Modification of a linear accelerator table top for non-coplanar conformal brain radiotherapy.

The use of non-coplanar conformal therapy necessitates the use of unusual beam projections that may not be accomplished with a conventional linear accelerator table top. Modification of the table top can increase the available combinations of gantry and couch rotation. A standard Philips table top, supplied with an SL 75-5 linear accelerator, was modified to increase available combinations of gantry and couch rotation. This was accomplished by shortening the length and decreasing the width of the table top. The modified table top increases the combinations of gantry and couch angles significantly, simplifying the delivery of non-coplanar conformal therapy without significant compromise to routine treatment. The modification of a standard linear accelerator table top has increased the available combinations of gantry and couch rotation to accommodate non-coplanar conforrmal radiotherapy.

Comparison of effectiveness of thermoluminescent crystals LiF:Mg,Ti, and LiF:Mg,Cu,P for clinical dosimetry.

This study compared the relative effectiveness of TLD crystals LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (TLD-700H) for clinical dosimetry, focusing on reproducibility, linearity, and energy response. Experimental results indicated that TLD-700H was superior to TLD-100 with regard to reproducibility, lack of supralinearity, and the absence of variation in TL signal with radiation quality. TLD-700H also had the additional advantages of higher sensitivity and immediate readability. The investigators conclude that this relatively new TLD crystal shows promising potential for clinical dosimetry.

A self-collimating convolution backprojection algorithm for optimizing dose distributions of I-125 prostate implants.

The algorithm presented here for optimizing brachytherapy dose distributions is based on the idea that the seed distribution can be modeled as an activity distribution determined analogously to gamma camera imaging. The peripheral dose to the tumor is converted to a set of uncollimated projection data that are then filtered and backprojected to produce an initial seed distribution. The actual doses resulting from the seed placement are used to correct the initial projection data for attenuation, scatter, and lack of collimation. The corrected projection data are backprojected a second time to yield the optimized but unconstrained seed distribution. Clinical constraints such as the number of different seed activities, the maximum seed activity, the minimum peripheral tumor dose, and the minimum percentage of the volume which receives less than a specified dose are then applied to the unconstrained solution. Through the entire process, the dose calculations are functions of source anisotropy, scatter, and attenuation. When applied to a set of elliptical contours, the algorithm produces elliptical peripheral dose isodose contours and reasonable dose volume histograms for a constrained solution. The results for actual patient prostate contours were not as good, primarily because of the difficulties encountered in dealing with the irregular geometry of the prostate. However, the algorithm shows promise for further research.

A method to reduce systematic spatial shift associated with magnetic resonance imaging.

To reduce the chemical shifts during magnetic resonance (MR) imaging, the authors replaced the petroleum gel in the Brown-Roberts-Well (BRW) MR localizer with chromium chloride. Computed tomography and MR scans were obtained of a phantom skull containing objects with known spatial coordinates. A 2-to 3-mm systematic spatial shift in the frequency-encoded direction was observed with petroleum gel, but not with CrCl3. Results were verified by reconstructing the three-dimensional spatial location of each object using X-Knife computer software. The authors conclude that spatial localization is more accurate with a CrCl3-filled than a petroleum-filled BRW-MR localizer.