3163 - Impact of Combined Respiratory and Gastrointestinal Motion on Dosimetric Interplay in Abdominal Proton Therapy

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 23

POSTER

Presenter(s)

Tian Liu, PhD - Icahn School of Medicine at Mount Sinai, New York, New York

J. Wang¹, X. Zhao², E. Khoudary³, B. Selvaraj⁴, Y. Wang¹, J. Zhang¹, K. Yang¹, C. B. Simone II^4,5, H. Lin^4,6, K. A. Goodman¹, T. Liu¹, X. Jia², S. Wei⁴, M. Kang³, and Y. Lei¹; ¹Department of Radiation Oncology, Icahn School of Medicine at Mount Sinai, New York, NY, ²Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University, Baltimore, MD, ³Department of Human Oncology, University of Wisconsin, Madison, WI, ⁴New York Proton Center, New York, NY, ⁵Memorial Sloan Kettering Cancer Center, New York, NY, ⁶Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY

Purpose/Objective(s):

Abdominal pencil beam scanning (PBS) proton therapy is sensitive to anatomical changes near the distal end of Bragg peak. While respiratory motion is well studied, stochastic gastrointestinal (GI) motility is rarely modeled. We developed a physiology-informed motion model integrating respiratory and GI dynamics. We hypothesize that combined motion produces dosimetric interplay, and that GI motility independently contributes to target degradation and organ-at-risk (OAR) dose variability.

Materials/Methods:

Nine patients (3 pancreas, 6 liver) treated with PBS were evaluated. We utilized a physiology-composed full motion model combining 4DCT-derived periodic respiration with stochastic GI motility (peristalsis, segmentations, high-amplitude contractions with respective amplitudes of 6, 6, 9 mm based on published physiologic measurements) along stomach, duodenum, and small bowel. A time-resolved Monte Carlo dose accumulation framework replayed clinical spot-scanning deliveries over 100 simulated scenarios per patient to capture dynamic spot timing. Interplay was quantified via deviance (?) relative to static plans and dose variation ranges (scenario bounds) for CTV D95% and OAR D2%. Cohort metrics are reported as mean [min, max].

Results:

Full-abdominal motion heavily degraded coverage and increased OAR risks. Across all patients, 1-fraction (1-fx) full motion resulted in a mean CTV D95% ? of -3.0 [-7.8, 1.0] GyRBE (-8.2% [-25.2%, 4.0%] of prescription doses). We evaluated the GI-motility component independently within the pancreas cohort (n=3). GI-only motion serves as a non-trivial source of interplay, independently degrading CTV D95% by a mean of -1.3 [-3.0, -0.4] GyRBE (-2.7% [-5.5%, -1.1%]) and producing measurable OAR dose variations, such as a distribution range of 5.9 [1.7, 13.8] GyRBE for the small bowel. In the liver cohort (n=6), target degradation yielded a mean full-motion CTV D95% ? of -2.7 [-7.8, +1.0] GyRBE (-8.3% [-25.2%, +4.0%]). Liver OAR hotspots also showed substantial variability, with maximum duodenum D2% variation ranges reaching 13.5 GyRBE, indicating overdose risk if close to constraints. Conventional fractionation (e.g., 30-fx) substantially narrowed the dose variations (74% variation range reduction), while hypofractionation (5-fx) stereotactic body radiotherapy (SBRT) only partially mitigated the variance (43% reduction).

Conclusion:

Combined respiratory and GI motion substantially impacts abdominal PBS dose delivery. GI motility alone contributes a non-trivial portion of this degradation, producing extensive bowel dose variations. Evaluating respiration alone may miss this submode of interplay and its associated upper-tail OAR risks. Combined GI and respiratory dynamics should be considered in motion-robust planning, particularly for SBRT near the bowel.