Improved Performance at Reduced Cost.

CHALLENGE

Severe dose drop off (over 50%) near the edge of the field of view.

SOLUTION

The design did not incorporate the physical spot size, just the field of view angle.  Although the spot is small, typically less than 2mm, neglecting it results in collimator geometry that reflects a pinpoint (idealized) spot.  Considering the physical size of the spot results in small changes to the collimation cuts, but eliminates this fall off.

Collimation shapes the useful part of the x-ray beam.

It forms the field of view, like the depictions here where the useful x-ray beam is shown in red and the collimators in grey.

As can be seen this collimation scheme uses disks with precision cut outs through all parts to shape the useful beam.

When I first arrived at Varian, one of my first tasks was to review the collimation scheme and drawings.  There was a problem with reduction in dose near the edge of the field of view, beyond what occurs naturally as dose falls off with angle from centerline (more on this under “Field Flattener”).  After review I concluded that there were two major issues with the design.

First, the physics was wrong.  Not by much, but I was able to show that this small error resulted in at least a 50% fall off at the edge of field.

Second, the part was dimensioned incorrectly.  Within a part, dimensions between key surfaces were dimensioned with 2 or 3 intermediate surfaces.  The tolerance stack up within a part was big, but considering the stack of parts in the assembly, the total stack up of accumulated error was quite large.  Further, because of the incorrect dimensioning scheme, several intermediate surfaces, that were not critical at all, were dimensioned quite tightly.  This was unnecessary and costly.  We were controlling things that we really did not care about (within reason).  I reworked the drawings accordingly and the new parts worked perfectly.  Improved performance and decreased cost.