This is what I found when working on a translation on the identical subject last week (pathology report of breast cancer biopsy, Mexico). I don't have time right now to show you my sources, but I think it's a safe translation.

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Note added at 10 hrs (2015-08-13 01:19:08 GMT)
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See the following description of how a breast biopsy is divided up and assigned to cassettes:

http://pathology2.jhu.edu/pathreport_breast.cfm
Gross Examination is done within a few hours of receiving your specimen. A physician (pathologist 1, a resident) will examine your specimen, measuring its size, describing its color and texture, and determining if any masses are present. If so, the size of the mass and its distance to the specimens’ edge (the margin-where the surgeon cut) will be recorded.

The pathologist then sections the specimen into thin slices with a scalpel and further examines the slices. **The pathologist then takes small pieces of the slices (about the area of a dime and about twice as thick) and puts them into small containers called cassettes. Cassettes are essentially small, 2cmX2cmX0.5cm, rectangular boxes with multiple small openings in their walls which let the formalin fixative in to bathe the tissue. Once fixed and processed (see below), these pieces of tissue will be the blocks of the case. **

It seems a coincidence that we both had breast cancer pathology reports at the same time. Here's the relevant passage in my translation:

"The following 18 cassettes were submitted for further study: A-B, xxx; C-D, xxx; E, xxx; F, xxx; G, xxx; H, xxx; J, xxx; K, xxx; L, xxx; M-R, 12 xxx."

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Note added at 13 hrs (2015-08-13 04:43:07 GMT)
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Just about any online explanation of a pathology report seems to mention putting the samples in cassettes.