ESTRO 2025 - Abstract Book

S2961

Physics - Image acquisition and processing

ESTRO 2025

974

Digital Poster Direct electron density reconstruction from spectral CT: Validation for radiotherapy dose calculation Poppy Nikou 1 , Helen Grimes 1 , Catharine Clark 1,2,3 , Callum Gillies 1 1 Radiotherapy Physics, University College London Hospital, London, United Kingdom. 2 Medical Physics and Biomedical Engineering, University College London, London, United Kingdom. 3 Medical Physics, National Physical Laboratory, Teddington, United Kingdom Purpose/Objective: Radiotherapy dose calculations for photon treatments require an electron density (ED) map of tissues. Conventionally, a single energy (120kVp) CT scan is taken to generate a Hounsfield Unit map of tissues, which is then converted to an ED map through stoichiometric calibration. However, in single energy (120kVp) CT scans regions of artifacts, beam hardening and contrast can lead to uncertainties in the calibrated ED map. Spectral CT scanners allow for acquisition at multiple energies, reducing these uncertainties [1,2]. The spectral acquisition data also allows for a direct reconstruction of an ED map which could be used for dose calculations [3]. Previous studies comparing photon dose calculations between calibrated and reconstructed ED maps showed no clinically significant differences in head and neck, chest, and pelvis cases [4]. This work extends the analysis to include brain and abdominal cases. Material/Methods: The study included 13 patients treated with VMAT: 3 brain, 3 head and neck (H&N), 2 thorax, 2 abdominal, and 3 prostate cases. All patients were scanned on a Phillips spectral CT 7500 scanner (Koninklijke Philips N.V., Amsterdam, The Netherlands). For each patient, an ED map was created by stoichiometric calibration of the CT scan and through direct reconstruction. The patient’s photon treatment plan was calculated onto both ED maps and compared. The clinical goals were calculated using the planning structure set, and a 3D local gamma analysis was performed between the two calculations. Results: An overlay of the dose difference map on the reconstructed ED map for a H&N patient is shown in Figure 1. Differences were observed in this slice due to reduced beam hardening in the reconstructed ED map.