Room: Exhibit Hall | Forum 7
Purpose: Paranasal sinus cancers are challenging to irradiate due to proximity to eyes, lenses, optic nerves, chiasm and brainstem. These tumors often have positive microscopic margins, necessitating doses of 60 Gray Equivalent (GyE) or more for cure, which may exceed tolerance of the optic pathways. This is the first known study of sinonasal cancer to evaluate potential dosimetric improvements by using a novel proton therapy modality: Spot-Scanning Proton Arc (SPArc).
Methods: Five patients with sino-nasal cancer undergoing high dose (60 GyE) Intensity Modulated Proton Therapy (IMPT) based on the Single-Field Optimization (SFO) were re-planned with SPArc technique, using Monte Carlo dose calculation in RayStation ver. 6.2. The same worst-case-scenario robust optimization parameters were used in both IMPT and SPArc planning groups (3.5% range and 3 mm setup uncertainties with 21 total scenarios). Clinical IMPT plans using 3 to 4 fields, including vertex beams, were compared to SPArc plans using an axial and/or vertex single arc with 2.5 degree control point sampling frequency. Statistical analysis was performed using the paired two-sided t-test.
Results: With similar target coverage, maximum point doses (in GyE, to 0.03 cm3) for organs at risk were as follows for IMPT versus SPArc plans, respectively: Ipsilateral Lens 12.14 vs 3.71, Contralateral Lens 7.63 vs 2.25, Ipsilateral Eye 46.15 vs 36.55, Contralateral Eye 35.59 vs 27.59, Ipsilateral Optic Nerve 53.69 vs 41.52, Contralateral Optic Nerve 49.44 vs 36.85, Optic Chiasm 39.49 vs 31.32, Brainstem 46.02 vs 31.01 GyE; all with p<0.05.
Conclusion: SPArc has the potential to significantly reduce maximal dose levels to the eyes by approximately 8 GyE, and to the optic nerves by 12 GyE beyond what was clinically delivered with IMPT plans. SPArc is capable of providing new and clinically meaningful optic pathway sparing, potentially reducing the risk of cataracts and vision loss.
Funding Support, Disclosures, and Conflict of Interest: Grant/Research Support: Beaumont Herb and Betty Fisher research seed grant award; Ion Beam Applications, Inc. (IBA) research funding; A patent related to the proton arc therapy is licensed to Ion Beam Applications, Inc.