SU‐FF‐J‐143: A Feasibility Study of Using O‐18 Enriched Water to Develop a Method of In‐Vivo Range Verification in Proton Therapy

Purpose: To report a feasibility study of using oxygen‐18 (O‐18) enriched water in combination with PET to develop a method of in vivo range verification in proton therapy. O‐18 enriched water is non‐toxic and non‐radioactive and can be safely administered to patients before proton treatment. When protons interact with O‐18, it is converted to F‐18 which decays back to O‐18 by positron emission with a half‐life of 110min. For the O18(p,n)F18 reaction, the interaction cross‐section is larger, the energy threshold lower, and the half‐life longer compared to most other inelastic proton interactions in patients leading to positron decay. Method and Materials: O‐16 water, 98% O‐18 enriched water, and 10% O‐18 enriched water samples were placed inside a polystyrene phantom and irradiated with a modulated 149 MeV proton beam with a 6 cm wide Spread Out Bragg Peak (SOBP) at three depths (mid‐entrance region, proximal, and distal third of the SOBP). The activity decay of each water sample was measured using a NaI detector for 1–2 hours. Additional information was obtained by scanning the irradiated water samples with a high‐resolution animal PET scanner. Results: As expected O‐16 irradiated water had a higher initial activity than 10% O‐ 18 and 98% O‐18 enriched water at each depth. However, at later times, the activity in the O‐18 samples was larger due to the short half‐times of positron‐emitting products resulting from O‐16 irradiation. Furthermore, there was a larger reduction of activity of irradiated O‐16 across the SOBP as compared to O‐18 irradiated water. Conclusion: Application of O‐18 enriched water has features that make it an attractive method for in‐vivo range verification of proton beams. It is likely that this method could also be useful for in vivo dose verification. © 2009, American Association of Physicists in Medicine. All rights reserved.