Gap Offset and Skew

The Gap Offset and Skew test procedure measures the alignment of collimator leaves to determine how accurately they are being positioned.

The Latest Gap Skew results show a summary of activity as shown below:

Technical Background

Edge measurements are acquired by selecting segments of the edge on the left and right ends. The segments are chosen to begin and end in the middle of a leaf, and span two leaves. They are positioned to be far enough away from the edge of the image and vertical jaws to avoid their effects.

The image below shows the detail of a measurement. The black horizontal line shows the length of the segment. The vertical gray lines (added for clarity) show the sides of the collimator leaves.

The angle of an edge is calculated with:

$ angle = atan((Y_{Right} - Y_{Left}) / (X_{Right} - X_{Left})) $

Where $X_{Right}$ and $X_{Left}$ are the horizontal centers of the right and left segments.

Angles are expressed both in degrees and mm/40 cm.

Summary Page

The Gap, Offset and Skew page lists all results for all machines and allows the user to navigate to specific results. The screen capture below provides and example.

Clicking on dates shows further details.
Results with white background are recent, and those with gray background are older.
There is one row of results for each treatment machine.
Results are ordered chronologically, with the most recently delivered data sets on the left.
Those results labeled "Missing Data" correspond to incomplete data sets. Results for individual images are reported, but the composite results are not.
This page does not update automatically. Use you browser's refresh button to get the latest results.

Composite Test Results

The following shows a report for one set of beams. Major sections describe:

Meta-information, including the machine and when the data was capture and when it was analyzed.

Table of composite results. Hovering the mouse over individual number will show (example for 0.182): As shown below, hovering over individual values will show (example for 0.182):

The name of the value.
How the value was derived from other values.
The precise value and the math used to calculate it.

Historical graphs for composite results for this machine. These are dynamically updated as more (future) results are captured. The points for this data set are orange.
Images for individual beams and their individual measurements. Clicking on the beam name will show more beam details.

Individual Beam Report

Clicking on the beam names in composite test results page will take you to a page like the screenshot below that shows detailed results for a single beam.

Clicking on the image will show a larger image and list the complete DICOM meta-data.

The report shows values from this beam only, and includes graphs over time of selected values. At the bottom of the page are selected DICOM meta-values providing relevant information.