Adjust Field Size

4.5g … Field Size off in QA

Scenario

Upon doing monthly QA, the radiation/light field coincidence is good, but the physical size of the radiation field is measured as 19.85, not 20.0 as expected.

Logic used to troubleshoot

If the radiation FS is physically wrong, but is coincident with the light FS then either a jaw(s) need recalibrating or the ODI is off.
To see which, first we test the ODI, which we compare to a mechanical reference. Then if it is OK we re-check jaws and recalibrate as needed.

Summery of process to remedy

  1. Chec the mechanical isocentre (collimator walkout with front pointer, determine mechanical isoc)
  2. Set couch to mech isoc height by transfering this plane from the stationary pointer used in determining the point to the front pointer to the couchtop.
  3. Check ODI is correct. If it is not, then that explains why the FS is wrong
  4. Now check physical field size (light) at this plane. If it is off, recalibrate the offending jaw(s)

The Fix

Assumptions

The G calibration is OK based off spirit level from earlier monthly QA in the same set.
Front pointer is straight

Find mechanical Isocentre

Set up the front pointer. Assume it is uncalibrated, but set to 100cm anyway to start with.
Do a collimator walkout test, determine the diameter. This gives us the limiting accuracy of future measurements
Set up a fixed point off the couch (ie film prick or pencil tip) aligned to the front pointer but with long. offset.
Start at G0 and G180, look for any lateral offset at these two points. If <1mm, then split the difference in the vertical displacement of the couch pointer from the end of the front pointer.
[Next Now compare G90 and G270, vertical offset should be <1mm if the difference split correctly above. The lateral offset should now be equal to the vertical offset at G0/G180. If so, the couch pointer is at mech isoc. This offset is the inaccuracy of the front pointer, and you can now recalibrate it to the couch pointer plane if desired.

Check ODI against mech isocentre

Transfer mech isocentre to the couchtop by setting the front pointer end to mech isoc, then bringing the couch to just touch (careful). Now check ODI to couchtop. If it is wrong then adjust the ODI (separate task.. not sure how)

Check the Jaws

Now that we know the setup is correct (true 100cm SSD), we can test the FS/jaws properly. Using the FS template has some uncertainty, as it has thick lines. Instead, set a blank sheet of A4 paper, and mark the shadow of a 20x20 field and the cross hairs. Now rule the points to draw the field. Now measure using a good ruler. If any jaw is >1mm off, it needs recalibration. 20x20 is a good FS to check at because it is big enough that it is near the 2mm tolerance so if it can be made good, then all should be good. You can slo mark smaller FS on the paper in a simialr manner and check these.

Calibrate the Jaws

Make sure you have someone to double check all machine calibrations
Set the FS template to ODI=100cm. Redirect the service mode control to the pendant. Go to the Utilities, Calibrations, Coll Y1 mode.
Go to each point (Y1=-9.0 to 19.0) and use the pendant down arrow and Enter to set. Look at the changes from previous, they should be small and similar each time. At the end, press Shift+Enter to End. The deviation value should be close to 1 (basically a fit of how linear the pot numbers are.. last time I did this both PRO and SPRO were ~1.5). Now check the Y1 jaw against different points (ie, -5, 0, 5, 10, 15, 20). If it is OK, save the changes as you quit service mode.

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