Dosimetric Effects Due To Inter-Observer Variability Of Organ Contouring When Utilizing A Knowledge-Based Planning System For Prostate Cancer

Purpose: Radiotherapy is a widely accepted standard of care for early-stage prostate\r\ncancer, and it is believed that the plan quality and treatment outcome are\r\nassociated with contour accuracy of both the target and organs-at-risk (OAR).\r\nThe purposes of this study are to 1) assess geometric and dosimetric\r\nuncertainties due to inter-observer contour variabilities and 2) evaluate the\r\neffectiveness of geometric indicators to predict target dosimetry in prostate\r\nradiotherapy. Methods: Twenty prostate patients were selected for this\r\nretrospective study. Five experienced clinicians created unique structure sets\r\ncontaining prostate, seminal vesicles, bladder, and rectum for each patient. A\r\nfully automated script and knowledge-based planning routine were utilized to\r\ncreate standardized and unbiased plans that could be used to evaluate changes\r\nin isodose distributions due to inter-observer variability in structure\r\nsegmentation. Plans were created on a “gold-standard” structure set, as well as\r\non each of the user-defined structure sets. Results: Inter-observer\r\nvariability of contours during structure segmentation was very low for clearly\r\ndefined organs such as the bladder but increased for organs without\r\nwell-defined borders (prostate, seminal vesicles, and rectum). For plans\r\ngenerated with the user-defined structure sets, strong/moderate correlations\r\nwere observed between the geometric indicators for target structure agreement\r\nand target coverage for both low-risk and intermediate-risk patient groups,\r\nwhile OAR indicators showed no correlation to final dosimetry. Conclusions: Target delineation is crucial in order to maintain adequate dosimetric coverage\r\nregardless of the associated inter-observer uncertainties in OAR contours that\r\nhad a limited impact upon final dosimetry.