CT slice index and thickness: Impact on organ contouring in radiation treatment planning for prostate cancer

Abstract

Objective: To assess the impact of CT slice index and thickness (3 mm versus 5 mm) on (i) prostate volume, dimensions, and isocenter coordinates, (ii) bladder and rectal volumes, and (iii) DRR quality, in the treatment of prostate cancer. Methods: 16 patients with prostate cancer underwent two planning CT‐scans using 3 and 5 mm slice index/thickness. Prostate, bladder, and rectum were outlined on all scans. Prostate isocenter coordinates, maximum dimensions, and volumes were compared along with bladder and rectal volumes. Bladder volumes and maximum diameters were further investigated using a second observer. A comparative analysis of DRR quality was conducted as well as a dosimetric analysis using DVH. Results: The differences in measurements of prostate volume, isocenter coordinates and maximum dimensions between the 3 and 5 mm scans, were small and not statistically significant. Similar finding was seen for rectal volume. However, bladder volume was always larger on the 3 mm scan (mean difference=27.9 cc; SE=4.8 cc; 95% CI: 17.7−38.2 cc; p<0.001) and the findings were reproduced with the second observer (mean difference=31.9 cc; SE=4.7 cc; 95% CI: 21.9−41.9 cc; p<0.001). The differences in volume are caused by a slight increase in (1) the measurement of the longitudinal dimensions on the 3 mm scans, and (2) the slice by slice measured bladder area on the 3 mm scans. The latter is due to partial volume effect. The 3 mm DRR were slightly better than the 5 mm DRR. The bladder DVH differed significantly in some patients. Conclusion: Bladder volume is significantly larger on the 3 mm scans. Differences in contoured areas may be accounted for, in part, by the partial volume effect.PACS number(s): 87.57.–s, 87.53.–j

Impact of Different CT Slice Thickness on Clinical Target Volume for 3D Conformal Radiation Therapy

“…The variation in the target volume is mainly due to spatial positioning of the volume scanned and the CT slice thickness used for planning. Berthelet et al, 15 studied the impact of 3 mm vs. 5 mm on prostate volume, dimensions, isocentric co-ordinates, bladder, and rectal volume and concluded that 3-mm scans showed slightly larger bladder volume and accounted the difference for partial volume effect. In our study, it is seen that the surface irregularity in the 3D reconstructed target volume gets evened out as the slice thickness increases.…”

“…The importance of target definition was studied by several authors. [11][12][13][14][15][16][17][18] The Photon Treatment Planning Collaborative Work Group (PTPCWG, 1991) recommended that accurate definition of the inferior and superior borders of the treatment volume for localization requires a close spacing between sections, and contiguous CT slices with a thickness in the range of 3 to 5 mm for the head and 5 to 10 mm for the body should be used to achieve a reasonable compromise between resolution and throughput. 19 PTPCWG recommended that further study is required for optimization of CT slice thickness used in radiotherapy.…”

Organ contouring for prostate cancer: Interobserver and internal organ motion variability

“…Moreover, organ motion (and, in particular, motion of the prostate itself) during the course of treatment may lead to variations in margin determination. In fact, organ motion as a geometrical variation in the determination of target volumes has been investigated by a number of researchers (see Berthelet et al [2], Wu [5], for instance), some of whose studies have been reviewed in [6].…”

“…The influence of the thickness of the CT slice in organ delineation and volume determination has been considered, for instance, in Berthelet et al [2] (see references therein). Here we report on a stereological study for prostate volume estimation which includes an error approximation of this estimation depending on the number of CT slices and the contour delineations.…”

“…The variability that exists in the determination of target volumes amongst radiation oncologists and institutions, that is, the interobserver variability inherent to manual delineation of normal tissue anatomical structures, such as the prostate or bladder, has been shown to influence the treatment process for prostate cancer (see Berthelet et al [1,2], Gual-Arnau and Masó [3] and Rasch et al [4]). Moreover, organ motion (and, in particular, motion of the prostate itself) during the course of treatment may lead to variations in margin determination.…”