Patient-Specific Planning Target Volume Margins for Liver Stereotactic Robotic Radiosurgery

Abstract

Purpose: To create analytical tools for a proposed clinical workflow, which implements patientspecific planning target volume (PTV) margins for liver radiation therapy treatments. Methods and Materials: Treatment log files are analyzed for liver treatments to assess tumour motion-tracking accuracy. A uniform PTV margin is estimated that considers motion-tracking errors and deformations, provided that the impact of uncorrected rotations is minimized. A supervised machine learning algorithm employing retrospective data, which emulates a dry-run session prior to treatment planning, is used to investigate if motion-tracking errors are less than 2 mm, and consequently, the standard PTV margins can be reduced by 2 mm. For a safer implementation in the clinic, we employ a warning system that quantifies the probability of a geographic miss if the PTV margin is reduced for every subsequent fraction. A dosimetric analytical tool is proposed to retrospectively assess the dose to a target against different types of delivery errors. The tool is validated by radiochromic film measurements for two very different types of treatments, liver and trigeminal neuralgia (a cranial nerve disorder). Case studies are conducted to access the suitability of a selected PTV margin based on the geometrical and dosimetric coverage of targets. The range of rotations that can be safely allowed for trigeminal neuralgia treatments is quantified since rotational corrections cannot be applied by the system for this specific disease site. Results: Isotropic 4 mm PTV margins are sufficient to account for tracking errors and deformations for 95% of patients. For patient-specific PTV margins, the accuracy of predicting if motion-tracking errors are less than 2 mm is 0.84 ± 0.06 using 5-fold cross-validation. Using the warning system, 11 out of 64 cases predicted to be treated with 2 mm reduced PTV margins might