During the past six months, the goals of the research program have been to develop a reliable beam delivery system for the eventual treatment of cancer patients and to characterize the beam by irradiating mice and cells to measure the RBE and other biological effects. We have had two 200 MeV proton runs (both polarized proton runs by the way) scheduled at the beginning of November and the beginning of January. Two beam spreading systems were tested, producing fields of 15 cm and 10 cm, a range modulator design was developed to produce an 8 cm SOBP, various methods of beam delivery and diagnostics were used, and cell samples and mice were irradiated.
The beam spreading system was designed using a program obtained from Harvard and was written by B.Gottschalk. Beam passes through a primary scattering foil and after some drift distance is collimated by a double annulus with a second scattering foil whose dimensions are dependent on the field size, beam energy, and relative positions of the scattering foils and target. The program requires the positions of the 1st and 2nd scatterers, and the target, the required field size, and the percentage flatness. A typical beam delivery geometry is shown in Fig.1. The 200 MeV beam can lose as much as 15 MeV for a 15cm geometry after passing through the scattering foils. Fields of 10cm and 15cm have been produced to a flatness of better than 10% and a field of 10cm will be used in the March run.
The range modulator is designed by a program written by C.Bloch and M.Fasano. Bow shaped Lucite attenuators with successively increasing subtended angles are stacked and mounted on a motor that rotates the bows across the beam (Fig.2). The beam loses energy (range) depending on the thickness of Lucite it passes through. The program determines the fraction of beam required at each range to give an integrated dose that is flat to better than 5% over the SOBP. An 8cm range modulator has been tested and the results are shown in Fig.2. A 4cm and an 18cm range modulator have been constructed and will be tested during the March run.
In order to deliver accurate doses, reliable non-destructive beam diagnostics are necessary. Since typical beam currents are on the order of 1 nA or less, normal electrostatic pick ups are too insensitive. The secondary electron emission monitor used in the proton therapy beam line consists of several layers of aluminized mylar. Alternate layers are biased with a negative voltage and the remaining layers act as electron collectors. The electron current is read on a current integrator. The beam quality for proton therapy is not affected as the energy loss in the thin mylar is less than 1 MeV compared with 15 MeV throughout the remainder of the beam spreading system. The SEEM is calibrated against a Faraday cup and an ion chamber so that integrated proton flux and the dose is measured simultaneously. A set of split ion chambers has been acquired from IUPUI and will also be used in the future.
Fifteen cell cultures and 67 mice have been irradiated with 150cGy, 400cGy and 800cGy of 200 MeV protons. Biological effects have been examined using micronucleation and cell and mouse death as an indicator. The results are being published by Dr. Rong-Nian Shen of the Department of Radiation Oncology at IUPUI and the proton therapy group at IUPUI and IUCF.
Successful results of this work have led to funding by the Lion's Club of Indiana. They will donate $300,000.00 for the development of the proton therapy facility to the point where first patient treatments can be done. We project that the first patient will be treated in about one year. More regular patient treatments (more than 3 or 4 per year) will require the installation of the L4 Lambertson magnet to enable us to split beam from the Cooler to the Gamma cave.