Topics covered in this article include:
- Measurement Summary
- Available Tests
- Image Acquisition Suggestions
- Detailed Discussion of Measurements
- Example File
The radiation light/field coincidence (aka Rad/Light) routine automatically analyzes the DoseLab phantom to provide accurate measurements for field size, shape and geometric coincidence of radiation and optical fields. The routine automatically detects the nominal field size and recognizes radiation fields as formed by the MLC or jaws.
Task Group 142 (TG-142) of the American Association of Physicists in Medicine (AAPM) recommends that the radiation light/field coincidence should be checked during monthly Quality Assurance (QA) in the Mechanical subsection of Table II.
The analysis provides the following results:
Radiation/Light Field Center Deviation Plot
- Light Field Center to Radiation Field Edge Distances
Radiation Field Dimensions
Radiation/Light Field Center Deviation
- Total Rad/Light Misalignment
- Radiation Field Orthogonality
MV Imager Scaling Discrepancy
The measurements are completely automated, requiring the user only to drag and drop the image set into the web-based software interface. A detailed report is created.
Radiation light/field coincidence images must be DICOM files. In some cases, it may be necessary to assist the automatic image recognition with manual naming conventions. If the images are not recognized automatically please try adding dlradlight somewhere in the file name or specifying the type as shown below:
Alternatively, limited capabilities to manually identify planar images using DICOM tag values have been added to the image processing system. This is an extension of the existing naming convention system. The PatientID, StudyID, and SeriesDescription DICOM tags are checked and if the text "dlradlight" (case insensitive) is found the image set will be processed as an asymmetric field test series. For more details see Manual Identification of RT Planar Images and Individual Catphan Slices through DICOM tags.
When imaging QA tests are added to templates an upload control will appear in the scheduled QA's data entry screen allowing the user to upload images for automated analysis.
To add files to the upload queue simply drag them from a Windows Explorer folder to the drag and drop folder and release them. Alternatively, by clicking on the Add Files button to the lower right of the control a windows file selection dialog will open and files can be selected for upload. Under either method, multiple files may be selected for upload at once.
If the automatically upload checkbox is checked (the default) then file uploading will start immediately as files are added.
If the automatically upload button is turned to off the file upload process must be started manually clicking the Start upload button on the lower right of the control. To clear the upload queue click the Clear button.
Once file series have been uploaded they will be displayed below the upload control. To remove a series from the queue click the Cancel button beside the series. To start processing click the Start Processing button. A description for the image series can be added at this point. Click the Edit button next to the series. Type a description for the series into the text box that appears below Description and either click Save or press the enter key. The description can also be edited after the images have been processed. Descriptions will appear in the report with the analysis of the series.
While files are being processed users may perform other tasks such as data entry.
Monthly Linac QA [TG-142 Table II]
Radlight Analysis (Isoalign and Doselab Phantoms)
Image Acquisition Suggestions
Below are the recommended steps to acquire the radiation/light field coincidence images:
- Move the couch to 100 cm SSD.
- Set the jaws or MLC so that the field size is 10 or 15 cm square.
- Place the phantom on the couch and align the edges of the phantom to the light field.
- Acquire the image.
- If desired, acquire more images at different field sizes.
Detailed Discussion of Measurements
The DoseLab radiation light field phantom does not contain a central BB marker. There are two sets of BBs at 10 x10 and 15 x 15 cm. For the two field sizes there is a second set of BB’s inset from BB’s at the edge of the field. These inset BB’s are located by the algorithm. The center of these BB’s is assumed to be the center of the light field.
Allowed field sizes for the DoseLab phantom are 10 and 15cm square fields. The software measures the field dimensions and assumes the nominal field size is the closest from the list above to the measured extent.
The measurement positions on the edges of the radiation field are at 40% of the field size from the center BB (e.g., if the field size is 10x10cm, the measurement lines are 4cm to the left, right, top, and bottom relative to the center).
Light Field Center to Radiation Field Edge Distances
The algorithm measures the radiation field edge at eight points, as shown below. The algorithm then calculates the absolute x distance between the x position of the centroid of the fiducial BB’s and the x-position of the field edge of the Left-Top, Right-Top, Left-Bottom, and Right-Bottom. The algorithm calculates the absolute y distance between the y position the centroid of the fiducial BB’s and the field edge position of the Top-Left, Right-Top, Bottom-Left, and Bottom-Right. The report includes length and deviation from the expected distance (½ field size). Positive deviations indicate the radiation field edge is outside the light field edge.
Radiation Field Dimensions
The radiation field dimensions table reports the distance between the radiation field edges for the 4 sides.
The deviation compares the lengths to the nominal field size.
Radiation/Light Field Center Deviation
The radiation field center is the centroid of the radiation field edges. The position of the radiation center is reported in terms of the coordinates set up by the RT Image Position Tag in the DICOM file.
For the DoseLab phantom, the light field center is assumed to be centroid of the located fiducial BBs. This center is also reported in terms of the coordinates set up by the RT Image Position Tag in the DICOM file.
The difference between the two centers in X and Y is also reported in mm. The Euclidean magnitude of the offset is displayed in mm.
Total Radiation/Lightfield Misalignment
The Rad/Light misalignment provides a single RMS style metric to quantify the overall misalignment of the field. The value is calculated as follows.
The difference in the deltas (difference from nominal) is calculated:
tlbl = topLeftDelta-btmLeftDelta
ltrt = leftTopDelta-ightTopDelta
lbrb = leftBtmDelta -rightBtmDelta
The squared vertical and horizontal alignments are calculated
vAlign = ((tlbl+trbr)/4)2
hAlign = ((ltrt+lbrb)/4)2
The total misalignment is calculated.
totaMisalign = sqrt(vAlign+hAlign)
The orthogonality measures the angle between the top and left sides of the field and the bottom and right sides (the top left corner and the bottom right corner) as a measure of the ‘squareness’ of the field.
Top to Left Side
The slope of the detected radiation field top edge is calculated (rise/run).
The slope of the detected radiation field left edge is calculated (rise/run). When the slope of the left edge is undefined (i.e perfectly vertical) 1099 is substituted.
orthogonality_top_left = | tan-1((slopeLeft-slopeTop)/(1+slopeLeft*slopeTop))|
Bottom to Right Side
The slope of the detected radiation field bottom edge is calculated (rise/run).
The slope of the detected radiation field right edge is calculated (rise/run). When the slope of the right edge is undefined (i.e perfectly vertical) 1099 is substituted.
orthogonality_bottom_rght = | tan-1((slopeRight-slopeBtm)/(1+slopeRight*slopeBtm))|
MV Imager Scaling Discrepancy
The scaling discrepancy is reported as a percentage and calculated as follows:
- The first step is to find the pixel size and account for any magnification due to the placement of the imager by dividing the SAD by the SID. We refer to this as the pxISO as dividing the SAD by SID gives the pixel size projected to the isocenter. pxISO = pixel size (SAD / SID). The following is pulled from the DICOM tags:
- pixel size in mm is pulled from the 3002,0011 Image Plane Pixel Spacing tag
- SAD is pulled from the3002,0022 Radiation Machine SAD tag
- SID is pulled from the 3002,0026 RT Image SID tag
- Find the BB pixel locations and calculate the length, diagonally in pixels, between the BB locations and multiplying by pxISO to produce the length of the sides in millimeters.
- Calculate the average side distance, referred to as measuredSideMM, by taking the mean of the four sides.
- Determine the physical BB spacing, referred to as bbDistance. For a 15x15 cm field the BB spacing is 166.4332 mm and for a 10x10 cm field the BB spacing is 96.933 mm.
- To calculate the percentage difference between the physical BB distance and the measured distance in the image, referred to as percentScalingErr, percentScalingErr = | bbDistance - measuredSideMM | / bbDistance * 100%.
The maximum discrepancy at isocenter is displayed in mm and is calculated by applying the scaling discrepancy against the longest axis of the imager. The maximum pixel dimension of the imager in pixels is referred to as maxImagerPx. Here is the formula:
maximum discrepancy at isocenter = maxImagerPx * pxISO * (percentScalingErr / 100)
The BB Location Detail Plot is included so that the BB position can be confirmed visually.
Below is an example of a completed report:
Below are example files for a asymmetric field test series for use in testing: