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22 prostate and 20 H&N plans were considered in the study. Clinical plans were optimized without limiting MU. Plans were then optimized with the same settings, clinical objective and constraint on DVH but enabling the limit MU option. MU per arc limit was chosen as compromise between MU reduction and DVH constraints quality. Plan data for analysis were obtained from the TPS system by means of ad hoc IronPython scripts.
The statistical comparison between MUs, segments sizes and dose-rates of plans optimized with or without MU limit was performed using Wilcoxon paired signed-rank test.
To quantify the effect of MU limit on dose distribution quality, analysis of dose homogeneity index, conformity index and DVH constraints was executed. Rectum V50Gy Gy, bladder V65Gy, average dose of right and left femoral head, body V5Gy and maximum dose, were considered for prostate plans. For H&N plans, spinal cord D2%, right and left parotid glands average dose, mandible V50Gy, body V5Gy and maximum dose were evaluated.
Plans were delivered on ArcCHECK phantom (Sun Nuclear Corporation, Melbourne, Florida) to evaluate dose delivery accuracy. Statistical analysis of the gamma passing rate with different thresholds (1% 1mm, 2% 2mm, 3% 3mm) obtained in MU reduction and no MU reduction plans was performed.
RESULTS AND DISCUSSIONS. Statistical significant differences in MUs were found for both prostate and H&N plans. Mean arc MU reduction of 25% and 19% were found for prostate and H&N plans, respectively. The mean variations of percentage number of leaves pairs with gap under 0.5 cm, 1 cm, 2 cm and 3 cm after MU reduction were equal to -7%, -15%, -13% and -10% for prostate plans and equal to -8%, -11%, -10% and -7% for H&N ones.
Statistical significant differences of dose-rate were found for both anatomical districts.
Rectum V50Gy and spinal cord D2% showed statistical significant differences: both these organs are critically involved in the PTV coverage - organ at risk dose sparing balance. The differences, 1.6 percentage points for rectum V50Gy and 0.67 Gy for spinal cord D2%, were not clinically relevant. No significant differences were observed for other DVH constraints.
Statistical significant increase in gamma passing rate was found for all the evaluated conditions.
Tesi di Specialità
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