3062 - Spot-Scanning Proton Arc as a Nephron-Sparing Strategy: Comparison with IMPT and VMAT for Kidney SBRT

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 24

POSTER

Presenter(s)

Chung-Tang Spencer Liu, DO - Corewell Health William Beaumont University Hospital, Royal Oak, MI

C. T. S. Liu¹, X. Cao², Z. Zhang³, X. Cong², N. Tayeb⁴, Y. Ramdas², X. Ding², and D. J. Krauss⁵; ¹Corewell Health East William Beaumont University Hospital, Royal Oak, MI, ²Department of Radiation Oncology, Corewell Health William Beaumont University Hospital, Royal Oak, MI, ³Corewell Health William Beaumont University Hospital, Royal Oak, MI, ⁴Corewell Health - William Beaumont University Hospital, Royal Oak, MI, ⁵Department of Radiation Oncology, Corewell Health Beaumont University Hospital, Royal Oak, MI

Purpose/Objective(s):

SBRT is an established treatment option for medically inoperable renal cell carcinoma (RCC). However, kidneys are radiosensitive, and low-dose baths from photon-based techniques (VMAT) have been associated with functional decline. While intensity-modulated proton therapy (IMPT) can improve renal sparing compared with photon techniques, further dose reduction may meaningfully impact toxicity risk. We hypothesized that Spot-Scanning Proton Arc (SPArc) therapy would reduce ipsilateral normal kidney dose compared with IMPT and VMAT while maintaining equivalent target coverage in kidney SBRT.

Materials/Methods:

We identified 18 patients who underwent 4D-CT simulation for abdominal radiotherapy and identified 29 well-circumscribed renal lesions amenable to dosimetric comparison. Treatment plans were generated in RayStation v14.0 for SPArc, IMPT, and VMAT using identical robustness settings (±5 mm setup, ±3.5% range). All lesions were prescribed 42 Gy (RBE) in 3 fractions. Endpoints included ITV V100, D0.03cc, ipsilateral normal kidney mean dose, and the volume of the ipsilateral normal kidney receiving 25%, 50%, and 75% of the prescription dose. Statistical analyses were performed using SPSS Version 26, and continuous variables were analyzed using repeated-measures ANOVA or Friedman tests based on normality (Shapiro–Wilk), with Bonferroni-adjusted post hoc pairwise comparisons as appropriate.

Results:

Target coverage was comparable among SPArc, IMPT, and VMAT (V100: 99.79% [99.04–100.00], 99.65% [99.01–100.00], and 99.76% [99.01–99.99], respectively; p=0.872). IMPT significantly reduced ipsilateral kidney mean dose compared with VMAT (638 [460–1008] cGy vs 895 [335–1083] cGy, p=0.017). SPArc provided further significant reduction, achieving the lowest mean dose (464 [357–768] cGy) and demonstrating superiority over both IMPT (p=0.017) and VMAT (p<0.001; overall p<0.001 when three arms are compared). For normal kidney sparing metrics, SPArc demonstrated significantly lower V25%, V50%, and V75% compared with both IMPT (p=0.001, 0.005, and 0.012, respectively) and VMAT ( p<0.001 in all three metrics).

Conclusion:

SPArc demonstrated the superior ipsilateral kidney sparing relative to IMPT and VMAT, while maintaining equivalent target coverage. This stepwise reduction in mean dose and low-dose bath exposure supports the dosimetric superiority of proton arc delivery in kidney SBRT. These findings suggest that SPArc may offer the lowest nephrotoxicity risk among evaluated modalities and warrant prospective clinical validation.