INFLUENCE OF CT SLICE THICKNESS ON VOLUME AND DOSE UNCERTAINTY FOR DIFFERENT ORGANS DURING TREATMENT PLANNING FOR EARLY PROSTATE CANCER

Abstract

The purpose of this study was to evaluate influence of different computed tomography (CT) slice thickness on gross target volume (GTV) and volumes of organs at risk (OAR), impact on the quality of digitally reconstructed radiographs (DRR) and dose volume histograms (DVH) during 3-dimensional conformal radiotherapy treatment planning. Ten patients with early prostate cancer were selected and CT scans with 2mm, 3mm and 5mm slice thicknesses were performed in sequence. The GTV, bladder and rectum were contoured in all scans. 3-dimensional planning was performed for all three CT datasets. The target coverage and isocenter shift between treatment plans for different slice thickness were correlated with tumor volume. Further comparative and dosimetric analysis was done for resultant DRRs and DVH respectively. For the prostate, no difference in mean volumes was seen for 2mm and 3mm scans (43.11cc, 40.2cc respectively) and were found larger as compared to 5mm scans (35.8cc) p value 0.0001. Similarly mean rectal and bladder volumes were found larger on 2mm and 3mm scans as compared to 5 mm scans p=0.002. The DVH data showed target underdosage was 8% for 5mm slice thickness as compared to 2mm and 3mm slice thicknesses. The quality of DRRs was found better for 2mm and 3mm scans. CONCLUSION: GTV and organs at risk (bladder and rectum) volumes were found larger on 2mm and 3mm slice thicknesses as compared to 5 mm scans, similarly better quality of DRRs for 2mm and 3mm scans. Significant tumor underdosage was seen on 5mm slice thickness.

Key Words : CT slice thicknesses, organ volumes, Isodose curves, dose volume histograms, early prostate cancer.