PubMed · 5022723
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1972-04-22. Measuring radiation.. https://pubmed.ncbi.nlm.nih.gov/5022723/
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The most commonly used technique for matching two adjoining parallel opposed fields has been geometric matching with a skin gap calculation. Many modifications have been suggested in order to minimize the shortcomings of this technique. Most of these, however, are more complex and time consuming in clinical practice, with little dosimetric improvement. We propose a simple new method for adjacent field abutment that eliminates hot and cold spots due to different beam divergences. This method is based upon the use of a single match plane for both the cephalad and caudad parallel opposed beams. The divergence angle of these opposed beams is forced to be equal at the junction through the use of the same field length and the same target axis distance (TAD). For the isocentric treatment case, the procedure is summarized as follows. After setting up the cephalad AP/PA fields to midplane in the conventional manner, the patient support couch is shifted longitudinally in the cephalad direction by a precise distance equal to the cephalad field length. The superior collimator of the caudad field is kept the same as for the cephalad field to ensure equal divergence angles. The actual irradiated volume of the caudad field is adjusted as needed by a large block or an asymmetric jaw. An analogous procedure is described for the nonisocentric case. This new isocenter shift method culminates in several distinct advantages over the other geometric methods.(ABSTRACT TRUNCATED AT 250 WORDS)
Three dosimetric methods for commissioning the dynamic wedges on a Varian 600C linear accelerator operating at 4.7 MV were compared. The techniques involved the use of ionization chambers, x-ray verification film and solid state detectors. Ionization chambers gave the most accurate results, as expected, but data acquisition was very time consuming. Film dosimetry was found to have good spatial resolution for beam profiles, but problems were encountered with the acquisition of depth doses. Solid state detectors generally showed reasonable agreement with the ionization chambers.
PURPOSE: Circular collimated X-ray beams rotated in various non-coplanar planes as commonly used in stereotactic linac-based radiotherapy are generating spherical dose distributions with typically steep dose gradients. In case of irregular, especially elongated lesions, this technique fails to conform the shape of the corresponding planning target volume, and a collimator aperture has to be selected which allows for an acceptable dose distribution only on the expense of an excess dose to a larger amount of normal tissue. Therefore we have investigated a bicentric irradiation technique with sequentially treating an elongated target volume over two separated isocenters. METHODS: For the computer simulation studies we used the planning system of the stereotactic unit SRS-200 by Philips. Selected combinations of different collimator aperture were examined systematically by varying the distance between the two isocenters. Dose distributions for a specially developed head-phantom were calculated and analysed. RESULTS: Optimization tables were generated taking into account the geometric quality criteria as well as the corresponding dose maxima. For a given size of the target volume, the appropriate parameters for the bicentric radiation treatment can be chosen. The calculated dose distributions turned out to be independent of the positioning of the two isocenters within the phantom. CONCLUSION: The optimization tables serve to select a set of starting values for the interactive planning process of an elongated target volume resulting in a significant decrease of the overall time for treatment planning of irregular shaped lesions.