Author: Adam P. Berdusco, Matthew R. Ceelen, Renata Farrell, Carri K. Glide-Hurst, Will Martin, Charles F. Maysack-Landry, Morgan A. McGauley, James Rice, Jordan M. Slagowski, Yuhao Yan 👨🔬
Affiliation: Departments of Human Oncology and Medical Physics, University of Wisconsin-Madison, University of Wisconsin - Madison, Department of Human Oncology, University of Wisconsin-Madison 🌍
Purpose: Upright radiotherapy (RT) is now available although this novel vertical CT and positioner presents unique challenges for quality assurance and testing methodologies. We have designed and built a novel deformable pelvis phantom, Standley, for clinical workflow and online adaptive validation in upright radiotherapy.
Methods: Pelvic anatomy was characterized between upright and supine orientations in three male volunteers with matched MRIs in terms of organ sphericity, point-to-point distances, and principal axis lengths. A pelvic phantom with modular components (bladder, bowel, rectum, pelvic bones) based on patent-informed volumes was constructed. Connective tissue-mimicking structures linked organs for realistic mobility and interactions. The prostate was secured with a urethra mimicking tube connecting to bladder neck for filling. Silicone cylinders representing the levator ani secured the prostate to the pubis bone, while a latex pelvic floor supported the rectum, urethra, and pelvis. Return-to-shape testing was conducted by adding 100-350 ml (50 ml increments) of fluid and measuring landmark distances. Upright phantom validation was conducted with empty, 120 ml, and 240 ml bladder volumes on an upright CT scanner at 120 kVp/250mAs. Mean organ CT numbers were computed.
Results: The bladder, prostate, and rectum were fabricated using Ecoflex 00-10 to yield CT numbers and elastic modulus (+/-5%) consistent with literature to ensure accurate dose calculations and deformation. The urethra and syringe pumping system successfully expanded the Ecoflex bladder. Return-to-shape bladder testing demonstrated no significant (p>0.05) deformation after repeated filling and emptying. Organ deformation/displacement were observed with bladder filling. Mean CT numbers were 256+/-61 HU (prostate), 225+/-52 HU (rectum) and 44+/-52 HU (bladder), compared to design goals of 45, 55, 45 HU, respectively.
Conclusion: A deformable pelvis phantom was designed for upright RT applications. Future work will integrate dosimeters for end-to-end testing of ART and perform comprehensive assessment of organ interactions in upright versus supine positions.