3477 - Evaluating the Impact of Bowel Gas Variations for Wilms Tumor in Pediatric Proton Therapy

O. Ates, F. Pirlepesov, J. Uh, C. H. Hua, T. E. Merchant, A. Boria, and M. Krasin; St. Jude Childrens Research Hospital, Memphis, TN

Purpose/Objective(s): Proton therapy is highly sensitive to changes in bowel content. These changes can impact the path of proton beams, leading to discrepancies in the intended proton range and the treatment plan quality. Materials/

Methods: Thirteen patients with Wilms’ tumor underwent proton therapy. The variations in bowel gas were systematically monitored using daily CBCT imaging. Air cavities identified in daily CBCT images were delineated and exported into the planning CT scans to construct daily verification plans and measure water equivalent path length (WEPL) changes. Dose accumulation of daily plans was analyzed against original planning metrics and dose volume histograms. A worst-case scenario simulation was also conducted to identify the safe beam angles, considering the variable nature of bowel gas distribution.
Results: The study revealed a maximum decrease in target dose (?D100%) of 8.0%, which corresponded to a WEPL variation (?WEPL) of 11.3 mm. The average reduction in target dose, ?D100%, was found to be 2.8%, with a standard deviation (SD) of 3.2%. The mean ?WEPL was observed as 3.3 mm, with an SD of 2.7 mm. Statistical analysis revealed significant correlation between the degradation of treatment plan quality and the ?WEPL, with a p-value of 0.01 (a=0.05). The worst-case scenario analysis suggested that gantry beam angles oriented towards the patients right and posterior aspects from 110° to 310°, particularly the right posterior quadrant, were associated with minimized WEPL discrepancies.
Conclusion: The study presented the most advantageous gantry angles that would reduce deviations from the treatment plan quality. The insights gained from this study are not only critical for improving treatment approaches for Wilms’ tumor but also hold potential applicability in other conditions where bowel gas variation affects proton abdominal irradiation. Abbreviations: LPO, left posterior oblique; RPO, right posterior oblique; AP, anterior-posterior; RL, right lateral; PA, posterior-anterior; LL, left lateral; RAO, right anterior oblique; LAO, left anterior oblique. R, right; L, left.

Abstract 3477 – Table 1: Patient characteristics and plan summaries

Patient Age (years)	Disease Site	Beam Arrangements	Prescription Dose (GyRBE)	Target D100% in original plan (%)	Target D100% in daily accumulation (%)	Target ?D100% (%)	?WEPL Mean ± SD (mm)
1	R Abdomen	RPO, PA	10.8	99.6	99.6	0.0	0.8 ± 0.9
2	R Kidney	RAO, LPO, PA	10.8	98.9	98.4	0.5	1.0 ± 0.8
3	L Abdomen	PA, LAO	10.5	97.9	92.8	5.1	5.7 ± 6.5
4	Partial Liver	PA, RL	10.5	99.6	99.5	0.1	3.9 ± 2.3
4	L Abdomen	LPO, LAO	10.8	99.0	92.1	6.9	3.4 ± 3.0
4	Partial Liver	PA, RAO	10.8	87.4	87.3	0.1	1.7 ± 1.7
4	L Kidney	PA, LAO	10.8	99.0	98.8	0.2	2.3 ± 2.5
5	R Abdomen	RL, PA	10.5	97.0	97.0	0.0	1.9 ± 2.2
5	L Abdomen	LPO, LL	10.8	97.6	96.1	1.5	1.3 ± 1.4
5	L/R Partial Kidney	PA, RAO, LAO	10.5	98.6	93.1	5.5	3.8 ± 2.5
6	Liver	PA, RAO, LAO	10.5	85.4	85.0	0.4	2.0 ± 0.8
9	L Abdomen	PA, LAO	10.8	97.0	88.8	8.2	3.4 ± 3.2
13	L/R Abdomen	AP, RPO, LAO	10.8	99.5	91.5	8.0	11.3 ± 9.6