31 Uveal Melanoma

Uveal Melanoma

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THE GEC ESTROHANDBOOKOF BRACHYTHERAPY | Part II Clinical Practice Version 1 - 15/04/2020

larger lesions. Ruthenium-106 plaques, being a beta-emitter, are to be preferred for small lesions, generally less than 5-6mm, in view of their more favorable toxicity profile delivering a lower dose to lens and opposite retina); they can be used for up to 7-8 mm tumour thickness. Similarly, Strontium-90 applicators feature even higher dose gradients andmay be used for small lesions < 3.5mm. (Fig. 14) Historically, treatment planning has been performed manually based on depth-dose curves provided by the manufacturers with the dose prescribed to the apex. However, the use of 3D treatment planning software is recommended, which incorporates medical imaging, an anatomical volumetric model of the eye and full 3D dose calculation. The software enables definition of target volumes as well as organs- at-risk. This in turn provides dose-volume metrics, such as target coverage and dose to critical structures (e.g. fovea, optical disc and lens), which can be used to drive the treatment planning. It has been shown, that performing 3D treatment planning can help in reducing dose to the macula and optic disk. Currently, only one dedicated TPS for plaque based brachytherapy of uveal melanoma is available commercially (Plaque Simulator, Eye Physics LLC, Los Alamitos, USA). This allows single images from different modalities (i.e. CT, MRI, fundus photographs and US imaging) to be fused and overlays them on a 3D eye model. MRI based planning is under development. Additionally, Plaque Simulator can help in planning the plaque placement and surgery by givingmore accurate information on tumour and consequently suture location before the actual surgical procedure. However, because only single slices from CT or MRI are used to represent the patient-specific anatomy, there can be geometric limitations to its use. The need for a modern image-guided approach is widely discussed in the literature. Some have proposed using a conventional TPS (Pinnacle, Phillips Health Care, Hamburg, Germany) and CT imaging to provide more accurate representations of the patient eye geometries. Others pointed out the advantages of the superior contrast of MR images in defining the tumour and eye geometry and demonstrated its feasibility using another commercial TPS (Eclipse BrachyVision, VarianMedical Systems, Palo Alto, CA). MRI can help in assessing more accurate tumour dimensions which could lead to significant changes in treatment prescriptions. Additionally, another group has developed a stand-alone 3D TPS with a focus on automated treatment plan optimization routines. (Fig. 15, 16) As eye plaque brachytherapy with Ruthenium-106 and Iodine-125 are the most widely used applicators, a description of these two isotopes is given here in more detail. Ruthenium-106 eye plaque brachytherapy The isotope Ruthenium-106 has a half life of 367 days and decays to Rhodium-106 while emitting beta rays with a maximum energy of 39 keV. Rhodium-106 with a half life of 36 seconds, decays to Palladium-106 with a maximum energy of 3.5 MeV and a mean energy of 1.5 MeV providing the effective therapeutic irradiation. Description of Ruthenium-106 applicators Ruthenium-106 applicators were developed by Lommatzsch and Vollmar in the 60’s in East Berlin (1966) Lommatzsch PK. Opthalmologische Onkologie. 1999 Enke, Stuttgart. The 106 Ru is almost equally distributed on the concave surface of the shell shaped applicator. (Fig. 17, 18)

The isotope is deposited on a thin (0.2 mm) target foil made of silver, which in turn is bonded to a 0.7mm silver backing. Towards the eye ball a thin (0.1 mm) silver foil covers the Ruthenium-106; this has almost no absorption effect on the beta ray emission. On the other hand, the convex surface absorbs more than 95% of all radioactivity. This fact is important for radiation protection of the personnel in the operating theatre with regard to the handling of the applicator, as only minor safety procedures have to be undertaken. The concave irradiating side of the applicator should always be protected as electrons have a limited range in tissue but much greater in air. There is an inactive edge at the peripheral margin of the applicator which is reported to be about 0.7 - 0.8 mm. The external diameter varies between 15 and 20 mm and the spherical radius between 12 and 14 mm. There are usually two lugs on each applicator, by which it can be sutured to the sclera. Small grooves can be put onto the convex side of the applicator, enabling fixation of the applicator against the sclera by a suture across the applicator. There are applicators with notches, enabling positioning near the optic nerve and applicators for brachytherapy of ciliary body melanoma.The nominal activity varies (dependent on the size of the applicator) between 13 and 26 MBq or 0.35 and 0.7 mCi. (Fig. 19) Iodine-125 eye plaque brachytherapy Iodine-125 is a gamma emitter with a half-life of 60 days. 125 I decays exclusively by electron capture to an excited state of Tellurium-125 which spontaneously decays to the ground state with the emission of 35.5 kV gamma photons. Characteristic x-rays in the range of 27-35 kV are produced also due to electron capture and internal conversion. The half value layer for gold is only 0.025 mm, hence 0.5mmof gold is sufficient to absorb 99.95%of the incident gamma rays. The low energy of I-125 results also in a lesser radiation safety problem than using other gamma emitting sources, such as Co-60 or Ir-192. Description of the applicators Iodine-125 was introduced to treat ocular tumours at the end of the seventies (30). Presently 125 I is predominantly used in the US to treat uveal melanoma. (Fig. 20) In many centres Iodine-125 plaques are individually fabricated using multiple seeds imbedded in a concave metal plaque. The most prominent models available are the COMS plaque, the ROPES (RadiationOncology Physics and Engineering Services, Australia) plaque and the EyePhysics plaque. These plaques follow the same concave designwith fixed inner radius. However, non-standard dome shaped designs have been developed recently for the treatment of iris melanoma. Iodine seeds are usually delivered in titaniumencapsulation (0.05mm) containing between 0.5 – 20 mCi of I-125, the (outer) seed dimension is about 5 x 1 mm. Because of the presence of titanium and end welds the dose distribution around iodine seeds is highly anisotropic. The seeds are adhered to the concave portion of the plaque with an adhesive and at completion of treatment they are removed by dissolving the adhesive and are re-used. Instead of using an adhesive, silicon acrylic plaque inserts may also be used to accommodate seeds (4 - 18, depending on plaque size) at fixed positions. Many centres report the use of customdesigned inserts, some of which are manufactured on 3D printers. The major difficulties in the design of Iodine-125 seed eye plaques result from the need to have a thin device to slip over the surface of the eye and the relatively bulky physical dimensions of the seeds.