2962 - Dosimetric Robustness of Key Metrics In Spatially Fractionated Radiation Therapy for Pediatric Tumors
Presenter(s)
B. L. Chau1, Y. Natsuaki1, S. Balik2, E. R. Zhang-Velten2, L. Lukas2, E. L. Chang2, H. Zhang3, K. Wong4, and B. P. Ziemer3,5; 1University of Southern California, Los Angeles, CA, 2Department of Radiation Oncology, University of Southern California Keck School of Medicine, Los Angeles, CA, 3Department of Radiation Oncology, University of Southern California, Los Angeles, CA, 4University of Southern California / Children's Hospital Los Angeles, Los Angeles, CA, 5Radiation Oncology Program, Children's Hospital Los Angeles, Los Angeles, CA
Purpose/Objective(s): Spatially fractionated radiotherapy (SFRT) is a novel therapy technique in the treatment of bulky tumors that distributes high radiation dose sub-volumes in a larger target volume receiving a lower dose. SFRT has shown excellent clinical response in adult bulky tumors of varying histologies, but there is limited clinical SFRT data in pediatric populations. Additionally, there is no data on the variation of key SFRT metrics, believed to be related to patient outcome, due to uncertainties present in treatment delivery.
Materials/Methods: A single institution retrospective review of pediatric patients who received SFRT between 2022-2026 was performed. In this analysis, all patients were prescribed 15 Gy to the high-dose sub-volumes (PTV) and 3 Gy to the low-dose (GTV) volume in a single fraction. The radiation treatment plans were modified by simulating patient displacements of ±1 mm and ±2 mm, and Hounsfield unit (HU) variations of ±2% and ±5%. The resulting metrics of 95%, 50%, 10%, 5% target coverage (DX%) and the equivalent uniform dose (EUD) were calculated as a percent difference from the original delivered plan. The EUD was calculated using the Niemierko model with a/ß of 10 and survival fraction of 0.3.
Results: A subset of 15 radiation plans from 10 patients and 13 treatment sites from the 37 identified plans are reported. The average mass of the PTV and GTV were 47.85±57.70cc and 505.53±182.28cc, respectively: a PTV/GTV ratio of 0.10±0.32. For the PTV target, a ±1 mm and ±2 mm shift yielded a -4.37±1.48% and -13.67±3.42% variation in D95%, respectively; for the GTV target, a -0.79±1.74% and -2.19±3.81% change in the D95% coverage was seen. For the PTV target, a ±2% and ±5% HU variation yielded a -0.04±0.42% and -0.04±1.05% difference in the D95% coverage, respectively; for the GTV target, D95% showed a -0.08±0.50% and -0.26±1.35% change. Further results of the low-dose GTV target are shown in the table below.
Conclusion: An approximate 14% reduction in PTV coverage was seen for the ±2 mm shift but is expected to have minor clinical significance because all high-dose shots were placed >1cm inside the GTV, away from any organs at risk, and due to the low PTV/GTV mass ratio. More noteworthy, the hypothesized outcome relevant SFRT metrics of GTV D50%, D5%/D95%, D10%/D90%, and EUD were all <±3%, and most <±1%, of the original values. HU variation had minimal impact on both PTV and GTV metrics. This initial investigation shows that SFRT treatments are robust to typical patient motion thresholds and changes in HU calibration curves. Future studies will use clinically observed patient offsets, larger HU variations, explore correlations in uncertainties and treatment site, and investigate how these uncertainties relate to clinical outcomes.
| D50% coverage | D5%/D95% ratio | D10%/D90% ratio | EUD | |
| ±1 mm | -0.22±0.30% | 0.81±1.82% | 0.71±1.89% | -0.41±0.66% |
| ±2 mm | -0.54±0.57% | 2.87±4.81% | 0.84±2.88% | -1.05±1.29% |
| ±2% HU | -0.09±0.45% | 0.11±0.24% | -0.37±1.57% | -0.03±0.35% |
| ±5% HU | -0.11±1.16% | 0.21±0.62% | 0.19±2.24% | -0.12±0.81% |