AbstractID: 7018 Title: CT/MR Volume averaging effects in treatment planning.
Conformal IMRT planning may generate sharp dose gradients in the boundary of target and surrounding tissues. Precise volume
definition is critical to obtain optimal treatment planning. The target volume and the Dose Volume Histogram may differ depending on the
slice thickness, image modality and the pixel size of the scan performed.
Commercially available High Precision Lucy Phantom was used for the study. The volume measurements were done using AcQSim and
Cadplan. Dose volume histograms(DVH) were calculated in Cadplan. Volume effects on radiotherapy conformal treatment planning of
a patient scanned in CT and MRI with different slice thickness were evaluated. Cadplan DVH volume and dose calculations were
evaluated with independent calculation on ideal geometrical shapes.
The volumes in the phantom were contoured manually in AcQSim. A variation up to 4% was observed for the volumes contoured using 3
and 5 mm scan. The volumes of 10mm CT scan were exaggerated up to 20%. The 3mm MR scan were closer to the real volumes. The
thicker the MR slices, the fuzzier the volume edges were. The volumes in 5 mm slice scan had an error of up to 17%. The volumes in
10mm slice scan were consistently exaggerated, with the maximum error of up to 36% was observed. A deviation of up to 5% was
typically observed for Cadplan DVH compared with an ideal calculations. The larger deviations > 5% were observed from small long
objects spanning multiple slices. An optimal slice thickness recommended is 3mm for both CT and MR Scan.