Influence of Calculation Algorithm on Dose Distribution in Irradiation of Non-Small Cell Lung Cancer (NSCLC)

Purpose:The influence of two different calculation algorithms (“pencil beam” [PB] versus “collapsed cone” [CC]) on dose distribution, as well as the dose-volume histograms (DVHs) of the planning target volume (PTV) and the organs at risk was analyzed for irradiation of lung cancer.Material and Methods:Between 10/2001 and 02/2002 three-dimensional treatment planning was done in ten patients with lung cancer (Helax, TMS®, V.6.01). The PTV, the ipsilateral lung (IL) and the contralateral lung (CL) were defined in each axial CT slice (slice thickness 1 cm). Dose distributions for three-dimensional multiple-field technique were calculated using a PB and a CC algorithm, respectively. Normalization was in accordance with ICRU 50. The DVHs were analyzed relating the minimum, maximum, median and mean dose to the volumes of interest (VOI).Results:Median PTV amounted to 774 cm3. Minimum dose within the PTV was 67.4% for CC and 75.6% for PB algorithm (p = 0.04). Using the CC algorithm, only 76.5% of the PTV was included by the 95% isodose, whereas 90.1% was included when the PB algorithm (p = 0.01) was used. Median volume of IL amounted to 1 953 cm3. Mean dose to IL was 43.0% for CC and 44.0% for PB algorithm (p = 0.02). Median volume of IL within the 80% isodose was 19.6% for CC and 24.1% for PB algorithm (p < 0.01). Median volume of CL amounted to 1 847 cm3. Mean dose to CL was 17.4% for CC and 18.1% for PB algorithm (p < 0.01). Volume of CL within the 80% isodose was 3.3% for CC and 4.1% for PB algorithm (p = 0.03).Conclusion:The CC and PB calculation algorithms result in different dose distributions in case of lung tumors. Particularly the minimum dose to the PTV, which may be relevant for tumor control, is significantly lower for CC. Since it is generally accepted that the CC algorithm describes secondary particle transport more exactly than PB models, the use of the latter should be critically evaluated in the treatment planning of lung cancer.Ziel:Der Einfluss zweier unterschiedlicher Rechenalgorithmen (“pencil beam” [PB] versus “collapsed cone” [CC]) auf die Dosisverteilung sowie die Dosis-Volumen-Histogramme (DVH) des Planungszielvolumens (PTV) und der Risikoorgane wird für die Bestrahlung des Lungenkarzinoms untersucht.Material und Methodik:Zwischen 10/2001 und 02/2002 wurde bei zehn Patienten mit Bronchialkarzinom eine dreidimensionale Bestrahlungsplanung durchgeführt (Helax, TMS®, V.6.01). Das PTV, die ipsilaterale Lunge (IL) und die kontralaterale Lunge (CL) wurden in jeder axialen CT-Schicht definiert (Schichtdicke 1 cm). Die Dosisverteilung für eine Mehrfeldertechnik wurde zunächst unter Verwendung des PB-Algorithmus optimiert. Anschließend wurde die Dosisverteilung der sich dabei ergebenden Bestrahlungspläne unter Beibehaltung der Feldparameter mittels des CC-Algorithmus erneut berechnet. Die Dosis wurde gemäß ICRU 50 normiert. Die DVH von PTV, IL und CL wurden analysiert.Ergebnisse:Das PTV betrug im Median 774 cm3. Die minimale Dosis im PTV war 67,4% für den CC- und 75,6% für den PB-Algorithmus (p = 0,04). Unter Verwendung von CC wurden lediglich 76,5% des PTV von der 95%-Isodose umschlossen, während dies unter Verwendung des PB bei 90,1% der Fall war (p = 0,01). Das mediane Volumen der IL war 1 953 cm3. Die mittlere Dosis in der IL betrug für den CC-Algorithmus 43,0% bzw. für den PB-Algorithmus 44,0% (p = 0,02). Das Volumen der IL innerhalb der 80%-Isodose betrug 19,6% für den CC- und 24,1% für den PB-Algorithmus (p < 0,01). Das mediane Volumen der CL lag bei 1 847 cm3. Die mittlere Dosis im Bereich der CL betrug 17,4% für den CC- und 18,1% für den PB-Algorithmus (p < 0,01). Das Volumen der CL innerhalb der 80%-Isodose war 3,3% für den CC- und 4,1% für den PB-Algorithmus (p = 0,03).Schlussfolgerung:Die Berechnung der Dosisverteilung mit dem CC- bzw. dem PB-Algorithmus führt bei gleicher Feldkonfiguration zu erheblich unterschiedlichen Ergebnissen. Insbesondere die sich dabei ergebende Minimaldosis im Bereich des PTV, welche für die Tumorkontrolle relevant sein kann, ist beim CC-Algorithmus signifikant niedriger. Da der CC-Algorithmus die tatsächlichen Streuungsverhältnisse im Gewebe unterschiedlicher Dichte genauer berücksichtigt als der PB-Algorithmus, sollte die Verwendung des PB-Algorithmus für die Bestrahlungsplanung des Bronchialkarzinoms sehr kritisch beurteilt werden.

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