Gamma Knife Dosimetry and Treatment Planning: Precision, Accuracy, and Beam Data, Lecture notes of Design of Wood Structures

Download Gamma Knife Dosimetry and Treatment Planning: Precision, Accuracy, and Beam Data and more Lecture notes Design of Wood Structures in PDF only on Docsity! Gamma Knife Dosimetry & Treatment Planning Jürgen Arndt Karolinska Hospital Stockholm Sweden AAPM 1999 Gamma Knife dosimetry & treatment planning J. Arndt 1 Introduction .................................................................................................................... 2 Irradiation technique ....................................................................................................... 3 Some principal consequences ...................................................................................... 3 Beam alignment .......................................................................................................... 4 Gamma Knife precision and accuracy ......................................................................... 5 Beam data stored in Gamma Plan.................................................................................... 6 The experimental beam channel .................................................................................. 7 Off Axis Ratios ........................................................................................................... 7 Percentage Depth Dose ............................................................................................... 8 OutPut Factors ............................................................................................................ 8 AAPM 1999 Gamma Knife dosimetry & treatment planning J. Arndt 4 To illustrate this hypothetical discussion above with reality, Off Axis Ratios (OAR:s) of the smallest and largest single beam of the Gamma Knife are compared with corresponding OAR:s of the radio lesion where the contribution from all single beams is combined. It can be seen that the penumbra of the two single beams is approximately the same in spite of the difference in beam diameter. In contrast differ the gradient at the borders of the resulting dose distributions considerably . The less steep dose gradient of the lesion irradiated by the larger beams is to a large extent a geometrical phenomenon. Beam alignment If the region where the radiation focus is said to be located is magnified, it can be seen that the beam axes do not cross exactly at one single point. This fact holds true for all treatment units independent of their technical design. The degree of magnification that is required to see this phenomenon depends however on the technical solution of the treatment unit as well as on its technical and radiophysical tolerances. This beam miss-alignment has two consequences for the quality of dose delivery. The “center of mass” of the radiation focus may be dislocated in relation to what is assumed, leading to a geometrical error in the dose delivery. The dose distribution may also differ from what is the ideal one; that is, the dose is smeared out over a larger region and its magnitude being less than calculated by the treatment planning system. The existence of these errors is acceptable, provided they are kept so small that they have no clinical significance during the life of the treatment unit. An eye must therefore be kept on these errors by means of QA-controls. It is obvious that it is easier to obtain and 1 0 Beam Axes AAPM 1999 Gamma Knife dosimetry & treatment planning J. Arndt 5 to maintain narrow geometrical tolerances with stationary beams as compared to a mobile system, subject to weare and tear. Gamma Knife precision and accuracy We may define the center of the smallest sphere through which all beam axes pass as the radiological Unit Center Point (UCP) or isocenter. The radius of this sphere may then be seen as a measure of the spread of the beam axes or the uncertainty of their location. This uncertainty is called “the precision of the Gamma Knife”. It is a time consuming task to calculate the precision of the Gamma Knife by using the tolerance measurements of all relevant parts made during the manufacture of the unit. Instead a more direct and practical solution may be chosen. Measured dose profiles are compared with those calculated, assuming identical conditions. The only disadvantage with this approach is that the experimental error of the Radiologically defined UCP PrecisionMechanically defined UCP 0 10 20 30 40 50 60 70 80 90 100 110 80 85 90 95 100 105 110 115 120 RELATIVE DOSE (%) REL. DOSE LGP (%) R E L A T IV E D O S E ( % ) Z-AXIS AAPM 1999 Gamma Knife dosimetry & treatment planning J. Arndt 6 used film dosimetry is by far larger than the radiological consequences of the beam alignment we are looking for. Thus, the experimental error rather than the beam alignment determine the tolerance set for the measurement of the precision. The used tolerance of ± 0.5 mm for each of the three axes is questionable as seen from the clinical perspective, but it is the best we at present can do in daily routine A spherical phantom simulating an adult human head is used for the film exposure. The films can be placed at the center of the sphere and the sphere can be rotated so that the film planes are correctly orientated in relation to the stereotactic system. The distance between the mechanically defined Unit Center Point and the one defined by radiological means is called “The Gamma Knife Accuracy”. This deviation is determined by measuring the distance between the radiological and mechanical Unit Center Points. This measurement is made along the three perpendicular axes of the stereotactic frame. The spherical phantom can not be manufactured with sufficient narrow tolerance to be used for this measurement. Instead are films exposed in a very accurately machined tool in which the mechanical Unit Center Point is simulated by a sharp needle. The needle is used to pierce the film. The hole in the film is then compared with the location of the center of the optical density distribution. Beam data stored in Gamma Plan All beam channels of the Gamma Knife are manufactured to very narrow mechanical tolerances. If allowance is made for the decay of the 60 Cobalt, these beam channels may be considered identical and unchanging from a radiophysical point of view. The design of the Gamma Knife is such that all beam channels of the same size are identical, independent of the individual unit or its model. This means that the storage of beam data in the treatment planning system GammaPlan can be greatly simplified. This is a great advantage since it would be difficult, to measure individual beams to the required accuracy inside the unit on site. Despite the simplicity of pre-storing the beam data, this approach has one disadvantage. It requires that the users, must trust in the reliability of this data at any specific installation. 0 0,2 0,4 0,6 0,8 1 1,2 1,4 94 96 98 100 102 104 106 O D Y-AXIS 3.57 mm3.32 mm Film holder AAPM 1999 Gamma Knife dosimetry & treatment planning J. Arndt 9 It is obvious that there are some experimental difficulties involved in measuring the dose rate in the 4 mm helmet. It is even more difficult to measure it on the beam axis of one single beam. The OPF for the 4 mm helmet is therefore subject to the largest uncertainty as compared to the other OPF:s. The numerical value is also subject to some controversy. Numerical values ranging from 0.63 to 0.93 have been measured. The size of the sensitive volume of many detectors and their alignment are two common errors. Most of the experimental errors will result in a value that is to low. The “new” OPF:s are based on fairly recent data measured at different sites in Europe. The data is averaged from measurements with TL-detectors, liquid ionization chamber and semiconductors. The extremes of these measurements have been excluded. The data and its average are shown in the diagram to the left. The only value that significantly deviated from the earlier recommended OPF:s is the one for the 4 mm helmet, which is changed from 0.80 to 0.87. How relevant is the assumption that the OPF:s are the same for all units, independent of their age or model. Careful measurements have been made in five B-model units of different age and also in single beams of the experimental device. These measurements confirmed that there are no measurable differences between the five units or between the measurements in the units and in the single beams. It can therefore be concluded that there are no measurable differences as long as the design of the beam channels is the same, which means that there should be no difference between the U-model and the B- model. The range of published calculated OPF:s for the 4 mm helmet is similar to those measured. The very selected data of the table to the left is chosen for two reasons. The analytical data is 0.80 0.82 0.84 0.86 0.88 0.90 0.92 0.94 0.96 0.98 1.00 0 5 10 15 20 OU T PU T FA CT OR COLLIMATOR 0.87±0.02 0.94±0.01 0.982±0.005 Collimator 4 8 14 Analytical 1) 0.871 0.943 0.979 Monte Carlo 2) 0.879 0.960 0.979 1) P.Nizin, Med. Phys. Dec. 98 2) S. Liu, Karolinska Hospital AAPM 1999 Gamma Knife dosimetry & treatment planning J. Arndt 10 the most recent publication. The data from the Karolinska Hospital is the only MC- calculation into which I had some insight how it was obtained. The OPF recommended by the manufacturer of the Leksell Gamma Knife are shown in the table to the left. The table to the left is an attempt to describe relevant characteristics of the used detectors with respect to dose measurements in very narrow beams. The number of plusses or minuses are by no means statements - rather indications. It can be seen, also with this crude classification that the liquid ionization chamber is superior for measurements in narrow beams. The only reason why the data for the OPF:s are not based only on measurements with the liquid ionization chamber is that there at present exists little practical experience in narrow beam application. Detector Si TLD Liquid Small volume +++ - ++ Energy independence -- + +++ Dose rate independence +++ ++ ++ Directional independence - + +++ High signal/volume ratio +++ + +++ The used detectors have all their limitations 18 mm helmet 1.00 14 mm helmet 0.98 8 mm helmet 0.95 4 mm helmet 0.87