| Summary: | 碩士 === 國立陽明大學 === 放射醫學科學研究所 === 88 === ABSTRACT
The incidence rate for primary malignant lymphomas in orbital tumors is about 10% and commonly presents in the aged people. Ultrasound, CT and MR image modalities have been used in localizing the lesions to provide better delineation of target volume in radiation treatment. The treatment goal in conjunctival lymphoma is to cure the disease while keeping the eyeball intact and preserving the vision of the patients. The survival rate for a prescribed dose of 2000-3500 cGy to lymphoma is 75-80% and for complete remission rate is about 90-95%. Although radiation has been proved to be an effective modality in treatment, radiation-induced cataract with 10% of incidence rate has been a great concern to the oncologists, as the tolerance dose to eye lenses is about 1000 cGy (with a fraction size of 200 cGy).
The purpose of this study is to develop a new radiation therapy technique for the conjunctival lymphoma. In the study, electron beam was delivered from a linear accelerator and passed through specially designed lens-blocks with variable diameters at the center of the beam, allowing 90% isodose curves surrounding the tumor bed. At the same time, the dose given to the lens was kept below 550 cGy (16.5~14.1%) to decrease the incidence of cataract, as a dose of 3000-3500 cGy delivered to the tumor. In addition, convenience of daily setup, accuracy of reproducibility and immobilization, and patient comfortness are also the major considerations of technique development.
To evaluate electron beam characteristics and dose distributions in conjunction with lens-block used in this technique, water phantom, ion chambers, solid phantom, TLDs and Kodak V-films were employed in the study.
There are several important properties in this technique: (1) no anesthesia is required, but the patients are need to be trained to focus their eye-ball movement within the shadow of the lens-block, (2) the lens-block is adjustable to the lesion size of the patient, (3) the treatment setup is relatively simple, convenience and reproducible, (4) the non-uniform off-axis dose distribution can be avoided as the lens-block is ceiling placed parallel to the electron beam, (5) acrylic holder is used to increase surface dose of the electron beam, (6) the selecting of lens-block size should cope with the electron beam energy.
Future improving directions are, developing a real-time system to prevent unnecessary dose to the lens by monitoring the eyeball movement during the treatment, the Monte Carlo algorithm may be applied to calculate the dose distribution patterns in treatment planning, and finally the polymer gel may be used to evaluate three-dimensional dose distributions of the beam.
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