For the aforementioned planning and delivery system and cranial lesions greater than 7 mm in diameter, multiple noncoplanar arc VMAT consistently provides accurate and high quality cranial radiosurgery dose distributions with low doses to healthy brain tissue and high dose conformity to the target. These qualities may make multiple noncoplanar arc VMAT suitable for a greater range of prescription doses or larger and more irregular lesions. For smaller and/or rounder lesions there are other clinically acceptable treatment techniques that may involve fewer couch angles or arcs and reduce treatment times.

A semi-automatic technique for the direct setup alignment of radiosurgical circular fields from an isocentric linac to treatment room laser cross-hairs is described. Alignment is achieved by acquiring images of the treatment room positioning laser cross-hairs superimposed on the radiosurgical circular field image. An alignment algorithm calculates the center of the radiosurgical field image as well as the intersection of the laser cross-hairs. This determines any alignment deviations and the information is then used to translate the radiosurgical collimator to its correct aligned position. Two detectors, each being sensitive to the lasers and ionizing radiation, were used to acquire the radiation/laser images. The first detector consists of a 0.3-mm-thick layer of photoconducting a-Se deposited on a 1.5-mm-thick copper plate and the second is film. The algorithm and detector system can detect deviations with a precision of approximately 0.04 mm. A device with gyroscopic degrees of freedom was built in order to firmly hold the detector at any orientation perpendicular to the radiosurgical beam axis. This device was used in conjunction with our alignment algorithm to quantify the isocentric sphere relative to the treatment room lasers over all gantry and couch angles used in dynamic stereotactic radiosurgery.

The dosimetric accuracy of radiosurgery treatments is limited by the treatment planning algorithm as well as dosimeter performance for small field sizes. In this work we focus on the small field dosimetry for both the single square fields (2.5mm to 30mm side length) and the composite arcs (5mm to 23mm diameter lesions) delivered using the HD120 2.5mm MLC and the 1000MU/min 6MV beam. The isocenter dose for the square MLC fields were measured at depths of 7cm and 15cm using EDR film, a radiosurgery diode (IBA) and a 0.01cc ion chamber (IBA). The measurements for all detectors agreed to within 3.4% down to a field size of 7.5×7.5mm2. Previous Monte Carlo studies have shown good agreement with film and the over response of diodes for sub‐1cm fields. Therefore, the dose calculated by the Eclipse AAA and iPlan PBC algorithms was compared to that measured using film. The doses for both algorithms agree to within 5% of the film measurements for field sizes of 7.5mm or greater. For the 5mm field, Eclipse has a −11% and −25% discrepancy for the depths of 7cm and 15cm, respectively, whereas iPlan has 6% discrepancy for both depths. This lower discrepancy may be a result of iPlan being commissioned down to a 5×5mm2 field size whereas Eclipse was commissioned down to 10×10mm2. Six composite HD120 arc planar dose distributions were measured in a Solid Water phantom using EDR film. Eclipse and iPlan show good spatial agreement for all lesion sizes (within 1mm). Central axis dose agreement is within 3% for PTV widths of 7.5mm or greater reflecting the single field results. However, for a PTV width of 5mm, Eclipse results improve compared to the single field results (2.5−12.5% agreement) whereas the iPlan results worsen (12.5% agreement).

This manuscript has been reproduced from the miadilm master. UMI films the text directly from the original or copy submiUed. Thus, m e thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer.The quality of this mproductkn is dependent upon the quality of the copy submitted. Broken or indistinct print, cobred or poor quality illustrations and photographs, print Meedthrough, substandard margins, and improper alignment can adversely affect repmduction.In the unlikely event that Ihe author did not rend UMI a complete manuscript and there are missing pages, Wse will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion.Oversize materials (e.g., mapsl dnwihgs, charts) are reproduced by sectioning the original, beginning at the upper lefthand comer and continuing from left to right in equal sedans with small overlaps.Photqpphs induded in the original manuscript have been teproducd xerographically in this copy. Higher quality 6 ' x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. Bll