int. Conf. on Application of Physics to Medicine and Biology, pp. 1-25. Editors: G. Alberi et al. World Scientific, Singapore, 1983. 7 Salter, J. M., Miller, D. W. and Archambeau, J.O. Development of a hospital-based proton treatment center. Int. J. Radiat. Oncol. Biol. Phys. 14: 761-775, 1988. 8 Tsunemoto, H., Morita, S., Ishikava, T., Furukawa, S., Kawachi, K., Kanai, T., Ohare, H., Kitagawa, T. and Inada, T. Proton therapy in Japan. Radiat. Res. 104: S-235-S-243, 1985. 9 Wilson, R.R. Radiological use of fast protons. Radiology 47: 487-491, 1946.
17
RADION 00640
A horizontal proton beam line for the development of a scanning technique H. B l a t t m a n n and A. C o r a y Medical Project, Paul Scherrer Institute - PSI, formerly SIN CH-5234 Villigen, Switzerland
(Received 5 October 1988, revision received 6 April 1989, accepted 23 May 1989)
Key words: Proton therapy; Spot scan; Pencil beam scan; Conformationtherapy
Summary A project for the development of a proton spot scan technique for deep-seated tumours, based on the experience of 6 years pion conformation radiotherapy, developed at SIN, is discussed. A horizontal proton beam line for the development of techniques and for treatment is presented.
Introduction Protons of energies of approximately 200 MeV have favourable physical characteristics for radiotherapy of deep-seated tumours, as has been pointed out by Wilson [9] already in 1949. Due to their large mass (980 MeV) they have very little multiple scattering and range straggling and are therefore of special interest, where not only an excellent collimation is needed, but also where the distal fall-off is of equal importance. At a few places, mainly Boston and Moscow, high energy proton therapy has been in use for many years predominantly for small field irradiations close to critical structures, at the base of the skull and near to the spinal cord. Proton therapy could prove Address for correspondence: H. Blattmann, Ph.D., Medical
Project, Paul Scherrer Institute - PSI, CH-5232 Villigen, Switzerland.
beneficial for a wide spectrum of indications, especially if treatment facilities with isocentric gantries, producing protons having a large range would be available. Today a growing interest is obvious with the first project of an accelerator situated in a hospital [7].
Dynamic treatment with protons At SIN, other charged particles, pions, are used for treatment of patients for more than 6 years with dynamic, three dimensional spot scan technique [2,6], conforming the dose distribution to the target volume. Although the primary reason for the use ofpions was the favourable depth dose distribution combined with the increased relative biological efficiency in the stopping region due to the various charged particles emitted after pion capture in a target nucleus, the development of
0167-8140/90/$03.50 © 1990 Elsevier Science Publishers B.V. (BiomedicalDivision)
18 dose shaping was given a high priority. Due to this technique an advantageous ratio of integral normal tissue dose to target volume dose can be achieved. This is probably the most important factor why with the P I O T R O N it is possible to treat, for example, retroperitoneal masses of soft tissue and bone sarcoma to an adequate target absorbed dose without overdose to the normal tissues [3 ]. The limits of the treatment technique with the P I O T R O N is the spot size of 55 mm F W H M , related to the size of the waterbolus and hence the energy of the pions necessary to treat abdominal tumours. This spot size determines the fall-off of the dose distribution outside the target volume. A special problem in some cases is the lack of the possibility to select a sharper fall-off in any direction - at the expense of other directions - which is of importance if the target volume is close to critical organs. Proton treatment is, up to now, performed with large fields produced by passive scattering [5,8], with the exception of the 70 MeV proton project an NIRS [4] where dynamic treatment is used. With the large field technique, especially if only one or two beam angles are used, the protons stopping outside the target volume may lead to an unnecessary high dose in some areas of surrounding normal tissues. This can be avoided with a spot scanning technique, similar to the pion therapy technique developed at SIN, where the stopped particles are confined as much as possible to the target volume, and where the dose distribution is optimized with mathematical methods. This three dimensionally shaped dose distribution might be able to improve significantly the therapeutic ratio for specific cases. Calculations have been made with a modified version of the pion treatment planning software [6] for proton pencil beam scanning. The target volume is outlined on each of a series of CT-slices, and the dose distribution optimized to yield a homogeneous dose inside the target volume, falling outside according to the characteristics of the beam used. For proton therapy the dose distribution is optimized for each port individually; several ports can be overlayed for one target volume. In the example used
Fig. 1. Examples of dose distributions in a single plane of an excentrically positioned target volume, irradiated (a) in the piotron with a sector of 30 beams centred around a 45 ° angle from the right upper quadrant (b) by two hypotheticalproton beams, entering horizontally from the right and under 45 ° from the right upper quadrant. The isodose lines are 95, 85, 65, 45 and 25% of the target dose, respectively.
19 I
120
I
I
I
I
I
I
I
I
i
!
I
I
!
!
protons. The dose distribution in the transverse cross-section demonstrates, that even using only a sector of beams the dose gradient is only moderate in every direction for pions (Fig. la). In contrast, using two ports for protons, from similar directions as the pion beams, results in a sharp fall-off distally, and laterally, allowing for sparing adjacent normal structures (Fig. lb). A pronounced difference can be seen in the dose volume histograms for the two radiation sources for the spinal canal (Fig. 2). An additional advantage of the spot scanning technique is the likely reduced time for positioning of the patient, as no bolus material is necessary. This is of special importance when multiple beam techniques are used, and when it will be possible to switch from one beam to the next without repositioning. At the same time the errors due to misalignment of bolus and inhomogeneity are avoided. As the time for patient positioning is usually large compared to the time for irradiation, this is an important step in making proton therapy cost-efficient. An inherent problem of the spot scanning technique is the movement of the patient and the movement of the organs inside the patient during
100 80.-~
60-
-
E
> 40-
I
.
Protons
200
i
0
|
2b
4b'6b
8o
Ioo
12o
Dose [ ~ )
Fig. 2. Dose volume histograms for the spinal canal in the involved section for pion and proton spot scanning treatment, demonstratinga marked reductionof radiationburden for protons. for intercomparison, an excentric target volume has been chosen to illustrate the differences between spot scanning with pions in the geometry of the P I O T R O N and pencil beam scanning with
~% ~
IOTR
~ c _ - ~ - proton-therapy-PreJ ect "
"
experimental
area
1
5m
Fig. 3. Floor plan of the experimental hall housing the energy degraded protons beam line. Indicated is the experimental area for the set up of the proton scanning experiment. As for the PIOTRON, a beam of up to 20/zA split from the main proton beam will be used.
20 treatment. To avoid cold or hot regions, due to incorrect overlapping, the spot size would have to be adjusted in relation to the size of the treatment volume, the expected movements of the patient, organ or inhomogeneities anticipated during one fraction, depending on the dose homogeneity or dose fall-off to be yielded. A selection of different methods are available to increase the spot size as defocussing, increasing straggling and multiple scattering by chosing higher energy protons or by introducing a drift distance between range shifter and patient.
The proton project at PSI In the proton accelerator at PSI, protons are accelerated to 590 MeV. From the main beam of 150-250 #A up to 20 #A are split off, guided into a separate beam line serving a material science irradiation facility and a nucleon physics area (Fig. 3). Included in this beam line is an energy degrader to reduce the energy down to the medically interesting range of 160 to 250 MeV. The degraded beam which will be used for physics experiments in the future is used to test a fast pencil beam scanning on various phantoms. The spot scanning technique and wobbling in connection with a multileaf collimator will be explored. Radiosurgery, for irradiation of arteriovenous malformations and other small field treatments, as performed for many years in Boston [1 ], would be possible in a scattered beam.
Conclusion Common use of proton radiotherapy is certainly reserved for hospital-based accelerators with isocentric beam delivery systems to one or several
rooms. A medical division of a physics laboratory as the one of PSI can contribute to this development by evaluating and testing treatment techniques from physical, biological and oncological standpoints.
References 1 Austin-Seymour, M., Munzenrider, J.E., Goitein, M., Gentry, R., Gragoudas, E., Koehler, A. M., McNulty, P., Osborne, E., Ryugo, D. K., Seddon, J., Uric, M., Verhey, L. and Suit, H.D. Progress in low LET heavy particle therapy: intracranial and paracranial tumours and uveal melanomas. Radiat. Res. 104: S-219-S-226, 1985. 2 Blattmann, H., Pedroni, E., Crawford, J. and Salzmann, M. Treatment planning for dynamic therapy at SIN. In: Pion and Heavy Ion Radiotherapy, Pre-clinical and Clinical Studies, pp. 119-128, Editor: L.D. Skarsgard. Elsevier, New York, 1981. 3 Greiner, R.H., yon Essen, C.F., Blattmann, H.J., Pedroni, E., Studer, U. E., Thum, P. A. and Zimmermann, A. Pion therapy at Swiss Institute for Nuclear Research (SIN). Int. J. Radiat. Oncol. Biol. Phys. 12, Supp. 1: 98, 1986. 4 Kanai, T., Kawachi, K., Matsuzawa, H. and Inada, T. Three dimensional beam scanning for proton therapy. N.I.M. 214: 491-496, 1983. 5 Koehler, A.M., Schneider, R.J. and Sisterson, J.M. Flattening of proton close distributions for largefield radiotherapy. Med. Phys. 4: 297-301, 1977. 6 Pedroni, E., Blattmann, H., Salzmann, M., Walder, E., Crawford, J. F., Dietlicher, R., Cordt, J. Schaeppi, K., von Essen, C. F. and Perret, C. Treatment planning and dosimetry for the pi meson therapy facility at SIN. In: Proc. Int. Conf. on Application of Physics to Medicine and Biology, pp. 1-25. Editors: G. Alberi et al. World Scientific, Singapore, 1983. 7 Salter, J. M., Miller, D. W. and Archambeau, J.O. Development of a hospital-based proton treatment center. Int. J. Radiat. Oncol. Biol. Phys. 14: 761-775, 1988. 8 Tsunemoto, H., Morita, S., Ishikava, T., Furukawa, S., Kawachi, K., Kanai, T., Ohare, H., Kitagawa, T. and Inada, T. Proton therapy in Japan. Radiat. Res. 104: S-235-S-243, 1985. 9 Wilson, R.R. Radiological use of fast protons. Radiology 47: 487-491, 1946.
