Spread-out Bragg Peak Width Research Articles

Irradiation System for HIMAC

Clinical trials of carbon radiotherapy started at HIMAC in 1994 using three treatment rooms and four beam ports, two horizontal and two vertical. The broad beam method was adopted to make a three-dimensionally uniform field at an isocenter. A spot beam extracted from an accelerator was laterally spread out by using a pair of wobbler magnets and a scatterer. A bar ridge filter modulated the beam energy to obtain the spread out Bragg peak (SOBP). The SOBP was designed to be flat in terms of the biological dose based on the consideration that the field consisted of various beams with different LET. Finally, the field of 20 cm in diameter with +/- 2.5% uniformity was formed at the isocenter. The width of the maximum SOBP was 15 cm. When treating the lung or liver, organs that move due to breathing, the beam was irradiated only during the expiration period in a respiration-gated irradiation method. This reduced the treatment margin of the moving target. In order to prevent normal tissues adjacent to the target volume from irradiation by an unwanted dose, a layer-stacking method was developed. In this method, thin SOBP layers which have different ranges were piled up step by step from the distal end to the entrance of the target volume. At the same time, a multi-leaf collimator was used to change the aperture shape to match the shape of each layer to the cross-sectional shape of the target. This method has been applied to rather large volume cancers including bone and soft-tissue cancers. Only a few serious problems in the irradiation systems have been encountered since the beginning of the clinical trials. Overall the systems have been working stably and reliably.

SU-FF-T-345: Measured Output Factors for Range-Modulated Spread-Out Bragg Peak Proton Beams

Purpose: The magnitude and the trend of variation of the output factor (OF) for range-modulated spread-out Bragg peak (SOBP) proton beams are investigated as a function of the range, modulated SOBP width and field size. Method and Materials: The proton delivery system of the fix horizontal beam line (FHBL) used at the Midwest Proton Radiotherapy Institute (MPRI) has been calibrated to give an output of 1.0 ± 1.0% cGy/MU at a reference condition. For field that deviates from reference is measured at center of the SOBP at the room “isocenter”. The OFs were measured with the fields that varied with range, SOBP width and size. Pristine curves for the various field sizes were also measured to study observed variation in the measured output factors. Results and Conclusion: For a field size of 10 cm in diameter, the OFs at the middle of a 10 cm SOBP vary 40% between 14 and 27 cm ranges in water and a similar trend with range is observed for a 6 cm SOBP. The OFs of a 17 cm range vary 60% between SOBP widths of 2.6 and 12.4 cm and a similar trend in SOBP width is observed for other ranges. For field sizes 2–10 cm diameters and a 10cm SOBP, 5% and 20% variations were observed for ranges of 12 and 27 cm, respectively. The trend in field size differs largely between different ranges. In theory, the trend in field size can be calculated from the ratios of Bragg-peak to entrance doses from measured pristine curves. However, the calculated output factors are significantly lower than the measured one for a small field size. Although the output factor can not be predicted theoretically, these measured output factors can be modeled analytically.

SU-FF-T-348: Study of Truncated Cone Filters Using GEANT4

Purpose: A ridge filter has been used for proton therapy to form a spread‐out Bragg peak (SOBP). As the SOBP width becomes larger and the distal falloff becomes smaller, more number of ridge elements is required for the ridge filter. Then extremity of the ridge filter becomes too sharp to machine for large SOBP widths. Since an aperture collimator shapes the proton beam, beam mixing by adjacent ridges is affected by scattering in the ridge elements in the peripheral region. Namely, the proximal part of the SOBP begins to fall at depth deeper than the expected depth. A purpose in this study is to achieve uniformities of both the SOBP and the lateral dose distribution by improving a conventional ridge filter. Method and Materials: We have designed a two‐dimensional array of truncated cone filters (TCF) in honeycomb structure. Since extremity of the TCF is moderate compared with a conventional ridge filter (CRF), manufacturing the TCF is relatively easy. Thus the TCF enables selection of lighter material with smaller scattering effect. In this study, a CRF (aluminum alloy) and a TCF (Lucite) with a SOBP width of 60mm were designed for a 155MeV proton beam. We simulated dose distribution in water for a therapeutic beam with GEANT4 to investigate the influence of both ridge filters. Results: While a few% dose decrease is observed in the proximal part of the SOBP region for the CRF, the dose distribution is more uniform in entire of SOBP region for the TCF. Conclusion: We designed a two‐dimensional array of TCF. It is easier to machine the TCF for SOBP filters with large SOBP widths especially for low‐energy protons. Overall dose uniformity of the SOBP region in the TCF was improved as compared with the CRF.