Abstract
Patients receive radiation therapy treatment at Strong Memorial Hospital for various medical conditions. Total body irradiation (TBI) therapy is a treatment necessary for a limited number of patients, an average of 12 people a year at the URMC, that would benefit from a simpler and more accurate procedure for their treatment. While the treatment requires the whole body to be accessible, the current bed only allows the machine access from the head to the top of the legs at most, requiring the patient to get up and rotate themselves for full body treatment. Unfortunately, not all patients have the capability to do this on their own, and must be treated with a different procedure.
The current method for this alternative TBI at the hospital involves having the patient lie down across the room with the radiation machine pointed at them. While this method allows the machine full access to the patient, the dosage is diluted and makes precision dosing harder than if the patient was directly under the machine. This method also has the side effect of exposing certain healthy tissue to unnecessary radiation. The Strong team was tasked with designing and creating a rotating table attachment for the current couch setup will allow all patients undergoing total body irradiation therapy to receive high quality treatment without needing to adjust mid-procedure.
Project Description
Problem Statement
Create a table top attachment to allow the radiology team at Strong Memorial Hospital to better apply total body irradiation therapy to patients.
Deliverables
- Rotational Table Attachment
- Testing Documentation/Report
- Theory of Operation Manual
Requirements
- Material used must be invisible to X-Ray
- Must be manually operated by two people
- Operated meaning it is able to be assembled and attached/detached from the couch
- Must safely support the weight of human plus a safety factor
- Must attach securely to the couch
- Must fit patients that otherwise would fit on the couch
Specifications
- Must lock into position at 0 and 180 degrees (with tolerance of 0.11 degrees) rotation around normal axis through center of the couch
- Must support a downwards force of 155 kg applied distributed along the surface of the couch attachment without material yielding
- The top table must have a width of 535 mm (tolerance of +25/-25mm)
- The top table must have a length of 2160 mm (tolerance of +50/-50 mm)
Concepts
The larger assembly was broken up into several different categories allowing each aspect of the project to be closely analyzed. These categories include beds, rotation mechanism, table attachment system, locking mechanism, and a latching mechanism.
Table Attachments/Bed: The bottom bed holds the rotational mechanism in place while being fixed to the current couch by utilizing the “lok-bar” design currently used a the hospital. This design uses off center cams to hold onto preexisting notches in the hospital table.
Locking Mechanism: The locking mechanism is broken into two distinct parts. First is the semi-circle ends that attach to the composite and protect the soft core while allowing the pin to stop the table. The second is the vertical block and pin. After several rounds of concepts, the team decided to go with a friction lock pin so that the locking mechanism could still work precisely after minor wear and tear. The large, tapered pin will be pushed and then turned as the locking pin will climb the ramp and lock into place. The angle of the ramp was calculated based on Newton’s second law and friction testing conducted on the material.
Rotating Mechanism: Imitating a “Lazy-Susan” rotating device, this mechanism is essentially a very large ball bearing. This allows the top table to rotate freely about the bottom. A race was added to ensure equal spacing between the balls.
Latching Mechanism: With no metal allowed in this design, much of the bonding relies solely on adhesives. This problem birthed the idea of a latching mechanism. This internal system attaches the top table to the bottom table, keeping the assembly together in the event of a patient getting up mid procedure or some unseen force tipping the top bed, while not interfering with the rotating mechanism. The mechanism, seen in bright green, has a rectangular bottom that can fit in the rectangular hole in the bottom bed (dark green) only when oriented at 0 and 180 degrees. The latching mechanism rotates with the top table and when the top table is rotating the rectangular holes no longer align. This will keep the assembly together while rotating even in the presence of an unknown force.
Manufacturing
Top Bed
The top bed was manufactured by cutting a large sheet of fiberglass Plastic Honeycomb composite into two identical layers using a table saw. These layers were then glued together using 3M Scotch-Weld DP 100 Plus and left to cure for 48 hours.
Bottom Bed
UHWMPE was used to manufacture the bottom table with the 3-axis CNC ShopBot. The material is heavier than other plastics used in the project, however it was made available to the team for free and the assembly still met the overall weight goal with this component.
Table Attachments
The table attachment pins were cut from an acrylic pin since it is light, cheap, and easy to cut. All the cylinders for the table attachment mechanism were laser-cut out of acrylic. The cylinders were then glued together in a stack and the holes were rimmed to size using a mill since the laser cut holes’ tolerance is not low enough to have a press-fit with the acrylic pin. The table attachment pieces were assembled by pressing the pin into the holes and then connecting this stack into the bottom bed’s locating holes by pressing and gluing the pins.
Locking Mechanism
The locking mechanism was made of acrylic since it is cheaper than alternative materials like polycarbonate and it passed the FEA load cases without plastically deforming. The locking mechanism block was cut to size and drilled using a mill and the locking pin gap was broached. The ramp was CNC milled and then solvent welded to the block. Two acrylic locating pins were turned on the lathe to get the right fit and pressed into the locking mechanism block. The block was then glued to the bottom bed via the locating holes in it. The tapered pin of the locking mechanism was first milled to locate the hole for the locking pin. It was then turned on a lathe to achieve its shape. Finally, an acrylic pin cut to size from stock was glued into the hole to give the tapered pin assembly its final shape. The locking mechanism semi-circle pieces were CNC milled and glued onto either edge of the top bed. All components of the LM were manufactured by Bill Mildenberger.
Rotational Mechanism
The balls for the rotating mechanism were purchased from McMaster. Torlon PAI balls were the strongest plastic balls available from this provider and their strength was verified with an in-house compressive test using the MTS machine. The top and bottom track of the rotating mechanism were CNC milled by Bill Mildenberger out of acrylic, because it is easy to work with and its strength passed verification based off finite element analysis performed for these components. The top track’s holes were tapped to allow for connection with the latching mechanism. The race for the rotating mechanism was laser cut using acrylic since it is easy to laser cut. To completely assemble the RM, #8-32 Glass filled Nylon screws were placed through the top track of the RM and into the latching mechanism.
Latching Mechanism
The latching mechanism piece was CNC programmed and machined by Professor Muir on the HAAS machine. To completely assemble the latching mechanism, #8-32 Glass filled Nylon screws were placed through the top track of the RM and into the latching mechanism.
Mechanical Analysis
FEA
Finite element analysis in Siemens NX software was used to simulate and understand the stresses present in the locking mechanism during worst-case situations. A reasonable worst-case scenario involved an operator applying force to rotate the top bed without unlocking and disengaging the locking mechanism pin. This was represented by applying a horizontal force of 50 lbf. on the tapered end of the pin, fixing the base of the locking mechanism block (as it is glued to the bottom bed), and applying surface contacts at all applicable locations. For this analysis, the displacement of the pin was not of peak importance. While it did slide forward in the slot, the donut grip at the back of the pin did not come into contact with the block. More importantly, the stresses experienced by the assembly were compared to the known failure stresses of acrylic (the material used for all parts of the locking mechanism). The peak stress in the system was less than 26 MPa, while the yield strength of acrylic is at least 65 MPa. This gave the team confidence that no aspect of the locking mechanism would fail during any reasonable operation or worst-case scenario. This analysis did not consider the possibility of the parts being dropped, crushed, or handled in some other unpredictable way that could lead to a force greater than 50 lbf being applied.
Tolerance Analysis
The tolerance analysis was conducted to see if the locking mechanism would still be operational under a worst-case scenario. This analysis spans several internal systems. Combining the possible offset of the peak of the tapered pin slot to the center of the top bed, the rotational mechanism glued improperly, the placement of the locating pins (for the locking mechanism) on both the bottom table and the locking block, and placement of the locking pin on the tapered pin. In the worst-case scenario, all errors are summed to show the additional vertical distance the locking pin must climb up the ramp. For all parts manufactured by Bill Mildenberger, a tolerance of 0.001in (or 0.0254mm) was placed. This is a fair estimate as he is a master machinist. For parts created by the team a tolerance of 0.05in (1.27mm). The sum of these tolerances is 3.86mm. Using Pythagorean’s theorem, a maximum ramp height of 6.885mm using a diameter of 39.05mm and an angle of 10 degrees, the assembly passes tolerance analysis as the maximum height of the ramp exceeds the worst possible error.
Testing
To ensure the completed table will satisfy all requirements, there are a series of tests that the team will conduct.
First, to ensure the table will fit into the CT machine, there are certain length and width requirements (Width: 535mm (+25mm/-25mm), Length: 2160mm (+50mm/-50mm)). Initially the requirement for the maximum width of the table was set to avoid interference between the completed table and the CT machine, but this maximum was increased to allow for the locking mechanism to have a solid bond with the composite tabletop as the bond with the soft internal material is softer. After a meeting with the mechanical team at UTSW it was determined the width of the bed could be much larger as their new design is and have recorded no interferences with the CT machine. The tolerance for length is much looser as it is less important for the fit, but more for ensuring the bed can rotate without obstruction. To test these tolerances, the team will measure the table from the correct drawing datum.
Regarding the table displacement requirement, the team placed the top composite bed on a wooden support that resembled the rotational mech in size, then recorded displacements of each corner after two team members (~290 lb. combined) and six jugs of water (8.48 lb. each) totaling approximately 340 lb. This load covered the entire surface area of the table. The maximum deflection was 7mm. Regardless of that slight deflection, the material was not close to yielding thus satisfying the team’s specification. If the assembly were to come up short, there are some ways to increase stiffness in the final days. Attaching bars of polycarbonate to the sides of the composite will increase the stiffness of the bed whilst also giving a nice finish to the bed.
Intellectual Property
The original concept design is based on the rotating table designed and built by a team located at the University of Texas Southwestern. The project’s sponsor at Strong Memorial Hospital, Dr. Webster, first asked the Strong team to contact this group in Texas to replicate the product they had made. The team met with project leaders from the original rotating table in Texas, where they learned the general concept of the Texas rotating table but could not be told any measurements or numbers due to liability issues. The Texas design consisted of two tables connected by a central cylinder block which allowed the top table to rotate due to the low friction coefficient of the material used: polyethylene. While the Strong team saw this design as a good place to start, they saw many places to improve upon the original design, instead of delivering the same product to Strong Memorial Hospital. Primarily, the Strong team saw improvements to be made to the rotating mechanism. Relying on a low coefficient of friction material was not as reliable as other methods of allowing rotational movement while keeping axial movement fixed. The Strong team designed a “lazy-susan” type mechanism, composed of two acrylic tracks and a few dozen small plastic balls sandwiched between the two tracks, which would allow these two caps to rotate freely. While the concept of a “lazy susan” is already patented and so is ball-bearing rotation, the team’s investigations have shown that there are no patents submitted combining the two to create a medical bed attachment to rotate patients without them moving. The most similar patent the team found was one where a ring gantry moved around the patient, a major difference being that the table where the patient lies remains completely still. The Strong team believes that a patent could be submitted for the final rotating bed attachment. Regardless, the team has not taken any further steps towards a patent as they feel that anyone should be free to pursue better treatment for their patients. The team is encouraged to think that others would like to reproduce their design. The Strong team would intend to support future similar projects as the Texas team supported this project.
Future Plans
Add several functionality components to the table to allow the technicians at Strong to more accurately and efficiently secure each patient. These include:
- Adjustable Sideboards
- Headrests
- Adjustable indexing system
Presentation
Final Document
Meet the Team!
- James Carl
- Jarod Forer
- Shira Hersch
- Carol Jerotich
- Lale Yilmaz
Mentor
Matt Webster