Radiation should hit the tumor and spare everything around it, so the beam has to be shaped to the tumor’s size. A collimator does that: heavy tungsten blocks, called jaws, that close in around the beam the way your hands can narrow a flashlight’s beam to a slit. Ours has two pairs of motorized jaws, so the software can set a rectangular field anywhere from 1 × 1 mm to 40 × 80 mm.
The motorized jaws began as a proof of concept at Johns Hopkins. I turned them into a product, working with mechanical and electrical engineers on the jaws while I owned the calibration and the motion controller. This was the only commercial research system of its kind with motorized jaws built in.
Changing the beam without changing parts
Fixed collimators are swapped by hand whenever the beam size changes. With two pairs of motorized jaws the software sets the field’s width and height, which also lets a plan cover a tumor with a few rectangles where a fixed collimator would need many small spots.
Tilted jaws for sharp edges
X-rays spread out from a tiny spot. A jaw set square to the beam would block some rays fully and others only partly, leaving a fuzzy edge, so the jaws are focused: each tungsten bar is tilted as a whole to the beam’s angle, its edge following the spread of the X-rays, which keeps the edges of the field sharp.
A unified robotics controller
The jaws run on the same controller as the rest of the robot, so one plan moves everything together. Every axis finds its home position at start-up, checks where it is against a second sensor before it moves, and has limits in software it cannot move past.