Clinical Proton Dosimetry Part I: Beam Production, Beam Delivery and Measurement of Absorbed Dose (ICRU Report 59)

International Commission on Radiation Units and Measurements Bethesda, MD: ICRU (1998) 60pp, ISBN: 0-913394-58-0 Proton therapy is a small but effective branch of conformal radiotherapy. New facilities, particularly with deeply penetrating proton beams, are being set up which build on the successful experience of existing proton centres. Thus, the need for this ICRU report, to provide an internationally acceptable dosimetry data set, is underlined. This, in turn, enables meaningful comparison of clinical results. To this end, the chapters within this report contain excellent reviews and clear descriptions. It may be useful to outline briefly the development of proton beams in therapy. The exploitation of the physical characteristics of proton beams, for the treatment of lesions, was suggested by the physicist Robert Wilson in 1946. The initial clinical application at Berkeley in 1954 was taken up at Uppsala and then at Harvard. In the UK, proton eye therapy commenced in 1989. The Bragg peak phenomenon, and the minimal scattering of protons, leads to the sharp lateral and posterior contours of the beam, quite unlike other therapy radiations. The Bragg peak can be 'scanned' in depth, by using a variable energy absorber, creating a constant dose region known as a modulated Bragg peak or a spread out Bragg peak (SOBP) as in this report. Unlike in conventional radiotherapy, the use of compensators or wedges alters the proton range and not the relative dose. The rationale for the clinical use of proton therapy rests on the application of these characteristics, which enables tumours to be treated with a high lethal dose. This in turn gives good local tumour control whilst minimizing the dose to adjacent, critical organs. The maximum tumour dose, when provided by other radiations, may be limited by the dose tolerated by healthy surrounding tissue. Protons beams can be considered an almost `tailor-made' form of radiation, where parameters, such as collimator shape, range and modulation, are optimized to conform the radiation to the lesion for each individual patient treatment. Chapter 1 of the report is eminently readable. It develops the rationale for using protons in clinical therapy, and mentions the clinical results that encouraged further development of this modality. The history section explains the early development of this therapy at different centres, with the technical improvements needed to attain varying clinical objectives. It's interesting to note the tendency of building newer facilities at hospital sites rather than at nuclear research centres. Chapter 2 describes the different particle accelerator machines that can be used to produce proton therapy beams. The choice of machine and beam producing techniques will correspond to local clinical requirements and economic constraints. Passive and dynamic scattering techniques used to produce beam flattening for a therapy beam would depend on beam duty cycle, available beam current and required field size. The newer spot (voxel) scanning techniques practised at PSI and GSI in Europe are described. These offer an extra element of proton dose conformity to the tumour and are, in effect, intensity modulated proton beams. Chapter 3 reviews dose definitions, including absorbed dose, lineal energy (due to the track structure of protons) and Cema, a heavy charged particle equivalent of Kerma. In the case of protons, the latter two units should be almost identical due to the short range of secondary electrons. The interaction of protons in matter is reviewed in chapter 4, where the factors affecting proton beams are concisely summarized. Chapters 5 and 6 describe absolute and relative dosimetry methods and contain extensive reviews of previous works, which reflect the experiences of the early pioneering proton treatment centres. Previously, many centres had evolved their own techniques according to local circumstances. Problems dealing with the dosimetry and monitoring of pulsed proton beams are discussed at length. The predominant method of absorbed dose measurement is based on ion chamber comparison in a 60 Co beam, with a reference chamber which is traceable to a national standards laboratory. A detailed review is given on proton values of W ( E ) and w ( E ) for ion pair production in gases, which leads to the recommended value of 34.8 J C -1 in air. The value seems a compromise between previous values and more recent obtained from calorimetry. It is not stated explicitly how this was obtained. The selection of suitable values of stopping power for polyenergetic, modulated beams is discussed but not explained. ICRU 49 data on stopping powers for relevant materials are usefully included. The limitations in proton dose linearity of conventional methods such as x-ray film, diodes, alanine and TLDs are discussed. Detailed description of obtaining absorbed dose from proton dose fluence measurement with Faraday cups is given; however it is known that this simple method is subject to significant uncertainties. The use of water or graphite calorimetry, although desirable, is not recommended as a reference standard due to difficulty of use and lack of availability. Chapter 7 summarizes the recommendations for physical parameters, measurement techniques and beam conditions for obtaining absorbed proton dose. This specifies proton dose absorbed to water and recommends using air-filled ion chambers. Additionally, inter-facility dose comparisons are recommended and, if possible, radiobiological or micro-dosimetrical measurements are encouraged, to ascertain differences in local beam characteristics. In the same vein, the suggestions for QA should correspond to local circumstances and beam conditions. Little prominence, however, is given to the low energy (i.e. <70 MeV) beams used in eye therapy, which represent the majority of proton therapy activity. Little information on small field area effects is offered and none on the use of wedges or boluses. No recommendations are made on the use of flat, parallel-plate ion chambers, which are widely used for shallow treatment depths or narrow SOBPs. Whilst nuclear effects may augment the apparent surface dose, especially at high proton energies, there is evidence to suggest that the apparent increase in dose effectiveness at the Bragg peak is due to increased ionization density effects, caused by very short range delta rays. This may be expanded upon in Part II of this report. The understandable, but long, lead-time to publication has led to the references, which are extensive, ceasing at 1995. A number are from conferences, and may be difficult to obtain, which reflects the pace of development of this small field. Recent work on 2D and 3D MR visualization of proton isodoses using polymer gels has thus not been included. Perhaps an occasion has been missed to regularize the ion chamber factor definitions with those used in IAEA dosimetry handbooks. The report states that it will fulfil a need for dose specification uniformity between growing numbers of proton therapy centres in the world, and does so by recommending factors and techniques. This report amply fulfils its title and is already in use at many centres. It also provides an informative overview of the subject for those interested, but not involved, in this field, of the technical aspects of this type of conformal radiotherapy. A fine balance between readability and scientific detail has been maintained.