S1679
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Poster Discussion 4080
A Highly Realistic Anthropomorphic Pelvic Phantom with a Fillable Dynamic Bladder Tim H.A. Stassen 1 , Lars H.B.A. Daenen 1 , Didier Lustermans 1 , Luana de Freitas Nascimento 2 , Frank Verhaegen 1 , Gabriel Paiva Fonseca 1 1 Department of Radiation Oncology (Maastro), GROW Research Institute for Oncology and Reproduction, Maastricht, Netherlands. 2 Research in Dosimetric Applications, Belgian Nuclear Research Centre (SCK CEN), Mol, Belgium
Purpose/Objective: The increasing adoption of online adaptive
radiotherapy (oART) systems demands more dose delivery verifications, especially in cases of large, rapid, or unpredictable intra-fraction anatomical changes (e.g., stomach emptying or bladder filling) (1). Commercial phantoms often lack morphological complexity and organ-movement realism, yielding unrealistic verification results. 3D-printed (3DP) phantoms can address this by providing realistic morphology and controllable organ motion. This study developed an anthropomorphic pelvic-region phantom with an inflatable bladder. Bladder filling was assessed using CT and CBCT, mimicking possible oART
Results: The pelvic phantom showed high realism in
anatomical geometry and mean CT-numbers: skeletal muscle (CT: 157 Hounsfield Units(HU); CBCT: 161 HU), adipose tissue (CT: 24.5 HU; CBCT: 19.0 HU), cortical bone (CT: 596 HU; CBCT: 761 HU). Relative bladder volume deviations decreased with increasing volume, likely due to reduced partial volume effects. All CT/CBCT volumes were lower (Figure 2): 10.1%/12.7% for 10 ml, 4.7%/4.0% for 40 ml, and 1.9%/1.3% for 70 ml. Absolute volume differences between the reference ranged from 0.9 to 1.9 ml.
treatment situations. Material/Methods:
A 187×287×70 mm pelvic phantom was printed consisting of two 3DP materials, including nine anatomical structures (e.g. skeletal muscle and bone). Printing settings optimization (Figure 1) was done for CT-number values using an in-house software AMIGOpy (2). The bladder was modeled as a cavity with two channels; one to insert a balloon connected to a peristaltic pump simulating bladder filling and one to drain excess cavity volume. Different liquids were tested for the bladder filling and the surrounding region. The cavity was filled with water-equivalent gel during imaging, moving as the bladder filled or emptied. Scans were performed with 120 kVp on CT (SOMATOM Confidence, Siemens) and 125 kVp on CBCT (Hypersight CBCT on Halcyon 4.0, Varian), focusing on volume changes as the balloon was reinserted between imaging modalities. The bladder was filled with 10, 40, and 70 ml saline-water (1:10 mass %) to improve imaging contrast to the surrounding gel. Mean CT numbers were measured using regions of interest for adipose tissue, skeletal muscle, and cortical bone structures using AMIGOpy. The CT and CBCT bladder volumes were segmented, calculated and compared with the actual filled volumes measured from a known volume syringe.
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