The development of fractionated stereotactic radiotherapy (SRT) has been hampered by slow and cumbersome relocation techniques. The Medtronic Sofamor Danek (MSD) frameless system is promoted as a fast, relocatable system. It relies upon the integrity of a bite tray system that is molded to the patient's upper dentition. Precise relocation of the bite tray is the key to accurate external placement of the fiducial reference frame. The optimum construction method for a stable reproducible tray is not described. We undertook a study to identify factors, which might influence the integrity of the bite tray system. Reprosil Fast Set Putty was used to construct three bite tray conditions to include: teeth only, teeth and alveolar sulcus, and teeth, alveolar sulcus, and the hard palate. The influence of dentition was also assessed: full dentition, partial dentition, and edentulous. Stability of the bite tray system was tested by assessing minimum displacement forces from known locations around a centrally placed maxilla jig. Dentition is important to stability, with progressively less displacement forces required for partial and edentulous conditions. Extension of the surface area of the bite tray to include teeth, gums, and also hard palate substantially increased stability across all dental conditions. Impression techniques that incorporate the teeth, alveolar sulcus, and the hard palate, result in significantly increased stability and are recommended to ensure maximal resistance to deviation.
Approximately 20% of patients with subfoveal choroidal neovascularization qualify for laser photocoagulation, a treatment associated with both disease control and reduced visual acuity. No proved treatment exists for other patients. Single-fraction proton therapy was investigated as an alternative for two groups of patients: 21 patients received 8 GyE; 27 received 14 GyE. Follow-up by fluorescein angiography, visual acuity, contrast sensitivity, and reading speed were done (mean duration: 16 months). Actuarial lesion control at 21 months was 36% for 8-GyE patients and 89% for 14-GyE patients; 77% of patients with controlled lesions achieved improved/stable visual acuity, compared to 44% with uncontrolled lesions. Actuarial mean visual loss for proton-treated maculas was zero at 24 months. No treatment-related morbidity supervened, based on Radiation Therapy Oncology Group criteria. Proton therapy of 14 GyE in one fraction appears to be more effective in controlling neovascular macular degeneration than 8 GyE in one fraction, for both laser-ineligible and -eligible patients.
It is recognized that multiple-shot radiosurgery using the Leksell Gamma Knife (LGK) can result in larger penumbra dose and less target dose homogeneity compared with a single shot treatment. In addition, the number of shots used increases significantly with increasing volume, which makes planning and optimization time consuming and complex. Although such complexity may not affect the accuracy of treatment delivery itself, it is difficult to assess the dosimetrical merits of a given plan. To deal with this complicated process, we have studied shot-related effects and target shape as separate issues. Shot-related effects include homogeneity, penumbra, shot weight, and prescription isodose line “shift.” Target shape is analyzed using morphological tools. Target volume and shape, as represented by the three-dimensional (3-D) medial axis transform or skeleton, are used to determine optimal shot positions, sizes, and number. These parameters are determined from successive 3-D skeletons of the unplanned target volume. Essentially, target shape is replicated by the arrangement of shots compromising the 3-D skeleton, forcing a conformal dose distribution. This planning process has been automated for multishot treatment. A clinical case in 3-D demonstrates this approach as a replacement for manual planning when a plan with multiple shots is required.
