Evaluation of stopping power ratio of artificial breast implants for carbon-ion radiotherapy

The number of patients requiring breast reconstruction with artificial implants has been increasing, and so is the use of carbon-ion radiotherapy (CIRT). Consequently, a growing number of patients with artificial breast implants are expected to undergo CIRT. Because artificial breasts are composed of a silicone polymer gel with a silicon-oxygen backbone, which differs significantly from human tissues, the stopping power ratio for carbon beams cannot be accurately converted from CT values using standard CT-to-stopping power ratio tables (CT-SP tables). Incorrect stopping power ratios can lead to significant problems in CIRT, including erroneous calculations of carbon beam range. To address this, we measured the CT values and stopping power ratios of three commercial artificial breasts using a 380 MeV/u carbon beam. Our results revealed significant deviations from the CT-SP table values. For instance, calculations for treating lung cancer with incorrect stopping power ratios resulted in errors of approximately 5 mm in range calculations, adversely affecting dose distribution to the target. Although further studies with various products are needed, it is crucial to conduct thorough patient consultations and develop treatment plans using accurate stopping power ratios.