Interobserver Variations in Target Delineation in Intensity-Modulated Radiation Therapy for Nasopharyngeal Carcinoma and its Impact on Target Dose Coverage

Abstract

Background: To investigate the differences between physicians in target delineation in intensity-modulated radiation therapy for nasopharyngeal carcinoma as well as their impact on target dose coverage.

Methods: Ninety-nine in-hospital patients were randomly selected for retrospective analysis, and the target volumes were delineated by 2 physicians. The target volumes were integrated with the original plans, and the differential parameters, including the Dice similarity coefficient (DSC), Hausdorff distance (HD), and Jaccard similarity coefficient (JSC) were recorded. The dose-volume parameters to evaluate target dose coverage were analyzed by superimposing the same original plan to the 2 sets of images on which the target volumes were contoured by the 2 physicians. The significance of differences in target volumes and dose coverage were evaluated using statistical analysis.

Results: The target dose coverage for different sets of target volumes showed statistically significant differences, while the similarity metrics to evaluate geometric target volume differences did not. More specifically, for PGTVnx, the median DSC, JSC, and HD were 0.85, 0.74, and 11.73, respectively; for PCTV1, the median values were 0.87, 0.77, and 11.78, respectively; for PCTV2, the median values were 0.90, 0.82, and 16.12, respectively. For patients in stages T3-4, DSC, and JSC were reduced but HD was increased compared to those in stages T1-2. Dosimetric analysis indicated that, for the target volumes, significant differences between the 2 physicians were found in D95, D99, and V100 for all the target volumes (ie, PGTVnx, PCTV1, and PCTV2) across the whole group of patients, as well as in patients with disease stages T3-4 and T1-2.

Conclusions: The target volumes delineated by the 2 physicians had a high similarity, but the maximal distances between the outer contours of the 2 sets were significantly different. In patients with advanced T stages, significant differences in dose distributions were found, stemming from the deviations of target delineation.

Keywords: dosimetric analysis; intensity-modulated radiation therapy; nasopharyngeal carcinoma; target delineation; target volumes.

Figure 1.

Box plot for DSC, JSC, and HD of PGTVn.

Figure 2.

Box plot for DSC, JSC, and HD of PCTV1.

Figure 3.

Box plot for DSC, JSC, and HD of PCTV2.

Figure 4.

A patient presented with stage T4N2M0 disease. The thin line indicates the target volume delineated by physician A. The thick line indicates the target volume delineated by physician B. The red line indicates PGTVnx, the green line indicates PCTV1, and the blue line indicates PCTV2. The orange-colored area indicates the dose of 70 Gy, the green-colored area indicates the dose of 60 Gy, and the blue-colored area indicates the dose of 54 Gy.

Figure 5.

(a) to (c) The target delineation for 3 different NPC patients. The thin line indicates the target volume delineated by physician A. The thick line indicates the target volume delineated by physician B. The red line indicates GTVnx, the green line indicates CTV1, and the blue line indicates CTV2. The overall size of each target volume between physicians A and B is similar, however, the deviation in delineation at the CT cross-sections is clearly seen.

Figure 6.

(a) to (c) The PCTV2 of the target volumes delineated by the 2 physicians. The thin line indicates the target volume delineated by physician A. The thick line indicates the target volume delineated by physician B. The blue line indicates CTV2, and the purple line indicates PCTV2. (c) The maximal distance between the outer contours delineated by the 2 physicians reached 3.64 cm.

Figure 7.

A patient presented with stage T4N2M0 disease. The thin line indicates the target volume delineated by physician A. The thick line indicates the target volume delineated by physician B. The red line indicates PGTVnx, the green line indicates PCTV1, and the blue line indicates PCTV2. Edema was observed in the external pterygoid muscle. Although MRI imaging had been integrated, it was still difficult to clearly identify the boundary of the tumor, and the differences in the target volumes delineated by the 2 physicians were clearly seen.