Impact of different CT slice thickness on clinical target volume for intensity modulated radiation therapy : A phantom study

Abstract

Purpose : To assess the accuracy of target volume with different computed tomography (CT) slice thickness-interval and the impact of different CT slice thickness on intensity modulated radiation (IMRT) treatment planning.

Objectives : To correlate the different CT slice thickness/interval with the variation of target volume on IMRT planning.

Design : Experimental study

Setting : Department of Radiology, Faculty of Medicine, Chulalongkorn University.

Materials and Methods : The IMRT thorax phantom (30 cm wide, 30 cm long and 20 cm thick), containing a tissue equivalent rod (2.5 cm diameter and 16 cm long) underwent CT scan with five different slice thickness/ interval (1.25 mm/1.25 mm, 2.5 mm/2.5 mm, 3.75 mm/3.75 mm, 5mm/5mm and 3.75 mm/1.25 mm). The target volume (tissuerod) was outlined on all scans by two observers. The computed volumes for each CT dataset were compared with the known true volume. IMRT planning was performed on all five CT datasets. The field size and dose-volume histogram (DVH) were compared.

Results : The difference of percentage volume between calculated and true volume of the tissue rod with CT slice thickness/interval 1.25/1.25 mm and 3.75/1.25 mm was less than 1%, whereas the 5/5 mm slice thickness/interval was 4.5%. The differences in the accuracy of the target volume among each CT slice thickness/ interval were small. However, the target volume was most accuracy on 1.25/1.25 mm and 3.75/1.25 mm slice thickness/ interval, respectively. Similar finding was seen by both observers. The differences in volume were caused by partial volume effect. The differences in field size of IMRT planning were small. However, the field size in measurements of Y axis was always larger on 1.25/1.25 mm and 3.75/1.25 mm CT scan, about 0.3 cm compared to other series. The DVH of each CT dataset was not different in term of the percentage of relative volume.

Conclusion : Accuracy of target volume delineation depended on CT slice thickness/interval. Target volume was most accurate on the 1.25/1.25 mm and 3.75/1.25 mm slice thickness/interval, respectively. Differences in contour volume may be due to partial volume effect. Different CT slice thickness had a small effect on field size in y axis and DVH of IMRT planning.