Impact Analysis of the Interobserver Variability in CBCT-Based Soft-Tissue Matching using TCP/NTCP Models for Prostate Cancer Radiotherapy

Background: Prostate alignment is subject to interobserver variability in cone-beam CT (CBCT)-based soft-tissue matching. This study aims to analyze the impact of interobserver variability in CBCT-based soft-tissue matching for prostate cancer radiotherapy.Methods: Retrospective data, consisting of 156 CBCT images from twelve patients with intermediate- or high-risk prostate cancer were analyzed in this study. To simulate interobserver variability, couch shifts of 2 mm relative to the resulting patient position of prostate alignment were assumed as potential patient positions (27 possibilities). For each CBCT, the doses of the potential patient positions were recalculated using deformable image registration-based synthetic CT. The impact of the simulated interobserver variability was evaluated using tumor control probabilities (TCPs) and normal tissue complication probabilities (NTCPs).Results: No significant differences in TCPs were found between prostate alignment and potential patient positions (0.944 ± 0.003 vs 0.945 ± 0.003, P = 0.117). The average NTCPs of the rectum ranged from 5.16 (%) to 7.29 (%) among the potential patient positions and were highly influenced by the couch shift in anterior-posterior direction. In contrast, the average NTCPs of the bladder ranged from 0.75 (%) to 1.12 (%) among the potential patient positions and were relatively negligible.Conclusions: The NTCPs of the rectum, rather than the TCPs of the target, were highly influenced by the interobserver variability in CBCT-based soft-tissue matching. This study provides a theoretical explanation for daily CBCT-based image guidance and the prostate-rectum interface matching procedure.Trial registration: Not applicable.

A Comparison of in-house and Shared Rapidplan Models for Prostate Radiation Therapy Planning

PurposeKnowledge-based planning (KBP) can increase plan quality, consistency and efficiency. In this study, we assess the success of a using a publicly available KBP model compared with developing an in-house model for prostate cancer radiotherapy using a single, commercially available treatment planning system based on the ability of the model to achieve the centre’s planning goals. Methods and MaterialsTwo radiation oncology centres each created a prostate cancer KBP model using the Eclipse RapidPlan software. These two models and a third publicly-available, shared model were tested at three centres in a retrospective planning study. Results The publicly-available model achieved lower rectum doses than the other two models. However, the planning-target-volume (PTV) doses did not meet the local planning goals and the model could not be adjusted to correct this. As a result, the plans most likely to satisfy local planning goals and requirements were created using an in-house model. For centres without an existing in-house model, a model created by another centre with similar planning goals was found to be preferred. ConclusionsVariations in local planning practices including contouring, treatment technique and planning goals can influence the relative performance of KBP. The value of publicly available KBP models could be enhanced through standardisation of planning goals and contouring guidelines, providing information related to the planning goals used to create the model and increased flexibility to allow local adaptation of the KBP model.