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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

Presentations

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

Purpose:
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.

Methods:
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).

Results:
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.

Conclusion:
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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