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The Effect of Slice Thickness On IMRT Planning

S Srivastava

S Srivastava1,2,4*, C Cheng1,3 , I Das1,2,4 , (1) Purdue University, West Lafayette, IN,(2) Indiana University Health Methodist Hospital, Indianapolis, IN, (3) University Hospitals Case Medical Center, Cleveland, OH, (4) Indiana University- School of Medicine, Indianapolis, IN


SU-E-T-319 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

The accuracy of volume estimated of a treatment planning system is investigated in this study. In addition, the effect of slice thickness on IMRT planning is also studied.

The accuracy in volume determination was investigated using a water phantom containing various objects with known volumes ranging from 1-100cm³. The phantom was scanned with different slice thickness (1-10 mm). The CT data sets were sent to Eclipse TPS for contour delineation and volume calculation. The effect of slice thickness on IMRT planning was studied using a commercial phantom containing four different shaped objects. The phantom was scanned with different slice thickness (1-5 mm). IMRT plans were generated for the different CT datasets to calculate TCP, homogeneity (HI) and conformity indices (CI).

The variability of volumes with CT slice thickness was significant especially for small volume structures. The minimum and maximum error in the volume estimation is in the range of -2.3% to 92%. On the other hand, with increasing slice thickness, the PTV mean dose and TCP values decreases. Maximum variation of ~5% was observed in mean dose and ~2% in TCP with slice thickness change from 1-5 mm. The relative decrease in target volume receiving 95% of prescribed dose is ~5% slice thickness change from 1-5 mm. HI increases up to 163% and CI decreases by 4% between 1-5 mm slice thickness change, producing highly inhomogeneous and least conformal plan.

Accuracy of volume estimation is dependent on CT slice thickness and the contouring algorithm in a TPS. During TPS commissioning and for all clinical protocols, evaluation of volume should be included to provide the limit of accuracy in DVH calculation. A smaller slice thickness provides superior dosimetry with improved TCP values. Thus, the smallest possible slice thickness should be used for IMRT planning

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