The sharp lateral penumbra and the rapid fall-off of dose at the end of range of a proton beam are among the major advantages of proton radiation therapy. These beam characteristics depend on the position and characteristics of upstream beam-modifying devices such as apertures and compensating boluses. The extent of separation, if any, between these beam-modifying devices and the patient is particularly critical in this respect. We have developed a pencil beam algorithm for proton dose calculations which takes accurate account of the effects of materials upstream of the patient and of the air gap between them and the patient. The model includes a new approach to picking the locations of the pencil beams so as to more accurately model the penumbra and to more effectively account for the multiple-scattering effects of the media around the point of interest. We also present a faster broad-beam version of the algorithm which gives a reasonably accurate penumbra. Predictions of the algorithm and results from experiments performed in a large-field proton beam are presented. In general the algorithm agrees well with the measurements.

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The dose distribution of a proton beam stopping in water has components due to basic physics and may have others from beam contamination. We propose the concise terms core for the primary beam, halo (cf. Pedroni et al. [1]) for the low dose region from charged secondaries, aura for the low dose region from neutrals, and spray for beam contamination.We have measured the dose distribution in a water tank at 177 MeV under conditions where spray, therefore radial asymmetry, is negligible. We used an ADCL calibrated thimble chamber and a Faraday cup calibrated integral beam monitor so as to obtain immediately the absolute dose per proton. We took depth scans at fixed distances from the beam centroid rather than radial scans at fixed depths. That minimizes the signal range for each scan and better reveals the structure of the core and halo.Transitions from core to halo to aura are already discernible in the raw data. The halo has components attributable to coherent and incoherent nuclear reactions. Due to elastic and inelastic scattering by the nuclear force, the Bragg peak persists to radii larger than can be accounted for by Molière single scattering. The radius of the incoherent component, a dose bump around midrange, agrees with the kinematics of knockout reactions.We have fitted the data in two ways. The first is algebraic or model dependent (MD) as far as possible, and has 25 parameters. The second, using 2D cubic spline regression, is model independent (MI). Optimal parameterization for treatment planning will probably be a hybrid of the two, and will of course require measurements at several incident energies.The MD fit to the core term resembles that of the PSI group [1], which has been widely emulated. However, we replace their T (w), a mass stopping power which mixes electromagnetic (EM) and nuclear effects, with one that is purely EM, arguing that protons that do not undergo hard single scatters continue to lose energy according to the Beth-Bloch formula. If that is correct, it is no longer necessary to measure T (w), and the dominant role played by the 'Bragg peak chamber' (BPW) vanishes.For mathematical and other details we will refer to [2], a long technical report of this project.

Whether an accurate formula for T is required depends very much on the problem at hand. For beam spreading in water, five of the six formulas for T give almost identical results, suggesting that patient dose calculations are insensitive to T. That is not true, however, of beam spreading in Pb. At the opposite extreme, the projected rms beam width at the end of a Pb/Lexan/air stack, analogous to the upstream modulator in a passive beam spreading system, is sensitive to T. In this case a simple experiment would discriminate between all but two of the six formulas discussed. Scattering power applies as much to Monte Carlo as to deterministic transport calculations. Using T in any of its forms will avoid step size dependence. Using the best available T could be important in general purpose Monte Carlo codes, which are expected to give the correct answer to many different problems.

Extremely high-dose-rate irradiation, referred to as FLASH, has been shown to be less damaging to normal tissues than the same dose administrated at conventional dose rates. These results, typically seen at dose rates exceeding 40 Gy/s (or 2,400 Gy/min), have been widely reported in studies utilizing photon or electron radiation as well as in some proton radiation studies. Here, we report the development of a proton irradiation platform in a clinical proton facility and the dosimetry methods developed. The target is placed in the entry plateau region of a proton beam with a specifically designed double-scattering system. The energy after the double-scattering system is 227.5 MeV for protons that pass through only the first scatterer, and 225.5 MeV for those that also pass through the second scatterer. The double-scattering system was optimized to deliver a homogeneous dose distribution to a field size as large as possible while keeping the dose rate .100 Gy/s and not exceeding a cyclotron current of 300 nA. We were able to obtain a collimated pencil beam (1.6 3 1.2 cm 2 ellipse) at a dose rate of ;120 Gy/s. This beam was used for dose-response studies of partial abdominal irradiation of mice. First results indicate a potential tissuesparing effect of FLASH.