Clinical adequacy assessment of auto-contours for prostate IMRT with meaningful endpoints

To determine if radiation treatment plans created based on auto-segmented (AS) regions-of-interest (ROI)s are clinically equivalent to plans created based on manually segmented ROIs, where equivalence is evaluated using probabilistic dosimetric metrics and probabilistic biological endpoints for prostate IMRT.

Manually drawn contours and auto-segmented ROIs were created for 167 CT image sets acquired from 19 prostate patients. Auto-segmentation was performed utilizing Pinnacle's Smart Probabilistic Image Contouring Engine. For each CT set, 78 Gy/39 fraction 7-beam IMRT treatment plans with 1 cm CTV-to-PTV margins were created for each of the three contour scenarios; PMD using manually delineated (MD) ROIs, PAS using auto-segmented ROIs, and PAM using auto-segmented organ-at-risks (OAR)s and the manually drawn target. For each plan, 1000 virtual treatment simulations with different systematic errors for each simulation and a different random error for each fraction were performed. The statistical probability of achieving dose-volume metrics (coverage probability (CP)), expectation values for normal tissue complication probability (NTCP), and tumor control probability (TCP) metrics for all possible cross-evaluation pairs of ROI types and planning scenarios were reported. In evaluation scenarios, the root-mean-square loss (RMSL) and maximum-absolute loss (MAL) of coverage probability of dose-volume objectives, E[TCP], and E[NTCP] were compared with respect to the base plan created and evaluated with manually drawn contours.

Femoral head dose objectives were satisfied in all situations, as well as the maximum dose objectives for all ROIs. Bladder metrics were within the clinical coverage tolerances except for the auto-segmented plan evaluated with the manual contours. Dosimetric indices for CTV and rectum could be highly compromised when the definition of the ROIs switched from manually delineated to auto-segmented. 72% of CT image sets satisfied the worst-case CP thresholds for all dosimetric objectives in all scenarios, the percentage dropped to 50% if biological indices were taken into account. Among evaluation scenarios, (MD, PAM ) bore the highest resemblance to (MD, PMD ) where 99% and 88% of cases met all CP thresholds for bladder and rectum respectively.

When including daily setup variations in prostate IMRT, the dose-volume metric CP, and biological indices of ROIs were approximately equivalent for the plans created based on manually drawn targets and auto-segmented OARs in 88% of cases. The accuracy of auto-segmented prostates and rectums are impediment to attain statistically equivalent plans created based on manually drawn ROIs. This article is protected by copyright. All rights reserved.

Medical physics. 2017 Feb 14 [Epub ahead of print]

Hamidreza Nourzadeh, William T Watkins, Mahmoud Ahmad, Cheukkai Hui, David Schlesinger, Jeffrey V Siebers

Department of Radiation Oncology, University of Virginia, Charlottesville, VA, 22908.