Author: Chase Cochran, Damodar Pokhrel, William St Clair 👨🔬
Affiliation: University of Kentucky, Department of Radiation Medicine, University of Kentucky, Radiation Medicine 🌍
Purpose: Currently, intraocular disease is primarily treated via COMS-plaques brachytherapy. Various stereotactic approaches via photons/proton beam have also been implemented (CyberKnife/GammaKnife/particle-therapy), but many of these advanced modalities are expensive and limited in their availability. Herein, we demonstrate the feasibility of utilizing fully-automated HyperArc stereotactic radiosurgery (SRS) delivery system to expand the access to care for this disease.
Methods: Seventeen previously treated COMS-patients with left/right intraocular melanomas were selected for retrospective study. Average GTV derived from MRI was 1.19+/-0.60 cc; PTV=GTV plus 1-2mm margin. HyperArc VMAT geometry with Encompass device, 6MV-FFF beam, and AcurosXB dose-engine was used. A single dose of 25Gy was prescribed to 70-80% isodose-line with each PTV95 receiving >25Gy via standard MLCs (5mm width) on a TrueBeam LINAC while sparing organs-at-risk (OAR), including the optic pathway. Each plan underwent patient-specific QA and an independent in-house Monte Carlo (MC) second check to ensure deliverability, safety, and accuracy.
Results: Generated HyperArc SRS plans were highly conformal (CI=1.12+/-0.07, PCI=0.74+/-0.06) with steep dose gradients, GI=3.23+/-0.41 and were able to adequately spare the optic nerve (Dmax=7.9+/-1.7 Gy vs 12.1 Gy, p<0.001) when compared to HyTEC standard. For sufficient PTV (D99%=24.59+/-1.05Gy) coverage, a mean dose to GTV >27.6 Gy vs 25 Gy prescription (p<0.001) was achieved. Maximum dose to brain, brainstem, optic chiasm, and ipsilateral lens were 6.3+/-1.6 Gy, 1.0+/-0.5 Gy, 0.9+/-0.5 Gy, and 10.1+/-5.8 Gy, respectively. Average beam-on time was 8.77+/-1.44 min. End-to-end QA indicated a patient-specific QA pass-rate of 99.6+/-0.5% (2%/2mm γ-criteria) with a standalone MC second check resulting in -1.8+/-2.0% agreement with AcurosXB.
Conclusion: HyperArc SRS plans provided sufficient target coverage for intraocular malignancies, adequately spared OARs including optic pathway, and offered a fast, safe, and efficient SRS method. Clinical implementation of HyperArc for intraocular/inter-ocular SRS, including fractionated-SRS is underway via development of a non-invasive eye immobilization method in our institution.