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The Beam Team

Aug. 17, 2006
Machining as a matter of life and death

One of the Proton Therapy Center's three rotatinggantrystyle proton-beam-delivery systems (above). A compensator and an aperture produced by the Beam Team (below).

The Beam Team's fully equipped in-house machine shop plays an integral role in treating cancer patients.

IF JOHN BARR MISSES A DELIVERY DATE, the cancer patient whose life depends on him has to wait to get treatment. Barr and his crew of machinists produce patient-specific compensators and apertures that are used to target proton beams in radiation treatment at cancerous tumors without harming surrounding healthy tissue. Barr's crew is known as the Beam Team. They work out of a fully stocked machine shop located on the lower level of the Proton Therapy Center at the University of Texas M.D. Anderson Cancer Center in Houston (, and while the team operates much like any other production shop, it faces unusual challenges. Job leadtimes are typically hours, not days; not weeks. Communication and data exchange with other treatment center departments are paramount. And, no two parts the shop makes are ever the same. Each part is individually and painstakingly made for each cancer patient.

Dealing with this kind of pressure takes a focused mindset. Every manufacturing decision the team makes and every piece of equipment it adds to the shopfloor must contribute to shortening delivery times that, in essence, will save lives.

"This shop is set up for production to knock out compensators and apertures," says Barr. "We are on a strict time line that must coincide with a patient's scheduled treatment appointment."

To hit its mandatory delivery times, Barr says the shop relies on efficient, modern machine tools that have the exact features, workholding and tooling the team needs. And, Barr says, his team works with outside vendors who provide some of the prep work needed to make the compensator and aperture blanks. Barr also emphasizes that a streamlined information system plays a key role in quickly getting jobs up, running and delivered.

A compensator controls and shapes the end of a proton beam and determines its range. The range is the distance from the proton-beam-delivery device to the tumor field inside the patient. Aperatures contour the outside of the proton beam by blocking protons from traveling beyond the field defined by the compensator.

Proton-beam therapy starts with a computed tomography image of the patient's tumor that is done through magnetic resonance imaging. An oncologist uses the image to define the lines of the malignant tumor that is to be treated, in effect, making a map of the cancerous tumor.

Meanwhile, treatment planners develop the patient's schedule as dosamatrists determine radiation dosage that the patient and tumor require. That dosage dictates the shapes of the compensators and the apertures, and each is specific to the patient. Data generated by the dosamatrists is fed into a special software interface that reconfigures it and scales point data that is transferred to the Gibbscam CAM system that the Beam Team uses.

The team machines compensators and apertures on four ethernet-networked Mazak Nexus 410A vertical machining centers equipped with through-spindle coolant, 12,000-rpm spindles, Renishaw tool setters, and Kurt vices. Two of the machines cut compensators, while the other two work on the mating apertures.

Compensators are made of acrylic plastic and come in three sizes (small, medium and large) that vary from 5.500-in. square to 11.500-in. square. On average, they are about 4-in. thick.

The compensators closely control the proton beam, allowing doctors to limit the beam's travel to within the tumor. In other radiation therapies, proton beams are not so controllable and travel into and through all of a patient's body parts. The compensator allows the beam to travel through other body tissues at low power, and its energy spikes when it is in the tumor. That leaves surrounding, healthy tissue unaffected.

Average cycle times to produce a compensator are 45 minutes to 60 minutes, but small-sized compensators, such as those used to treat tumors in eyes, can be made in 5 minutes to 10 minutes. On the other hand, compensators used to treat large tumors may require several hours of machining.

Compensator shapes are plunge milled. They resemble a honeycombed mold cavity, and each plunge-cut depth must be held to within 0.001 in. of a specific thickness dimensioned from the compensator's bottom surface. That close dimension and the depth of the plunge pattern regulates the proton beam. For example: The deeper the plunge pattern, the further the beam projects inside the patient's body.

A compensator's honeycombed pattern lets Barr and his team accurately drive to specific X/Y machine coordinates for quickly and easily checking the dimensions that must be held from the bottom of the Z-axis plunge cut to a zero datum on the compensator's bottom surface. For this, they typically use a three-axis mill equipped with a digital readout and touch probe — touching off the machine's table, zeroing the digital readout, then checking specific plunge-cut holes.

The Beam Team machines apertures out of 932 bearing bronze or 660 bronze, and two or three apertures often are required per compensator, depending on the amount of beam blocking needed.

Outside aperture dimensions match those of the mating compensator, but aperture thicknesses never exceed 0.787 in. (2 cm.). This is because a thick aperture, as opposed to multiple thin ones, would be too heavy for technicians to load into the proton-beam-delivery systems.

The shapes cut in apertures go all the way through the brass plates and frame the outside parameters of compensator fields. Since the machining cycle time for one aperture is three to five minutes, the Beam Team easily cuts the required number of apertures on one of the Nexus machines in the time it takes to produce one compensator. The team uses the other Nexus machine to square up raw aperture blanks, but Barr says he will eventually farm out this prep work to free the machine for compensators.

To speed setups, Barr's team uses the same holding devices and work offsets to preserve zero settings from part to part. To go from medium to large-sized apertures the team's machine operators clamp both sizes in the same workholding vice and assign their different centers to G54 and G55 machine offsets.

"No one gets treated if we aren't producing, so we constantly look for ways to streamline the process. One way we do that is by experimenting with different types of tooling," Barr says. For instance, the team recently incorporated a special roughing tool from Iscar Metals that eliminates having to run a finish pass on apertures and lets operators machine at 90 ipm. In addition, Barr says the new cutters do not need coolant and last longer.

While no two proton-beam treatment fields are the same, compensators and apertures do have a common rough format that the Beam Team capitalizes on to further streamline production. The team shortens part cycle times by using saved templates that preserve the processing steps for the three compensator and aperture sizes.

In the Gibbscam software, the team pulls up the process and selects one of several milling tools without having to supply data for that particular tool. In addition, they overwrite previous programs, so all that they need to do is to edit the individual patient information that is engraved on each compensator and aperture. Also, as part of the editing process, the team chooses a compensator block thickness, deletes old points and adds new ones.

After programming, Barr runs a computer simulation of the machining process, then sends a post-processed program to the appropriate machine. At the machine, operators pull up the program, move it to the machining side, load a blank and start the machine.

Currently, the Beam Team produces a couple hundred treatment sets per month, and it expects that number to increase dramatically in the future as the number of patients treated at the center increases. Eventually the team will service one fixed-beam and three rotating-gantry-style proton-beam-delivery rooms. When that happens, Barr expects to add a second shift at the shop.

Each of the Center's three-story, 190-ton steel gantries revolve 360 degrees around patients so that the proton beam can be delivered at any angle. In the fixed-beam room, patients sit in a special chair or couch that moves around the beam to a precise position.

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