ESTRO 2025 - Abstract Book

S2957

Physics - Image acquisition and processing

ESTRO 2025

895

Proffered Paper Real-time in vivo range verification based on N-12 imaging at beam spot-level in proton therapy Peter Dendooven 1 , Brian Zapien Campos 1 , Zahra Ahmadi Ganjeh 1 , Giuliano Perotti Bernardini 2 , Jeffrey Free 2 , Stefan Both 2 1 PARTREC, Department of Radiation Oncology, University Medical Center Groningen, Groningen, Netherlands. 2 Department of Radiation Oncology, University Medical Center Groningen, Groningen, Netherlands Purpose/Objective: Intensity Modulated Proton Therapy (IMPT) can reduce dose to organs-at-risk without compromising tumor dose coverage. Due to steep dose gradients near the tumor edges 1,2 , its effectiveness is influenced by range and setup uncertainties. Therefore, proton range monitoring is crucial for verifying beam delivery in order to mitigate treatment side effects. In-beam positron emission tomography (PET) offers potential for range verification by imaging the treatment beam-induced positron emitter activity. Real-time feedback is hindered by the relatively long half-lives of the positron emitters conventionally used: 11 C ( T 1/2 = 20 min), 15 O ( T 1/2 = 2 min). We have shown the potential of short-lived emitters, particularly 12 N ( T 1/2 = 11.0 ms), for real-time range verification 3-5 . We present a proof-of-concept for real-time range verification based on 12 N imaging at beam spot-level in a close-to-clinical scenario. Material/Methods: Spherical targets of 4 cm diameter were defined in an anthropomorphic head phantom (CIRS-731 HN) and IMPT treatment plans were generated in the RayStation TPS. The plans were optimized to deliver a uniform dose of 4 Gy RBE – twice the standard clinical fraction dose to compensate for the current non-optimized PET scanner sensitivity. During irradiation, a 60 ms delay between beam spots was implemented, allowing PET data acquisition of 12 N activity, and image reconstruction, after each spot (Fig. 1). Additionally, 2 and 5 mm thick solid water slabs placed in front of the phantom induced dose range shifts of 1.8±0.5 and 4.3±0.4 mm, enabling to assess the 12 N image accuracy to detect treatment deviations. For the most distal spots, delivered in the first energy layer, the positron activity range (PAR) was determined as the 50% distal fall-off position of the 1D longitudinal positron activity profiles.

Results: Range measurements show a 1.5σ-uncertainty of 1.5-3.5 mm for a single spot, comparable to or smaller than the 3 5 mm margins used in clinical robust IMPT plan optimization. For the most distal spot (of 146.5 MeV and 5.3×10 8 protons) the measured range shifts were 1.6 and 4.6 mm for the 2 and 5 mm shifted irradiations, respectively (Fig. 2).