Orientation of radiation treatment machine jaws

[Jim Irrer]

Greetings -

I am doing some work with radiation treatment machine
testing, and need to interpret RTPLAN files.

The work is being done with Varian machines, which have
two pairs of jaws and one pair of collimator leaves.
The collimator can rotate, but the jaws are fixed.
I am interested in the jaw, not the collimator.

The DICOM spec for 300A,00B8 (RT Beam Limiting Device
Type) in the case of jaws will give two numbers which
indicate the position of each jaw in the pair.

These are referred to as X1,X2 for one jaw pair, and
Y1,Y2 for the perpendicular pair. My question:

Does the diagram below (as if captured in an RTIMAGE)
reflect the proper designation for the jaws? In other
words, is X2 on the right and Y1 on the bottom? (which
puts X1 on the left and Y2 on the top).


Y2
|__________________|
|__________________|
|__________________|
|__________________| X2
|__________________|
|__________________|
|__________________|
Y1

I suspect that this is true for the jaws because this
is how they are labeled for the collimator when it is
rotated at 0 degrees.

Thanks for any input - Jim

For reference, a fragment of an RTPLAN below showing the
positions for the X jaw, Y jaw, and MLCX collimator:

300A,011A SQ 0 BeamLimitingDevicePositionSequence :
Item: 1 / 3
300A,00B8 CS 1RTBeamLimitingDeviceType : X
300A,011C DS 2-2NLeafJawPositions : -100 \ 100
Item: 2 / 3
300A,00B8 CS 1RTBeamLimitingDeviceType : Y
300A,011C DS 2-2NLeafJawPositions : -195 \ 195
Item: 3 / 3
300A,00B8 CS 1RTBeamLimitingDeviceType : MLCX
300A,011C DS 2-2NLeafJawPositions : 0.6 \ 0.6 \ 0.6 \ 0.6 \ 0.6 \ (repeats 60 times) 100 \ 100 \ 100 \ 100 \ 100 (repeats 60 times)

[Ulrich Busch]

Dear Jim,

I can confirm that your assumption is correct.

The Beam Limiting device Positions (here X1 and X2) are given in the order following the direction of the x or y-Axis respectively.
Together with the central beam axis z, they build a right-handed 3-dimensional coordinate system, with the positive z-axis pointing to the imaging radiation source.
When viewed from the Imaging Radiation Source, the viewing direction is the negative z-axis. Therefore the y-axis is rotated against the x-axis by -90 Deg (when viewing in the positive z-axis, it would be +90 Deg). In your drawing, where the y-axis points "upwards", the x-axis points from "left" to "right".

So the identification of X1/X2 and Y1/Y2 is correct.

Best Regards,
Uli

[Jim Irrer]

Thank you Uli - Very much appreciated!

It would be nice if the DICOM specification had an illustration both the jaw, and collimator. :)

[Ulrich Busch]

Dear Jim,

nice drawings and the whole background you can find in the standard document IEC 61217 "Radiotherapy equipment – Coordinates, movements and scales", on which DICOM is based on in respect the device geometry in this context. The document can be found on the IEC page, but it not free of charge (not too expensive). In case take Edition 2.0 2011-12. I can strongly recommend to anyone dealing with device geometries in RT to get access to that document (disclaimer: ...even in not all parts are perfect).

Best Regards,
Uli