3617 - Assessing Clinical and Dosimetric Benefits of Spot-Scanning Proton Arc Therapy in Ocular Melanoma Treatment

X. Cao¹, P. M. Liu¹, R. L. Deraniyagala Jr¹, A. H. Zureick¹, M. Mamalui-Hunter², R. Dagan², M. S. Rutenberg³, L. Zhao⁴, G. Liu⁵, C. W. Stevens¹, X. Li¹, and X. Ding¹; ¹Department of Radiation Oncology, Corewell Health William Beaumont University Hospital, Royal Oak, MI, ²University of Florida Health Proton Therapy Institute, Jacksonville, FL, ³Department of Radiation Oncology, Mayo Clinic, Jacksonville, FL, ⁴Department of Radiation Oncology,Stanford University, Palo Alto, CA, ⁵Cancer Center, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China

Purpose/Objective(s): To evaluate the feasibility and dosimetric superiority of spot-scanning proton arc therapy (SPArc) for ocular melanoma, in comparison to Intensity Modulated Proton Therapy Materials/

Methods: Six ocular melanoma cases, varying in size (large/small) and anatomical locations (posterior/anterior/optic disc), were planned with 3-field IMPT (3F-IMPT) and SPArc for dosimetric comparison. With a target dose normalized to 50 cGE, 3.5% range uncertainty were used in both groups for robust optimization. Dose to organs-at risk (OARs) were analyzed. The mean dose averaged linear energy transfer (LET_d) were calculated with dose threshold of voxels receiving no less than 20% of the maximum dose.
Results: SPArc demonstrated a consistently superior dose distribution compared to 3F-IMPT in all cases. Notably, SPArc achieved improved dose sparing of lens and cornea in tumor situated in the posterior segment and optic disc region, where the lens and cornea were in the beam path. Specifically, for the four cases involving tumors in the posterior segment or optic disc, the median maximum dose (Dmax) to the lens with IMPT and SPArc was 30.61 CGE and 10.62 CGE, respectively. SPArc achieved a 65.3% reduction in the lenss Dmax. A similar outcome was observed for the mean dose (Dmean) to the cornea, with SPArc achieving a 76.7% reduction in the Cornea Dmean. Additionally, SPArc presented an increased LET_d within the target area while reducing the LET_d in OARs abutting the target. Particularly, the median of mean LETd for tumors was 3.90 keV/µm with IMPT compared to 4.14 keV/µm with SPArc. For the lens, the median of mean LETd was significantly lower with SPArc at 0.60 keV/µm, compared to 1.92 keV/µm with IMPT.
Conclusion: As an innovative proton therapy approach, SPArc without utilizing the compensator and blocks or MLCs has demonstrated dosimetric superiority and adaptability in treating ocular melanomas situated in the posterior segment and optic disc region, especially in scenarios where sparing of the lens and cornea is crucial. Abstract 3617 – Table 1

Tumor Size/Location	Plan	Cornea (mean)cGE	Lacrimal Gland (mean) cGE	Retina (mean) cGE	Optic Nerve 0.03cc (max) cGE	Lens 0.03cc (max) cGE
Anterior Large	3F IMPT SPArc	35.00 14.82	20.66 10.42	36.40 29.55	38.04 28.91	42.48 28.39
Anterior Small	3F IMPT SPArc	46.20 44.65	17.03 0.82	6.89 5.15	0.20 1.83	53.81 51.79
Optic Disc Large	3F IMPT SPArc	26.67 6.77	28.75 8.80	37.65 29.98	50.68 44.69	34.97 13.54
Optic Disc Small	3F IMPT SPArc	19.56 3.72	14.52 4.32	34.54 23.67	52.09 50.34	26.25 7.69
Posterior Large	3F IMPT SPArc	33.68 5.95	33.00 15.12	41.72 29.95	34.51 30.45	42.74 16.73
Posterior Small	3F IMPT SPArc	16.86 1.96	20.80 11.40	29.10 15.44	38.22 29.33	23.41 3.87