Variations in volume and spatial distribution of contour delineation on GTV among different facilities

Purpose: To verify operator-based contour delineation on GTV (Gross Tumor Volume) among different facilities using radiotherapy planning systems (RTPS), and to compare their results in terms of volume and spatial distribution. Method: GTV outlines were delineated for two pulmonary carcinoma (T1N0M0) cases in six radiation therapy facilities using the Radiotherapy Treatment Planning System (RTPS) capability of each facility. Image sizes and resolutions were different among each facility. Therefore, helped by anatomical landmarks, the tumor and surrounding regions were excised using sequential CT images in order to standardize all of the images to equal size (500 × 500 pixels) and position. Image magnifications (pixels/mm) was calculated from the image scale shown on the RTPS. A summation of the GTV areas for each section were calculated and multiplied by the slice thickness to obtain the calculated value of GTV (GTVc). GTV images were analyzed to study differences in GTV among the various facilities. The mean and variance of the GTV images for all the facilities were obtained. Result: The GTVc values were 7.0 ± 0.7 ml (case 1) and 12.3 ± 1.8 ml (case 2). The values obtained directly from RTPS (GTVr) were 7.0 ± 0.7 ml and 12.1 ± 1.7 ml, respectively. Differences were seen among the volumes and spatial distributions of GTV Variation of the image in case 2 indicated that GTV differences were large where the bronchus and the vessels were close to the tumor. Conclusion: Image areas that overlap vascular structures tend to indicate relatively large variations in GTV.