Abstract
Background and Purpose:
Elective pelvic nodal irradiation using intensity-modulated radiation therapy (IMRT) is widely utilized in selected prostate cancer patients. Interobserver variability in pelvic lymph node contouring may affect target dose coverage and organs at risk (OARs) sparing. This study systematically evaluated the dosimetric consequences of interobserver contouring variability among three independent radiation oncologists.
Materials and Methods:
In this retrospective treatment planning study, three radiation oncologists independently delineated the clinical target volume (CTV) for pelvic lymph nodes in 60 prostate cancer patients. For each observer-specific contour set, dose–volume histogram (DVH) parameters were generated for the planning target volume (PTV) and selected OARs (bladder, rectum, small bowel, and femoral heads). Primary endpoints included pelvic nodal volume, PTV V95%, PTV D_mean, as well as OAR D_mean and D_max. Interobserver differences were evaluated using repeated-measures ANOVA, and agreement was quantified using the intraclass correlation coefficient (ICC). Bland–Altman analysis and boxplots were employed to characterize interobserver variability.
Results:
Mean pelvic lymph node contour volumes were 259.18 ± 11.65 cm³ for Observer 1, 263.70 ± 11.41 cm³ for Observer 2, and 267.16 ± 11.40 cm³ for Observer 3 respectively. Target coverage remained highly robust across observers, with no statistically significant difference in PTV V95% (p=0.178). Regarding OARs, bladder and rectum D_mean remained stable across observers, whereas bladder and rectum D_max differed significantly (both p<0.001). The small bowel exhibited the greatest dosimetric sensitivity, demonstrating significant differences in both D_mean and D_max (both p<0.001). Femoral head D_mean also varied significantly p<0.001), whereas femoral head D_max remained stable (p=0.851).
Conclusions:
Interobserver contouring variability had minimal impact on PTV coverage, but significantly influenced PTV D_mean and several OAR dosimetric parameters, particularly small bowel exposure and peak doses to the bladder and rectum. These findings emphasize the necessity of implementing standardized contouring guidelines and structured peer-review processes to mitigate clinically relevant dosimetric variation.