3712 - Gantry-Based Pencil Beam Scanning (PBS) Proton Therapy for Uveal Melanoma: IMPT vs. Proton Arc Therapy

H. Qi¹, L. Hu², S. Huang³, Y. P. Lee⁴, F. Yu¹, Q. Chen¹, Y. Yang⁵, M. Kang¹, H. Zhai¹, A. Shim¹, P. Park¹, X. Ding⁶, J. Zhou⁷, C. B. Simone II¹, C. A. Barker⁵, and H. Lin¹; ¹New York Proton Center, New York, NY, ²Inova Schar Cancer Institute, Fairfax, VA, ³Tianjin Medical University Cancer Institute&Hospital, Tianjin, Tianjin, China, ⁴University of Iowa Health Care, Iowa City, IA, ⁵Memorial Sloan Kettering Cancer Center, New York, NY, ⁶Department of Radiation Oncology, Corewell Health William Beaumont University Hospital, Royal Oak, MI, ⁷Department of Radiation Oncology, Emory University, Atlanta, GA

Purpose/Objective(s): Ocular proton therapy is often delivered in a dedicated scattering beamline. This study reports multi-field PBS intensity-modulated proton therapy (IMPT) for uveal melanoma (UM) in a regular clinical PBS gantry room. We hypothesize that volumetric modulated proton arc therapy (pArc) can offer superior conformity in comparison to IMPT Materials/

Methods: Twelve consecutive UM patients were treated with IMPT to 50 Gy in 5 fractions. A customized gaze-fixation device attached to the thermoplastic mask was used to reproduce globe position for each patient. The delivered plan was robustly optimized with perturbations of 3mm setup and 3.5% range uncertainties using 3-4 fields without apertures. Each plan was robustly reoptimized (same perturbation parameters) using a single arc in a Monte Carlo-based TPS. Anterior beams were avoided to minimize dose to the anterior segment of eye, eliminate the use of an eye retractor and reduce potential range uncertainty induced by the eyelid. Plan quality was assessed with metrics including target coverage (D95%) and mean/maximum doses (Dmean/Dmax) to the adjacent organs.
Results: The plan quality of pArc plans was consistent with IMPT plans (p > 0.1). Similar target coverage was achieved by pArc and IMPT, with D95% equal to 100.8%±1.2% and 100.9%±1.1%, respectively. The pArc plans achieved an improved conformity index (RTOG) relative to IMPT plans from 2.48±0.53 to 2.13±0.48 (p < 0.05). Since most targets exhibit a concave shape and have a small volume (<1cc), the conformity index exceeds one in both plans. The pArc successfully met the clinical goals for critical organs-at-risk (OARs), and a comparison of dosimetry parameters can be seen in Table 1. The comparison of dose distribution between pArc and IMPT plan showed that pArc was more likely to accumulate higher entrance dose within the arc field, but the doses to adjacent organs were lower, resulting in a lower Dmean.
Conclusion: Gantry-based multi-beam PBS proton treatment for ocular cancer was successfully delivered. This study demonstrated that single arc plans are capable of delivering comparable dosimetric metrics to those of IMPT plans, while achieving improved dose conformity. Abstract 3712 – Table 1: Dosimetric comparison of critical OARs between IMPT and pArc with Dmean and Dmax in Gy

Retina

Lens

Cornea

Conjunctiva

Lacrimal Gland

Optic Nerve

Eyebrow

Dmax

Dmean

Dmax

Dmax

Dmean

Dmax

Dmean

Dmax

Dmean

Dmax

Dmean

Dmax

IMPT

54.5±0.6

38.4±5.2

23.2±9.2

24.5±8.7

10.9±5.3

44.0±7.9

13.8±4.4

42.6±8.6

26.0±12.0

52.7±1.0

19.1±4.1

16.2±7.8

pArc

54.4±0.6

36.6±6.7

24.7±8.7

26.0±8.7

10.2±5.0

44.9±7.5

12.2±4.8

40.9±9.0

22.4±12.5

53.4±0.9

18.7±4.3

16.7±7.6