ESTRO 2025 - Abstract Book

S4371

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Keywords: Volume, deliniation, prostate

References: [1]: Prostate volume changes during extreme and moderately hypofractionated Magnetic Resonance Image-guided Radiotherapy, S.E. Alexander et al, Clinical Oncology (2022) e838-e391

3014

Mini-Oral Photon-counting CT for treatment planning of head-and-neck cancer Enkelejda Lamaj 1 , Serena Psoroulas 1 , Lotte Wilke 1 , Patrick Wohlfahrt 2 , Hatem Alkadhi 3 , Niccolo Bertini 1 , Hubert Gabrys 1 , Matthias Guckenberger 1 , Panagiotis Balermpas 1 , Stephanie Tanadini-Lang 1 1 Radiation Oncology, University Hospital Zurich, Zurich, Switzerland. 2 R&D, Siemens Healthineers, Forchheim, Germany. 3 Radiology, University Hospital Zurich, Zurich, Switzerland Purpose/Objective: Photon counting detector CT (PCD-CT) is an innovative imaging modality providing high-quality images, better soft tissue contrast, and artifact reduction, compared to energy-integrating detector (EID-CT). In this work, we analyzed the accuracy of dose calculation, artifacts caused by high-density materials, and the advantage of using different image reconstructions of PCD-CT scans, in patients with head-and-neck cancer (HNC). Material/Methods: We selected six patients (11 plans) enrolled in an observational clinical study at our institute, scanned between June 2022 and August 2023 with PCD-CT (Siemens NAEOTOM Alpha). Selection criteria were referral for HNC radiotherapy and the presence of metallic implants in close proximity to the target volume. Raw data were collected and reconstructed as 70 keV virtual mono-energetic images (70 keV VMI, resembling a spectral 120 kVp image obtained with EID-CT) and electron density (ED) images, both with iterative metal artifact reduction. For 70 keV VMI a dedicated CT calibration protocol was applied, while ED images directly provide the material parameter of interest. Plans were optimized in Eclipse on ED images according to the prescribed treatment, and then recalculated on 70 keV VMI with preset number of monitor units. Auto-planning was used following our clinical practice. Artifact override was performed if needed, as in our standard clinical procedure. Results: We evaluated PTV coverage and dose maximum for all plans on the ED images (average D95% = (96.2±0.5)%). The dose distribution to the PTV was clinically acceptable, even in cases with strong artifacts, with hotspots being within acceptable limits (median DMax = (108±2)%). When comparing to the 70 keV VMI, we found a higher maximum dose, and lower conformity and coverage, in the 70 keV VMI (Figure 1). Figure 1 depicts the dose difference between the 70 keV VMI and ED calculation, for the patient with the most pronounced differences. The 70 keV VMI calculation underestimated the dose around the dental implants (dose deviation < 5% for all organs at risk), and inside the CTV (dose deviation < 2.5%).