A device () was developed to hold and section formalin-fixed cancerous prostate glands removed by prostatectomy. The prostate sections were then processed using whole mount histology protocol and digital images of the whole mount sections were compared to corresponding MRI images of the gland acquired in vivo. The device which is referred to as the “Multi-bladed Cutting Assembly” holds the prostate in the correct orientation for matching the histology and MRI sections, makes cuts at the same 4 mm interval as the MRI slices, performs perfectly parallel cuts without shearing and minimizes damage to the prostate specimen in the process.
Figure 1 Photograph of the assembled cutting device with walls (LW – lateral walls, SIW – superior-inferior walls) inserted in the base (A), multi-bladed knife used to cut the specimen (B), and the rack used to hold the blades while they are cleaned (more ...)
In order to keep the orientation of the prostate gland fixed during the sectioning process, the device needs to apply a holding force from all sides. An adjustable box was designed and constructed that could accommodate a prostatectomy specimen of any size between 30 and 85 mm in the lateral direction (). The application of slight pressure on the specimen from every direction succeeds in fixing it in place during the cutting process. Since the box is open from the top, a “plunger” tool was used to press on the excised gland from the top in between blades, preventing the gland from rolling or sliding and allowing the user to adjust pressure as needed during the cut.
The walls of the device, located laterally to the specimen, and the plunger, each include vertical slots spaced at 4 mm intervals that allow the blades of the knife to pass through the specimen. These slots (0.5 mm wide) are just slightly wider than the blades (0.25 mm), and slots on opposite walls are directly lined up with each other to ensure parallel cuts and to prevent any side to side movement of the blades. The specimen is held in place in the superior-inferior direction by two aluminum plates that fit through the slots in the lateral walls.
A separate multi-bladed knife is used in conjunction with the adjustable box. The knife assembly includes openings at 4 mm intervals for a maximum of twenty-one blades to be held (). The blades of the knife fit through the vertical slots in the adjustable box and cut simultaneously through the sample. This multiple-blade design ensures that all cuts are made in a perfectly parallel fashion.
Individual Part Design and Construction
All materials used to construct the multi-bladed cutting assembly were purchased from McMaster-Carr Supply (Cleveland, Ohio) except when noted.
The base of the cutting assembly acts as the main body from which the rest of the device is assembled (). The main part of the base is made from 3/4″ nylon with a width of 4″ (102 mm) and an approximate length of 4.2″ (107 mm). A custom made tool was used to cut blade slots lengthwise in the nylon. Wall slots running perpendicularly to the blade slots at 3/8″ (9.5 mm) intervals 11 mm deep were milled into the base with a 4 mm diameter end mill. 4×10 mm aluminum bars act as slats that fill empty slots beneath the prostate sample to help support it during cutting. Aluminum pieces 1/8″ thick were screwed into each side of the nylon sheet to close each end of the wall slots and to act as feet for the device.
The walls of the device hold the prostate by adjusting to the smallest separation distance allowed by the device without seriously deforming the prostate. The two lateral walls are made from 1/4″ aluminum cut out using an Omax Water Jet Cutting Machine (Omax Corporation, Kent, Washington). The outline of the walls and the vertical slots were cut using the water jet, and the final width was finished to 4″. The bottom 10 mm of each wall was thinned from 1/4″ (6 mm) to 4 mm using a mill. The thin bottom allows the walls to be inserted into the slots in the base of the device. The superior-inferior plates are made from 0.02″ aluminum and the locations of these plates can be adjusted at 4 mm intervals to conform to the superior-inferior size of the specimen ().
Photograph of the cutting device with lateral (A) and superior-inferior (B) walls adjusted to immobilize a prostatectomy specimen.
The slit walls of the plunger are made from 1/2″ polypropylene plastic cut using a water jet cutter and milled to a 45° angle on the tips. The two walls were then hot glued to a flexible 1/16″ piece of polypropylene bent in three places on a break press. Polypropylene was chosen for its low cost, durability and chemical resistance. The two slit walls touch at the tips when at rest, while when squeezed at the top, they open up to fit a wide range of prostate sizes. The plunger holds the prostate in place by wedging its slit walls between the lateral walls of the holding box and the prostate specimen ().
Figure 3 Photograph of the cutting device showing lateral walls and plunger holding fixed beef mock-up. Knife blades have been inserted through the lateral walls and are ready for cutting. One foot and one superior-inferior plate have been removed for better visualization. (more ...)
The multi-bladed knife holds the required number of blades for each specimen in tension to prevent them from deflecting during the cutting process (). The No. 260 Type (L) Feather Trimming Blades (Fisher Scientific, Ottawa, Ontario) are spaced at 4 mm intervals to match the slots in the lateral walls. Each blade has one hole at both ends which are used to affix the blade to the knife frame. One end of each blade is held in an assembly of three layers of 1/4″ aluminum cut on the water jet and joined together by screws. A 1/8″ diameter aluminum rod is slid between two of the layers and through one of the two holes in each blade. Arms made of 3/8″ thick aluminum milled 18 mm high and 245 mm long extend parallel to the blades and hold another 1/4″ aluminum plate that screw tightens to hold a 1/8″ nylon rod through the second hole in each blade and pull the blades tight. The tensioning plate is attached and tightened after the blades have been inserted through the slots in the lateral walls. The blades are inserted from the side of the lateral walls and the slots in the plunger are used to guide the blades through.
A rack was also designed to hold the blades so that they could be cleaned simultaneously in solution. Cleaning of the blades allow them to be reused for subsequent specimens. The blade rack is made from 1/4″ aluminum legs attached by 1/8″ nylon rod. Two 1/8″ diameter aluminum rods extend horizontally to hold the dirty blades with 3/8″ nylon spacers used to keep blades from sticking to each other ().
Simulated Tests on Formalin Fixed Beef
During the design process, various prototypes were tested using formalin fixed meat. Beef was decided on as it was easy to obtain and had a similar consistency to a prostatectomy specimen. Beef steak was purchased at a local grocery store and cut using a utility knife to the same shape and size as a typical prostatectomy specimen. The beef was then injected with and submerged in formalin for over 24 hours. After removal from the solution, the beef phantom had very similar texture to a formalin fixed prostate gland.
Prostate Specimen Tests
Once the specimen has been prepared by the pathologist, with inking of the resection margins and removal of the seminal vesicles and vas deferens, and the apical and bladder neck resection margins as thin tissue dough-nuts, the set up of the multi-bladed cutting assembly can be started. The first part of setting up the cutting assembly requires placing the prostate specimen on the base where the spacing of the lateral walls can be determined. The base slats are arranged so that the walls will fit in the correct slots and the lateral walls are then inserted into the base on each side of the specimen. Once held in place, the specimen is then rotated manually to the orientation that will produce the proper slice direction. The superior-inferior plates are then slid through the lateral walls so that they hold the prostate tightly, as shown in .
The blades of the multi-bladed knife assembly are loaded so that they match the open slots whose plane passes through the specimen. Once inserted through the lateral walls of the cutting assembly, the blades are inserted into the tension plate and tightened. It is important that the blades are sufficiently taut to avoid lateral deflection or twisting while cutting.
The final step before cutting is to insert the plunger between the walls and wedge it against the prostate. This is demonstrated in where the inferior plate has been removed for better visualization. Once the prostate is fully immobilized with the plunger, the knife can be lowered into the prostate specimen and sectioning can be completed.
The average superior-inferior length of each prostate specimen is around 44 mm requiring an average number of 10 blades per procedure.
The processing and paraffin embedding of the prostatectomy specimen was carried out in a routine fashion, except oversized baskets and blocks were used to accommodate the intact sections of the prostate gland. Whole mount sections were cut using Lieka RM2245 whole body rotatory microtome, and submitted as intact transverse sections mounted on oversize glass slides for hematoxylin and eosin (H&E) staining. The oversized slides were scanned at 2.5x and 20x magnification using digital image analysis system from Bacus Lab (Olympus Global). Digital images of the H&E stained sections were subsequently registered to the corresponding T2-weighted MRI images. The registration was carried out with a software procedure developed in house using b-splines algorithm. The software carries out non-rigid body registration using the boundary of the entire prostate gland manually outlined on MRI and histology images.