vision 20∕20: proton therapy
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Vision 20 20 : Proton therapyAlfred R. Smith Citation: Medical Physics 36, 556 (2009); doi: 10.1118/1.3058485 View online: http://dx.doi.org/10.1118/1.3058485 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/36/2?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Effectiveness of robust optimization in intensity-modulated proton therapy planning for head and neck cancers Med. Phys. 40, 051711 (2013); 10.1118/1.4801899 Feasibility of Using Laser Ion Accelerators in Proton Therapy AIP Conf. Proc. 740, 414 (2004); 10.1063/1.1843524 Treatment planning and verification of proton therapy using spot scanning: Initial experiences Med. Phys. 31, 3150 (2004); 10.1118/1.1779371 Dosimetry for ocular proton beam therapy at the Harvard Cyclotron Laboratory based on the ICRU Report 59 Med. Phys. 29, 1953 (2002); 10.1118/1.1487425 Advances in charged particle therapy AIP Conf. Proc. 610, 157 (2002); 10.1063/1.1469926
Vision 20/20: Proton therapyAlfred R. Smitha
Department of Radiation Oncology, The University of Texas M. D. Anderson Cancer Center,1515 Holcombe Boulevard, Houston, Texas 77030
Received 6 August 2008; revised 5 December 2008; accepted for publication 8 December 2008;published 26 January 2009
The first patients were treated with proton beams in 1955 at the Lawrence Berkeley Laboratory inCalifornia. In 1970, proton beams began to be used in research facilities to treat cancer patientsusing fractionated treatment regimens. It was not until 1990 that proton treatments were carried outin hospital-based facilities using technology and techniques that were comparable to those formodern photon therapy. Clinical data strongly support the conclusion that proton therapy is superiorto conventional radiation therapy in a number of disease sites. Treatment planning studies haveshown that proton dose distributions are superior to those for photons in a wide range of diseasesites indicating that additional clinical gains can be achieved if these treatment plans can be reliablydelivered to patients. Optimum proton dose distributions can be achieved with intensity modulatedprotons IMPT, but very few patients have received this advanced form of treatment. It is antici-pated widespread implementation of IMPT would provide additional improvements in clinicaloutcomes. Advances in the last decade have led to an increased interest in proton therapy. Currently,proton therapy is undergoing transitions that will move it into the mainstream of cancer treatment.For example, proton therapy is now reimbursed, there has been rapid development in proton therapytechnology, and many new options are available for equipment, facility configuration, and financ-ing. During the next decade, new developments will increase the efficiency and accuracy of protontherapy and enhance our ability to verify treatment planning calculations and perform qualityassurance for proton therapy delivery. With the implementation of new multi-institution clinicalstudies and the routine availability of IMPT, it may be possible, within the next decade, to quantifythe clinical gains obtained from optimized proton therapy. During this same period several newproton therapy facilities will be built and the cost of proton therapy is expected to decrease, makingproton therapy routinely available to a larger population of cancer patients. 2009 AmericanAssociation of Physicists in Medicine. DOI: 10.1118/1.3058485
Key words: proton therapy, particle therapy, hadron therapy, radiation oncology
I. ABBREVIATED HISTORY OF PROTON THERAPY
E. Rutherford proposed the existence of protons in 1919;1
however, it was not until 1955 that the first patients weretreated with proton beams at the Lawrence Berkeley Labo-ratory LBL in California.2 In the years between 1919 and1955, two events occurred that had an important impact onproton therapy. First, in 1930, E. O. Lawrence built the firstcyclotron, paving the way for future particle acceleratorswith energies high enough for cancer treatment applications.In 1946, Robert Wilson then proposed that because of theirphysical properties, beams of protons would be advanta-geous for the treatment of deep-seated cancers stating, Itwill be easy to produce well collimated narrow beams of fastprotons, and since the range of the beam is easily control-lable, precision exposure of well defined small volumeswithin the body will soon be feasible.3 Wilson also de-scribed the use of a rotating wheel of variable thickness, i.e.,a range modulation wheel RMW, interposed in a protonbeam as a method of adding together several Bragg peaks ofvariable energies and weights in order to spread the protonstopping region in depth to deliver a uniform dose to anextended target volume. Figure 1 illustrates the concepts pro-posed by Wilson.
Further progress in proton therapy was rather slow in the35 years following the first patient treatments at LBL. Dur-ing this time, patients were treated in a few research facili-ties, notably in the United States, Sweden, and Russia, wherethe methods and techniques for proton therapy were furtherrefined and new technology was developed. The clinical re-sults obtained in these research facilities demonstrated thefeasibility and efficacy of proton therapy. In 1990, the mod-ern era of particle therapy then began when the first hospital-based proton therapy facility was opened at the Loma LindaUniversity Medical Center LLUMC in California.4 Nowthere are approximately 25 facilities around the world treat-ing cancer patients with proton beams with more than 55,000patients treated.5
II. PROTON BEAM DELIVERY TECHNIQUES
In order for the reader to understand the material in thefollowing sections, it is necessary to understand the variousmethods used to deliver proton radiation therapy, which fallinto two general categories: passive scattering and spot scan-ning also called pencil beam scanning techniques. A basicoverview of the techniques is given here; for further reading,good general descriptions can be found in the literature.6,7
556 556Med. Phys. 36 2, February 2009 0094-2405/2009/362/556/13/$25.00 2009 Am. Assoc. Phys. Med.
The passive scattering technique uses scattering devicesin the treatment delivery nozzle usually double scatterers forlarge fields and single scatterers for small e.g., radiosurgeryfields to spread the beam laterally and an RMW or ridgefilter7 to create a spread-out-Bragg peak SOBP in the targetvolume. Between the nozzle exit and the patient surface, atreatment field-specific collimator is used to shape the fieldlaterally to conform to the maximum beams-eye-view extentof the target volume and a range compensator is used tocorrect for patient surface irregularities, density heterogene-ities in the beam path, and changes in the shape of the distaltarget volume surface. The size of the SOBP is chosen tocover the greatest extent in depth of the target volume. TheSOBP size is constant over the entire target volume; there-fore, in general, there is some pull back of the high doseregion into normal tissues proximal to the target volumeFig. 2. Because several treatment fields are usually used,
each directed from a different angle, this high dose pull backinto normal tissue is not additive over all fields. In principle,the use of an RMW to obtain an SOBP in passive scatteringtechniques modulates both energy and beam intensity. How-ever, this should not be confused with intensity modulatedproton treatment IMPT achieved with beam scanning tech-niques as described below.
The second type of beam delivery utilizes spot scanningtechniques also often called pencil beam scanning. Figure 3illustrates how a target can be scanned by placing Braggpeaks throughout the volume by the use of scanning magnetsand energy changes. Energy changes in scanning techniquescan be carried out with various methods including: 1 en-ergy changes in the accelerator when a synchrotron is used;2 energy changes made with an energy selection systemwhen a cyclotron is used; or 3 either of the above methodsplus energy absorbers in the treatment nozzle.
15 MV Photonsvs.
Photons FIG. 1. Comparison between the depth dose curves for15 MV photons and a proton spread-out-Bragg peakSOBP. A target volume is shown in red. Shown alsoin red lines is an ideal dose distribution for the targetvolume, which provides uniform, maximum dose to thetarget volume and zero dose outside the target volume.The proton dose distribution approaches the ideal caseto a much greater extent than does the photon dosedistribution. Notably, the proton dose stops abruptlydistal to the target volume and delivers less dose to theregion proximal to the target volume.