PHYSICS Bragg Peak Flattening Filter for Dose Delivery Utilizing Energy Stacking region (Fujitaka, Takayanagi, Fujimoto, Fujii, 1' G. Warrell Terunuma 3101 ; Weber, Kraft 2765). Abstract Previous ripple filters have either used multiple Proton beam radiotherapy is attractive for cancer treatment because of the unusually sharp Bragg simply consisted of single sheets of material wit peak exhibited by proton beams. However, many beam energies are needed to cover the entire thicknesses (Fujitaka, Takayanagi, Fujimoto, Fr volume of the tumor, increasing the sensitivity of the treatment to target motion. It is therefore Terunuma 3 JO I ; Weber, Kraft 2767). The goal ' desirable to have a flattening filter that necessitates fewer beam energies to cover the target by single-layer flattening filter, consisting of an ak spreading out the Bragg peak. A prototype aluminum grid flattening filter was designed, proton Bragg peak in a clinically useful way. Tl manufactured, and tested at the Indiana University Cyclotron Facility (IUCF). It has been found the Indiana University Cyclotron Facility (IUCI feasible to use such a filter to treat the patient with fewer beam energies. However, further work is necessary to generate a sufficiently uniform dose for clinical purposes. Materials and Methods Introduction Aluminum, an easily machined material with a 1 filter in order to minimize the lateral scattering' obert Wilson initially proposed proton radiotherapy for cancer treatment in his seminal for manufacturing the flattening filter had a thic 1946 paper, Radiological Use of Fast Protons (Wilson 487). Since then, twenty-six clinical first step in designing the prototype flattening fi R find the proper weighting factor, defined as the proton therapy facilities have been built worldwide, and nearly as many are planned to be built (Kraft 1083). Although the infrastructure for proton therapy is far more expensive than photon of empty space in it, as seen by the unmodulate1 or electron therapy, the unusually sharp Bragg peak of protons makes them very attractive for with and without being passed through the alurr cancer therapy. The short Bragg peak means that at energies typically used for clinical purposes, energies: 6 cm in water and 8 cm in water. (As t most of the beam energy is deposited in the last few millimeters of travel. This makes precision material is directly proportional to its energy, p1 targeting of the tumor possible, but also necessitates a large number of beam energies to provide a in water.) As shown in Figure 1, the data were t uniform dose in depth over the target region. Moreover, the small irradiated region sensitizes the phantom. This apparatus was used to find the d< treatment to target motion. A single proton energy may be split up by means of a propeller and the beam passed through the aluminum plat modulator (Koehler, Schneider, and Sisterson 438) or a series of binary in-beam degraders to pull back the maximum penetration range in carefully weighted energy steps (Farr, Mascia, Hsi, Allgower, Jesseph, Schreuder, Wolanski, Nichiporov, Anferov 4845). In order to cover the entire lateral area of the tumor, beams may be spread transversely either by passing the beam through thin lead foils or by utilizing a wobbling magnet to scan the beam across the target area. The latter method is used by the beam nozzles at the Midwest Proton Radiotherapy Institute (MPRI) (Anferov 3560). A possible alternative to traditional methods of beam spreading is to use a Bragg peak flattening filter, also known as a ripple filter, to split up the proton beam. Flattening filters typically consist of multiple sheets, each containing alternating regions ofmater!al and empty space. As the beam passes through varying thicknesses of material, it is split into multiple beams. The number of beams depends on the number of layers in the ripple filter. As the beams reach the target, they add together to produce a dose profile with a flattened Bragg peak. The result is that a single beam irradiates a large portion of the tumor, and fewer beams are needed to cover the entire targeted 62 PHYSICS Delivery Utilizing Energy Stacking region (Fujitaka, Takayanagi, Fujimoto, Fujii, Nishiuchi, Ebina, Okazaki, Hiramoto, Sakae, :ell Terunuma 310 I; Weber, Kraft 2765). 1ct Previous ripple filters have either use? multiple layers of material with slits cut into them, or have ·eatment because of the unusually sharp Bragg simply consisted of single sheets of material with grooves cut into them of depth-varying beam energies are needed to cover the entire thicknesses (Fujitaka, Takayanagi, Fujimoto, Fujii, Nishiuchi, Ebina, Okazaki, Hiramoto, Sakae, the treatment to target motion. It is therefore Terunuma 3101 ; Weber, Kraft 2767). The goal of this project was to determine if a much simpler :s fewer beam energies to cover the target by single-layer fl attening filter, consisting of an aluminum grid, could be used at MPRI to flatten the 1minum grid flattening filter was designed, proton Bragg peak in a clinically useful way. The filter was designed, manufactured, and tested at Cyclotron Facility (IUCF). It has been found the Indiana University Cyclotron Facility (IUCF). fewer beam energies. However, further work is clinical purposes. Materials and Methods Aluminum, an easily machined material with a relatively low density, was chosen for the fl attening filter in order to minimize the lateral scattering effects of the protons. The aluminum sheet chosen •therapy for cancer treatment in his seminal for manufacturing the flattening filter had a thickness of0.063 inches (approximately 1.6 mm). The is (Wilson 487). Since then, twenty-six clinical first step in designing the prototype flattening filter was to do computer simulations of it in order to ·ldwide, and nearly as many are planned to be find the proper weighting factor, defined as the ratio of the area of material in the filter to the area oton therapy is far more expensive than photon of empty space in it, as seen by the unmodulated proton beam. To do this, data on the proton beam Jf protons makes them very attractive for with and without being passed through the aluminum sheet were taken at two clinically significant nergies typically used for clinical purposes, energies: 6 cm in water and 8 cm in water. (As the penetration depth of a proton in a uniform nillimeters of travel. This makes precision material is directly proportional to its energy, proton energies are typically measured in centimeters a large number of beam energies to provide a in water.) As shown in Figure 1, the data were taken by means of an ionization chamber in a water rer, the small irradiated region sensitizes the phantom. This apparatus was used to find the dose versus depth profile of the unmodulated beam ay be split up by means of a propeller and the beam passed through the aluminum plate. r a series of binary in-beam degraders to pull ighted energy steps (Farr, Mascia, Hsi, , Anferov 4845). In order to cover the entire ·ersely either by passing the beam through thin b.e beam across the target area. The latter 'roton Radiotherapy Institute (MPRI) spreading is to use a Bragg peak flattening ton beam. Flattening filters typically consist s of material and empty space. As the beam plit into multiple beams. The number of beams 's the beams reach the target, they add agg peak. The result is that a single beam ns are needed to cover the entire targeted 63 PHYSICS Figure 1. The upper image shows the proton Once the dose-versus-depth data of the beam hi beam nozzle and the water phantom used to Microsoft Office Excel to obtain a preliminary • find the dose-versus-depth data for the water equivalent thickness of the aluminum pl ai modulated and unmodulated proton beams, travel through water to undergo the same energ and later to test the completed flattening filter. The lower image is a schematic of the determined. Using these data, Excel was used t< gantry beam nozzle. The beam is spread out defined as the area of aluminum in the plate div laterally by the wobbling magnet, on the far weighting factor, the spreadsheet predicted that left, and passed through the range modulator, a series of binary in-beam energy degraders. least 80% of the maximum dose would be depo The X and Y jaws were formerly used to energies. The spreadsheet also predicted that th• shape the beam in the X-Y plane (under the some ripple, while the 8 cm in water beam wou convention that the beam travels along the Z­ axis), but their function has been replaced by simulation suggested that at the chosen weighti1 the aperture, placed on the nozzle for of the flattened 6 cm in water Bragg peak woul< treatments. The prototype flattening filter flattened 8 cm in water peak would have a sligh was affixed between the wobbling magnet and the range modulator, in order to provide sufficient distance between the filter and the target for scattering effects to conceal the grid structure of the flattening filter. The proton beam used at MPRI is accelerated by a cyclotron to a fixed extraction energy of208 mega-electron volts (MeV), and sent to one of three treatment rooms. Two of the treatment rooms contain 90-ton beam gantries that rotate the beam nozzle around the patient to obtain nearly 4p geometry around the tumor. The patient is secured to a carbon-fiber treatment couch attached to a large robotic arm, which moves so that the tumor is at the isocenter of the beam nozzle. Control of the beam energy (and therefore the penetration depth) is accomplished by energy degrading range 6 modulators in each treatment room and built into each of the beam nozzles.