Chulalongkorn Medical Journal


Purpose : To assess the accuracy of target volume with different computedtomography (CT) slice thickness-interval and the impact ofdifferent CT slice thickness on intensity modulated radiation(IMRT) treatment planning.Objectives : To correlate the different CT slice thickness/interval with thevariation of target volume on IMRT planning.Design : Experimental studySetting : Department of Radiology, Faculty of Medicine, ChulalongkornUniversity.Materials and Methods : The IMRT thorax phantom (30 cm wide, 30 cm long and 20 cmthick), containing a tissue equivalent rod (2.5 cm diameter and16 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 computedvolumes for each CT dataset were compared with the knowntrue 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 andtrue volume of the tissue rod with CT slice thickness/interval1.25/1.25 mm and 3.75/1.25 mm was less than 1%, whereasthe 5/5 mm slice thickness/interval was 4.5%. The differences inthe accuracy of the target volume among each CT slice thickness/interval were small. However, the target volume was mostaccuracy 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 alwayslarger on 1.25/1.25 mm and 3.75/1.25 mm CT scan, about0.3 cm compared to other series. The DVH of each CT datasetwas not different in term of the percentage of relative volume.Conclusion : Accuracy of target volume delineation depended on CT slicethickness/interval. Target volume was most accurate onthe 1.25/1.25 mm and 3.75/1.25 mm slice thickness/interval,respectively. Differences in contour volume may be due topartial volume effect. Different CT slice thickness had a smalleffect on field size in y axis and DVH of IMRT planning.


Faculty of Medicine, Chulalongkorn University

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