31 Uveal Melanoma

Uveal Melanoma

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

the largest tumour diameter to create a tumour-free margin of safety to prevent geographic miss. Dose rate calculation and treatment plan approval represent the last phases of the workflow. All patients should be discussed in the Multidisciplinary Tumour Board before treatment.

sonographic measurement of the tumour thickness itself. In view of the specific growth pattern of uveal melanoma, there is debate over the target cells in uveal melanoma.The whole tumour volume and its microscopic extensions form the basis of the CTV, therefore the apex tumour dose represents the minimum target dose to tumour cells at the apex. In contrast, the uveal layer carrying the main blood supply is where tumour growth takes its origin and is continuously fed via the uveal blood supply. This layer receives a significantly higher dose, which is close to the scleral surface dose.

7. TUMOUR AND TARGET VOLUMES

The gross tumour volume (GTV) is determined based on pretreatment findings fromophthalmoscopy and ultrasound scan. The ophthalmologist provides a diagram of the fundus oculi with tumour border orientation and reports the distance from optic nerve and fovea. Basal diameters and apex height (thickness) of the disease are also carefully measured for gross tumour volume (GTV) definition. The use of both A and B ultrasound scans is strongly recommended. Tumour diameters are defined with B-scans while disease height assessment is evaluated with A-scans, and their information cannot be replaced by other US techniques, even if with higher nominal resolution. Sometimes MRI could be used if ultrasound cannot measure the tumour in a precise way. The pretreatment GTV measurements are mandatory for 3D pretreatment plan and to define the type and size of plaque. A confirmation of tumour diameters is done using the tumour’s transillumination shadow during the intervention in the operation theatre. (Fig. 2, 3, 4) Base diameter of the CTV The base diameter of the CTV is determined by the tumour base diameters . A safetymargin of 1 - 2mmfor subclinical disease is added in all directions, which accounts for microscopic spread along the ocular tunicae, mainly the uvea and the sclera. In some situations, uncertainties in tumour delineation (e.g. in the exact drawing of the tumour shadow during transillumination) or in plaque localization (e.g. in posterior pole locations) may be observed, which can be taken into account by adding an extra safety margin (PTV) to deal with these uncertainties.The amount of such a safetymargin should be determined individually according to the given situation. No safety margin for eyeball movements is taken into consideration, as the eye plaque is sutured tightly onto the outer sclera. Safety margin considerations have to be taken into account particularly if the tumour is located in the vicinity of critical structures, such as the optic disc, the macula, or the ciliary body. Height of the CTV (prescription point) The height of the CTV (prescription point – distance between apex of the tumour and the internal surface of the plaque) is determined by the tumour thickness plus 1mm (thickness of the sclera) or the measured distance from external scleral surface to tumour apex (A and B-scan). The thickness is usually taken from sonography findings (A-scan) with 1 mm added for the sclera. The thickness is measured at themost prominent point of the dome shaped tumour (tumour apex) with a line drawn towards the uvea which has to be perpendicular to its basis. For ciliary body tumours, the distance from the tumour apex and the surface of the plaque needs to be considered. (Fig. 5, 6) Therefore, in dome-shaped tumours these dimensions usually represent the maximum tumour thickness. Uncertainties in the determination of tumour target thickness are correlated to the

8. TECHNIQUE

The surgery is usually performed under general or local anaesthesia or intravenous sedation by a subspecialty-trained surgeon. The conjunctiva is opened at the limbus and the rectus muscles are isolated for traction with silk sutures to expose the quadrant (Fig.7). Localization of the tumour borders is achieved by transillumination placing a fiber optic light 180° away from the tumour. The tumour base shadows its subjacent sclera (Fig. 8). A sterile marking pen is used to mark the outline of the tumour border plus 2-3 mm free margin around the tumour base (Fig.9). If an extraocular muscle is overlying the tumour, the muscle should be detached for proper positioning of the plaque. The selection of the size of the plaque is based on the dimensions of the clinical target volume, which usually exceeds the GTV at each side by 1 - 2 mm. Furthermore, it will also take into account that there is an inactive edge at the outer margin of the plaque of 0.6 - 0.7 mm. The preplanning for iodine-125 eye plaques (See further section treatment planning) must be precise so that themanufactured plaque fulfils the demands for dimensions as outlined during the brachytherapy procedure. A template (dummy) of the same size and shape as the treatment plaque is used to position sutures on the sclera and to verify the position before placement of the radioactive plaque (Fig.10). The template (dummy) is then removed and replacedwith the radioactive plaque secured to the sclera with the preplaced sutures (Fig. 11). Any disconnectedmuscle is temporarily reattached to its insertion by using two preplaced sutures. A temporary knot is placed to hold themuscle to the rectus insertion.The conjunctiva is closed, and the eye is patched and, for iodine plaques, covered with a lead shield left in place throughout the treatment period. Plaque location can be confirmed intraoperatively by ultrasonography (Fig. 12). After a few days, the patient returns to the operating room for plaque removal. Under local anesthesia, the plaque is visualized, the anchoring sutures are cut, and the plaque is removed. The displaced muscle should be reattached into its insertion. (Fig. 7, 8, 9, 10, 11, 12, 13)

9. TREATMENT PLANNING

Different types of radioisotopes are available for ocular brachytherapy. Ruthenium-106 and Iodine-125 are the most widely used radioisotopes. Plaques loaded with Iodine-125 seeds can be used both for the treatment of small lesions and for the treatment of