Helikal Tomoterapi ile Prostat Işınlamalarında Kritik Organ Dozlarının Cihaz İçi Parametrelere göre Değerlendirilmesi
Yürekli, Ahmet Fatih
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The treatment planning of Tomotherapy depends on three device parameters, i.e. field width (FW), pitch (P) and modulation factor (MF). Various treatment plans can be generated with different combinations of these parameters. In this study, the effect of these parameters on dose distribution and treatment time were analyzed to determine the optimum parameter set for the treatment planning of prostate cancer. Ten high risk prostate cancer patients already treated with radiotherapy were included in the study. CTV consisted of gland and seminal vesicles while PTV was created by giving 4mm margin from posterior and 7 mm margin from the other sides of CTV. 48 different plans were created for each patient by using combinations of the three parameters (FW = 1.05, 2.5, 5.0 cm; P = 0.172, 0.215, 0.287, 0.430; MF = 1.8, 2.0, 2.2, 2.4). The plans of each patient were created at the end of 1000 iterations using constant intrinsic constraints. Constraints were set not to break the conditions D_(PTV,2) ≤ 81.3 Gy and D_(PTV,95) ≥ 76 Gy. Conformity number (CN), homogeneity index (HI) and treatment time (t) based comparisons were performed by analyzing dose volume histograms (DVH). When FW=5.0 cm, the shortest treatment time was obtained. However, dose fall off was not sharp and the V_x values of organ at risk (OAR) were high. In contrast, while the best CN, HI and OAR V_x values were obtained in FW= 1.05 cm, the treatment time was almost 3 times by FW= 5.0 cm. The effects of P variations on the difference of average maximum-minimum OAR V_x values are founds in the range of 0.1 cc and 0.9 cc, while the effects of MF variations were in the range of 0.7 cc and 3.6 cc. Throughout this study, the optimal parameter set for obtaining optimum values of OAR V_x , CN, HI and treatment time is suggested as FW= 2.5 cm, MF=2.4 and P=0.215.