ESTRO 2023 - Abstract Book

S566

Sunday 14 May 2023

ESTRO 2023

fillings is considered. The open ring design of the detector will allow for in-beam and post irradiation imaging of head and neck patients. Materials and Methods We simulate the production of positron emitting isotopes in the phantom using the GPU Monte Carlo code FRED (v3.63.2). We consider the delivery of a treatment plan made on the assumption of an initial CT with full nasal cavities, as well as the same plan delivered to the phantom with empty nasal cavities. Delivery of such a treatment would lead to overdosing the region distal to the tumour. The treatment is delivered using reversed energy ramping, such that the highest energy layers are delivered last. We simulate the imaging of the phantom between delivery of each energy layer and following delivery of the first field using GATE (v9.1) and use in-house coincidence processing and reconstruction software to reconstruct the images. The PET reconstruction uses attenuation correction based on the initial planning CT for both cases. Results Both on-line and post irradiation imaging indicate that an overdosage occurred. In-beam imaging shows a systematic shift of the distal edge even with low count rates, Fig. 1, giving the possibility of interrupting treatment before delivery of the highest energy layers and preventing overdosage of normal tissue. Post irradiation imaging of the first field for 60 seconds provides 3D information indicating the location of the anatomical change, Fig. 2. Along the line profile indicated in Fig. 1 the 20% activity fall off is shifted by 37 mm, equivalent to the shift in d80% along the same profile.