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Correlation of Pathological Finding with Dose Distribution in a Case of Intraocular Choroidal Melanoma Treated by Stereotactic Radiosurgery

Malignant melanoma is a radioresistant tumor and surgical resection has been established as a treatment of choice for large tumors. Recently, the effectiveness of stereotactic radiosurgery (SRS) for intraocular choroidal melanoma has been reported, but the proper radiation dose has not yet been established. We report a case of intraocular choroidal melanoma treated by SRS using a conventional linear accelerator as preoperative radiation therapy (RT) to prevent operation-induced metastases, and correlate the pathological finding in the resected specimen with the dose distribution obtained by SRS to establish the pathologically effective radiation dose. Case report. A 65-year-old woman who complained of decreasing vision of the left eye was admitted to our hospital. An orbital postcontrast computed tomography (CT) done on admission demonstrated a slightly enhancing tumor, 16×16 mm in size, attached to the posterior wall of the left eyeball. Ophthalmoscopy revealed a brown tumor at the posterior part of the left eyeball. T1and T2-weighted magnetic resonance (MR) images (Fig. 1) revealed a tumor of high and low signal intensities, respectively. Clinically, a malignant melanoma was highly suspected. Scintigraphy with gallium-67 revealed a high accumulation corresponding to the left intraocular lesion but did not demonstrate any other abnormal uptake suggesting distant metastases. The clinical stage was T3NOMO (Stage III; 1987, UICC). SRS was chosen as a method of preoperative RT to prevent operation-induced metastases. At first, a head ring (BRW; Radionics, Inc., Burlington, MA, USA) was attached to the outer table of the cranium of the patient using a four-point fixation under local anesthesia. After achieving satisfactory akinesia of the left eyeball by injection of local anesthetic to the retrobulbar space, CT with contrast enhancement was performed with continuous 3-mm thick slices through the affected eye to the top of the skull to define the target for SRS planning. The tumor and the adjacent critical organs, such as the optic nerves, the optic chiasma, the brain stem and the major vessels, were marked according to the CT data and three-dimensional radiation planning was performed by a treatment planning workstation (X-Knife III software system; Radionics, Inc.). The entire tumor was within the 80% isodose contour border. A marginal dose of 20 Gy with a central dose of 25 Gy with five arcs using a narrow beam collimator with a 3-cm diameter was planned (Fig. 2 and Fig. 3). This dose was decided on from doses previously reported as preoperative RT for intraocular choroidal melanoma. The average irradiation doses to the adjacent critical organs were as follows: the left optic nerve, 6.17 Gy; the right optic nerve, 0.12 Gy; the optic chiasma 0.096 Gy; and the brain stem, 0.051 Gy. All of these doses seemed to be tolerable. The dose volume histogram (DVH) of the left intraocular lesion is shown in Fig. 4. After completion of the 3D radiation planning, the patient was placed on the treatment couch to align the center of the tumor with the isocenter of the linear accelerator (Mevatron 77DX 67; Siemens, Germany), and a retrobulbar injection of local anesthetic was administered again for immobilization of the left eyeball during irradiation. Stercotactic irradiation was performed with combined rotations of the gantry and the treatment couch. Two weeks after SRS, the left eyeball was surgically enucleated. The pathological examination (Fig. 5) revealed spindle B cell type tumors in the Callender classification (1) containing a high amount of melanin at the periphery of the tumor. Necrotic areas were observed mainly around the center of the tumor, and cystic changes were also observed. These pathologic changes were similar to those of tumors treated with iodine-125 brachytherapy (2). The distribution of necrotic areas in the tumor corresponded to the areas irradiated by more than 24.9 Gy in dose distributions obtained by SRS. Three years after the treatment, there was no local recurrence, nor was any distant metastases noted. Discussion. Uveal melanoma is a relatively rare disease in Asian countries, but is the most common primary ocular malignancy in adults in Western countries. It occurs in approximately six to seven cases per million people per year according to US figures (3). Enucleation has been the standard treatment for