RESUMEN
The dosimetric leaf gap (DLG) is a parameter used by Eclipse to model the rounded leaf ends of Varian MLCs. The DLGs were determined for the Millennium (M120) and High-Definition (HD120) model MLCs and taken as the difference between measured (0.6mm diode, IBA) and nominal MLC-defined profile FWHM values. Configuring the Eclipse pencil beam algorithm with the measured DLG gave poor agreement between measured and calculated IMRT dose distributions for the HD120 but not the M120. Agreement was optimized by adjusting the DLG for the HD120; 0.3mm changes in DLG were enough to cause significant variations in field dose agreement. Optimal DLG values of 0.04cm and 0.05cm were found for the 6MV HD120 and 10MV HD120, respectively, and 0.135cm 0.175cm for the 6MV M120 and 18MV M120, respectively. Agreement between measured and calculated dose distributions worsened for the AAA algorithm indicating separate DLG values may be required. A leaf calibration software upgrade also reduced agreement by changing the physical leaf position for a given location value. The change was detected using film and the picket fence MLC-pattern which places the two banks of opposing leaves at the same position but at different times. The DLG value can be adjusted from its measured physical value to improve the dosimetric accuracy of Eclipse IMRT plans and compensate for the effects of treatment planning algorithm and varying leaf calibrations. Since leaf calibrations are variable it is important to define the dosimetric leaf gap for each accelerator and clinic.
RESUMEN
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.
Asunto(s)
Aceleradores de Partículas/normas , Radiocirugia/normas , Algoritmos , Fenómenos Biofísicos , Biofisica , Estudios de Evaluación como Asunto , Humanos , Aceleradores de Partículas/instrumentación , Aceleradores de Partículas/estadística & datos numéricos , Radiocirugia/instrumentación , Radiocirugia/estadística & datos numéricos , Reproducibilidad de los Resultados , RotaciónRESUMEN
The potential for radiosurgery with an isocentric teletherapy cobalt unit was evaluated in three areas: (1) the physical properties of radiosurgical beams, (2) the quality of radiosurgical dose distributions obtained with four to ten noncoplanar converging arcs, and (3) the accuracy with which the radiosurgical dose can be delivered. In each of these areas the cobalt unit provides a viable alternative to an isocentric linear accelerator (linac) as a radiation source for radiosurgery. A 10 MV x-ray beam from a linac used for radiosurgery served as a standard for comparison. The difference between the 80%-20% penumbras of stationary radiosurgical fields in the nominal diameter range from 10 to 40 mm of the cobalt-60 and 10 MV photon beams is remarkably small, with the cobalt-60 beam penumbras, on average, only about 0.7 mm larger than those of the linac beam. Differences between the cobalt-60 and 10 MV radiosurgical treatment plans in terms of dose homogeneity within the target volume, conformity of the prescribed isodose volume to the target volume, and dose falloffs outside the target volume are also minimal, and therefore of essentially no clinical significance. Moreover, measured isodose distributions for a radiosurgical procedure on our Theratron T-780 cobalt unit agreed with calculated distributions to within the +/- 1 mm spatial and +/- 5% numerical dose tolerances, which are generally specified for radiosurgery. The viability of isocentric cobalt units for radiosurgery will be of particular interest to centers in developing countries where cobalt units, because of their relatively low costs, provide the only megavoltage source of radiation for radiotherapy, and could easily and inexpensively be modified for radiosurgery. Of course, the quality assurance protocols and mechanical condition of a particular teletherapy cobalt unit must meet stringent requirements before the use of the unit for radiosurgery can be advocated.