The third object above, also shown in close-up below, is a hard plastic piece that was added to the stack after the collimators. It's about the same diameter, about twice as thick, and weighs about three pounds. It was sculpted to modulate the intensity of the beam specifically for my prostate. The central part is very thin, and lets most of the protons through. The rest of the sculpted-out part is thicker where a less-intensive beam is needed. The final beam that reached me matched the three-dimensional shape of my prostate.
Each piece is coded with H (head), F (foot), P (posterior), and A (anterior) to ensure that they are inserted into the apparatus in the correct orientation. Each piece also has a label on the edge, seen below, with my name and a bar code. The therapists scan each piece, as well as the body pod and my PTI ID, to make sure that everything matches.
Two more brass collimators and another plastic beam modulator were made to fit the shape of my prostate as seen from the right side. After my series of treatments was finished, the four brass collimators were melted down and recast for another use. The two plastic modulators can't be reused, and I got them as souvenirs of my treatment. I'm not sure yet what I'll do with them. Other recipients of these souvenirs have mentioned some possible uses, including bookends (when mounted on pedestals), ashtrays, nut/candy dishes, and large jello shot molds. I may just hang them on a wall. If located in the living room, they would make excellent conversation starters.
The collimator precision is wasted if the beam isn't aimed accurately. First I settled into the body mold that was prepared for me before treatment started. Then laser beams, pre-programmed for me, were aimed at a target on my hip, marked with permanent marker and covered with a 1" diameter plastic film. The marker locations were established the same day the body mold was prepared, using an x-ray to first establish my exact prostate location (using gold markers previously installed in my prostate). The green marker is used to adjust the proton beam so that hits to within a millimeter or so of its intended target in the prostate. The beam where it enters is offset from the marker. In the picture below, the reddish patch that is mostly below and to the left of the marker, and about three inches across, is where the beam actually hit. This is the "proton sun burn" and various from man to man, depending on skin type and sensitivity. Mine is hard to see without looking carefully, but some men have much redder patches that may itch and flake off. Like a sun burn. The "proton sun burn" will fade away before long, leaving me with only the plastic modulators as souvenirs.