Practical considerations for handling implants in proton therapy

CT simulation scan is typically used for proton therapy planning. However, non-tissue-like materials are often not correctly characterized by the CT conversion curve, and medical implants of high-Z materials, such as metals, can introduce artifacts in CT images. These can negatively affect the accuracy of dose calculations, ultimately impacting both treatment planning and dose delivery.

To present our clinical procedures for managing implants in patients treated with proton therapy.

We categorized implants into two groups-'small' and 'large'-based on their size relative to the CT voxel and their effect on image quality. To assess the influence of different implants on dose calculations, we conducted dose evaluations in a few patients with different implants. Our key considerations included the selection of appropriate CT window-level settings for implant contouring, the necessity of performing material overrides weighing the tradeoff between the time spent on contouring and the impact on the dose distribution, as well as artifact mitigation strategies. The proposed procedures were derived from these dose evaluations and our clinical experience.

For small implants, made of titanium or tantalum, we can rely on our CT conversion curves, without requiring material overrides. Conversely, larger implants, such as silicone breast implants, metal hip prostheses, or air pockets, require material overrides to ensure an accurate representation of stopping-power ratios and to minimize dose discrepancies. The general rule is to avoid proton beams passing through metal implants when possible.

We have presented a comprehensive and standardized strategy for managing implants in our routine clinical practice. These strategies have been specifically developed for our clinical setup based on pragmatic considerations and may not be universally applicable. However, we believe they can offer valuable insights for other proton therapy centers facing similar challenges and looking to enhance their protocols for implant management.