Beispiel für die gestaltung der abstracts für die tagung 2006 (14 pt, fett)
Comparison of breast cancer treatment plan properties using two different dose calculation algorithms
M. Sassowsky, D. Frauchiger, E. Born, R. Mini Division of Medical Radiation Physics, Inselspital Berne and University of Berne, Switzerland mail: Introduction Today different dose calculation algorithms are available in treatment planning systems (TPS). Two examples are the analytical anisotropic algorithm (AAA) and the pencil beam algorithm (PBC). In the presence of pronounced tissue inhomogeneities, the dose distribution calculated by AAA is generally considered closer to the reality than the one calculated by PBC, as it includes a modelling of the lateral energy transport. From the physicist’s point of view, this would imply using AAA instead of PBC. However, past clinical experience was gathered with PBC, and a change in the calculation algorithm might lead to a different tumor control probability (TCP) and/or normal tissue complication probability (NTCP). It is thus advisable to compare the properties of treatment plans calculated with AAA and PBC – preferably for a larger set of plans instead of a single plan. In the treatment of breast cancer, the lung is an important organ at risk. Breast cancer treatment plans involve the boundary between low density lung tissue and other tissue of higher density. The aim of this study is to investigate possible consequences when changing the dose calculation algorithm from PBC to AAA. Material and Methods 72 clinically applied breast cancer plans calculated with PBC were recalculated with AAA. The plans were stratified into 4 groups (left/right; with/without suprax fields). For each patient the difference of the DVHs calculated with AAA and PBC, respectively, was determined. Average DVHs were calculated to obtain information representative for each group of plans. The shapes of the averaged DVHs were compared between the four groups of plans, and between AAA and PBC. The number of MUs to obtain the prescribed PTV D50% was compared between AAA and PBC. Furthermore, the deviation between the lung V20Gy and the lung Dmean, calculated with PBC and AAA, respectively, was calculated. Results From the set of the 72 plans, the average changes of the abovementioned quantities have been determined. Compared to AAA:
PBC underestimates the number of MU to obtain the prescribed PTV D50% by:
(uncertainties are given as 1 standard deviation)
Discussion When changing the calculation algorithm from PBC to AAA, one could either adapt the dose prescription and the accepted limit on V20Gy. Or, given the fact that the magnitude of observed differences in the abovementioned quantities appears small, one might prefer to keep the prescription dose and limit of V20Gy. This approach would lead to a marginally increased TCP and a more cautious evaluation of the plans with respect to the lung exposure.
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