PubMed · 4205748
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
J van de Geijn. 1972. Computational methods in beam therapy planning.. https://doi.org/10.1016/0010-468x(72)90026-8
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
PURPOSE: The purpose of this study was to evaluate the characteristics of a new rotating gamma system for stereotactic radiosurgery by comparison with a well accepted system. METHODS AND MATERIALS: A novel gamma unit for stereotactic radiosurgery has been developed and distributed to 15 hospitals in China. The unit contains 30 cobalt-60 gamma radiation sources with initial activity of 200 Ci (7.4 x 10(12) Bq) each. The sources are positioned along 30 arcs, and rotate continuously as a group in an axis orthogonal to the patient's body. Measurements have been made on a representative unit installed in the Auhai Radiosurgery Center at the Beijing Navy General Hospital in the People's Republic of China. Ionization chambers calibrated by an American accredited dosimetry calibration laboratory were used for these measurements, as well as radiochromic film and thermoluminescent dosimeters. The unit tested utilizes collimators of nominal diameters of 4, 8, 14, and 18 mm. Radiochromic film samples from a Leksell Model U Gamma Knife were evaluated by the same laboratory and are presented for comparison. The treatment planning system was not evaluated. RESULTS: Radiation-absorbed dose rates and profiles measured for this unit are comparable to those previously measured with the same techniques for the Leksell Model U Gamma Knife units in San Diego and Atlanta. CONCLUSION: This unit is capable of producing well collimated beams of high energy photons, suitable for stereotactic radiosurgery. It has similar physical characteristics to those previously reported for the Leksell Model U Gamma Knife unit.
Myelopathy is a feared consequence of radiation therapy. Risk factors are multifocal; therefore, total dose calculation is crucial. We evaluated the contribution of scatter radiation to obtain an accurate cumulative spinal cord dose. Twenty patients undergoing three field head and neck radiation by Cobalt or 6 MV Linac had a total cord dose calculated from direct and scatter radiation. The cord was removed from the radiation field at tumor doses no higher than 4,400 cGy. Total tumor dose ranged from 5,400-7,400 cGy (mean 6060). All patients achieved the prescribed dose and none were lost to follow up (mean 36 months). It was found that scatter radiation can contribute as much as 20% extra dose to the spinal cord. Mean extra dose was 9% (range 1%-20%). This additional dose ranged from 52-810 cGy (mean 339 cGy). No apparent difference was seen with Cobalt or Linac source. Our conclusion was that significant additional dose is delivered to the spinal cord by scatter radiation and that scatter may contribute more to the development of myelopathy than previously believed.
A coherent system for the use of scatter correction factors, determined at 10 cm depth, is described for dose calculations on the central axis of arbitrarily shaped photon beams. The system is suitable for application in both the fixed source-surface distance (SSD) and in the isocentric treatment set-up. This is in contrast to some other proposals where only one of these approaches forms the basis of the calculation system or where distinct quantities and data sets are needed. In order to derive the relations in the formalism, we introduced a separation of the phenomena related to the energy fluence in air and to the phantom scatter contribution to the dose. Both are used relative to quantities defined for the reference irradiation set-up. It is shown that dose calculations can be performed with only one set of basic beam data, obtained at a reference depth of 10 cm. These data consist for each photon beam quality of measured collimator and phantom scatter correction factors, in combination with a set of (percentage/relative) depth-dose or tissue-phantom ratio values measured along the central axis of the beam. Problems related to measurements performed at the depth of maximum absorbed dose, due to the electron contamination of the beam, are avoided in this way. Collimator scatter correction factors are obtained by using a mini-phantom, while phantom scatter correction factors are derived from measurements in a full scatter phantom in combination with the results of the mini-phantom measurements. For practical reasons the fixed SSD system was chosen to determine the data. Then, dose calculations in a fixed SSD treatment set-up itself are straightforward. Application in the isocentric treatment set-up needs simple conversion steps, while the inverse approach, from isocentric to fixed SSD, is described as well. Differences between the two approaches are discussed and the equations for the conversions are given.