Quantifying false positional corrections due to facial motion using SGRT with open-face Masks.

PURPOSE: Studies have evaluated the viability of using open-face masks as an immobilization technique to treat intracranial and head and neck cancers. This method offers less stress to the patient with comparable accuracy to closed-face masks. Open-face masks permit implementation of surface guided radiation therapy (SGRT) to assist in positioning and motion management. Research suggests that changes in patient facial expressions may influence the SGRT system to generate false positional corrections. This study aims to quantify these errors produced by the SGRT system due to face motion. METHODS: Ten human subjects were immobilized using open-face masks. Four discrete SGRT regions of interest (ROIs) were analyzed based on anatomical features to simulate different mask openings. The largest ROI was lateral to the cheeks, superior to the eyebrows, and inferior to the mouth. The smallest ROI included only the eyes and bridge of the nose. Subjects were asked to open and close their eyes and simulate fear and annoyance and peak isocenter shifts were recorded. This was performed in both standard and SRS specific resolutions with the C-RAD Catalyst HD system. RESULTS: All four ROIs analyzed in SRS and Standard resolutions demonstrated an average deviation of 0.3 ± 0.3 mm for eyes closed and 0.4 ± 0.4 mm shift for eyes open, and 0.3 ± 0.3 mm for eyes closed and 0.8 ± 0.9 mm shift for eyes open. The average deviation observed due to changing facial expressions was 1.4 ± 0.9 mm for SRS specific and 1.6 ± 1.6 mm for standard resolution. CONCLUSION: The SGRT system can generate false positional corrections for face motion and this is amplified at lower resolutions and smaller ROIs. These errors should be considered in the overall tolerances and treatment plan when using open-face masks with SGRT and may warrant additional radiographic imaging.

An open-source tool to visualize potential cone collisions while planning SRS cases.

PURPOSE: To create an open-source visualization program that allows one to find potential cone collisions while planning intracranial stereotactic radiosurgery cases. METHODS: Measurements of physical components in the treatment room (gantry, cone, table, localization stereotactic radiation surgery frame, etc.) were incorporated into a script in MATLAB (MathWorks, Natick, MA) that produces three-dimensional visualizations of the components. A localization frame, used during simulation, fully contains the patient. This frame was used to represent a safety zone for collisions. Simple geometric objects are used to approximate the simulated components. The couch is represented as boxes, the gantry head and cone are represented by cylinders, and the patient safety zone can be represented by either a box or ellipsoid. These objects are translated and rotated based upon the beam geometry and the treatment isocenter to mimic treatment. A simple graphical user interface (GUI) was made in MATLAB (compatible with GNU Octave) to allow users to pass the treatment isocenter location, the initial and terminal gantry angles, the couch angle, and the number of angular points to visualize between the initial and terminal gantry angle. RESULTS: The GUI provides a fast and simple way to discover collisions in the treatment room before the treatment plan is completed. Twenty patient arcs were used as an end-to-end validation of the system. Seventeen of these appeared the same in the software as in the room. Three of the arcs appeared closer in the software than in the room. This is due to the treatment couch having rounded corners, whereas the software visualizes sharp corners. CONCLUSIONS: This simple GUI can be used to find the best orientation of beams for each patient. By finding collisions before a plan is being simulated in the treatment room, a user can save time due to replanning of cases.

A dosimetric comparison between volumetric-modulated arc therapy and dynamic conformal arc therapy in SBRT.

PURPOSE: The purpose of this study was to investigate the dosimetric equivalency of dynamic conformal arc therapy (DCAT) against volumetric modulated arc therapy (VMAT) plans in stereotactic body radiation therapy (SBRT) of lung and liver lesions and to examine if efficiency can be increased. METHODS: Nineteen patients previously treated for lung and liver cancer lesions with SBRT were included. Organs at risk (OAR) and targets were contoured by a single radiation oncologist. All plans were optimized by the same dosimetrist using ELEKTA Monaco treatment planning system version 5.0 for 6MV flattening filter free (FFF) photon beam in a VersaHD (ELEKTA, Crawley, UK). A VMAT and DCAT plan was optimized using the same objectives using coplanar arcs of 225o arc span. RESULTS: All plans have achieved the target and OAR planning objectives. The target dose conformity was comparable (mean VMAT PTVr=1.3 and DCAT PTVr=1.4), and the low dose spillage were similar (mean VMAT R50=4.5 and DCAT R50=4.6). However, monitor units (MU) for DCAT plans were lower by 2.5 times on average than VMAT plans. It was observed that in 75% of cases where OARs overlapped with the PTV, maximum doses to OAR were higher in VMAT than DCAT plans, but the difference was not significant. Patient specific quality assurance (QA) plans were measured using the Scandidos Delta4 phantom and gamma analysis performed using 2mm distance to agreement (DTA) and 2% dose difference yielded more than 95% passing rates on both VMAT and DCAT plans. CONCLUSIONS: DCAT delivery for lung and liver SBRT is a dosimetrically equivalent and an efficient alternative to VMAT plans.