Proton Beam Ocular Treatment in Eyes with Intraocular Silicone Oil: Effects on Physical Beam Parameters and Clinical Relevance of Silicone Oil in EYEPLAN Dose-Volume Histograms

Proton beam therapy (PBRT) is an essential tool in the treatment of certain ocular tumors due to its characteristic fall-off and sharp beam parameters at critical structures. Review of clinical cases in our ocular PBRT program identified patients with silicone oil used as an intraocular tamponade following pars plana vitrectomy for repair of retinal detachment. Patient’s eye may be filled with silicone oil prior to PBRT for an ocular tumor. The objective of this study was to extend our knowledge of the physical characteristics of proton beams in silicone oil by measuring dose within a silicone tank itself, hence better representing the surgical eye, as well as applying the range changes to EYEPLAN software to estimate clinical impact. The relevant proton beam physical parameters in silicone oil were studied using a 67.5 MeV un-modulated proton beam. The beam parameters being defined included: 1) residual range; 2) peak/plateau ratio; 3) full width at half maximum (FWHM) of the Bragg peak; and 4) distal penumbra. Initially, the dose uniformity of the proton beam was confirmed at 10 mm and 28 mm depth, corresponding to plateau and peak region of the Bragg peak using Gefchromic film. Once the beam was established as expected, three sets of measurements of the beam parameters were taken in: a) water (control); b) silicone-1000 oil and water; and c) silicone-1000 oil only. Central-axis depth-ionization measurements were performed in a tank (“main tank”) with a 0.1cc ionization chamber (Model IC-18, Far west) having walls made of Shonka A150 plastic. The tank was 92 mm (length) × 40 mm (height) × 40 mm (depth). The tank had a 0.13 mm thick kapton entrance window through which the proton beam was incident. The ionization chamber was always positioned in the center of the circular field of diameter 30 mm with the phantom surface at isocenter. The ionization chamber measurements were taken at defined depths in increments of 2 mm, from 0 to 35 mm. To define the effect of silicone oil on the physical characteristics of proton beam, the above-defined three sets of measurements were made. In the first run (a), the Bragg-peak measurements were made in the main tank filled with water. In the second run (b), a second smaller tank filled with 10 mm depth silicone oil was placed in front of the water tank and the measurements were repeated in water. In the third run (c), the water in the main tank was replaced with silicone oil and the measurements were repeated in silicone directly (no second tank in runs “a” and “c”). Finally, the effects of change in range on dose distribution based on the EYEPLAN® treatment planning software of patients with lesions in close proximity to the disc/macula as well as ciliary body tumors were studied. The uniformity of the radiation across the treatment volume shows that the radiation field was uniform within ± 3% at 10 mm depth and within ±4% at 28 mm depth. Parameters evaluated for the three runs (a, b, c) included: 1) residual range; 2) peak/plateau ratio; 3) FWHM of the Bragg curve; and 4) distal penumbra. The measured data revealed that the un-modulated Bragg peak had a penetration at the isocenter of: a) 30 mm in water; b) 31.5 mm in silicone and water; and c) 32 mm range in silicone oil. The peak/plateau ratio of the depth dose curve is 3.1:1 in all three set-ups. The FWHM is: a) 9 mm in water; b) 10 mm in silicone and water; and c) 11 mm in silicone oil. The distal penumbra (from 90% to 20%) was: a) 1.1 mm; b) 1.4 mm; and c) 2 mm. Clinical relevance of the extended distal range in silicone was studied for impact in EYEPLAN treatment software, including cases in which tumors were in close proximity to the optic disc/nerve and macula as well as cases in which anterior ciliary body tumors were treated. The potential change of range by 2 mm in silicone would impact the dose-volume histograms (DVH) importantly for the posterior structures. In ciliary body/anterior tumors, an increase in distal range in silicone could result in optic disc/macula dose and length of optic nerve treated, compared with original EYEPLAN model DVHs. The use of silicone oil as a surgical tamponade in the treatment of retinal detachments has important implications for PBRT treatment planning. In patients with intraocular silicone oil, the physical parameters of the beam should be closely examined and DVHs for posterior structures should be analyzed for potential increased doses to the macula, disc, and length of optic nerve in the field. The change in beam parameters due to silicone oil is essential to consider in treatment planning and DVH interpretation for ocular patients with posterior as well as anterior ocular tumors.