Page 95 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

74 PART I The Biology and Pathogenesis of Cancer

result in excessive nonspecific background staining. A skilled TEM can help identify specific cellular features, such as inter-
pathologist who is familiar with the IHC stain should be asked cellular junctions or basal lamina in epithelial cells, melanosomes
in melanocytic cells, granules in mast cells, neurosecretory gran-
to differentiate background stain from tumor-specific stain and to
VetBooks.ir navigate technical difficulties. As with histochemical stains, IHC ules in neuroendocrine cells, mucin droplets in certain epithelial
17,137,138
These
cells, and villous projections of mesothelial cells.
does not distinguish between neoplastic and nonneoplastic tissue.
For example, normal bladder mucosal epithelium (urothelium), features may be useful in distinguishing carcinoma from lym-
urothelial hyperplasia, and urothelial carcinoma (TCC) would all phoma and identifying melanoma, MCT, neuroendocrine tumor,
be immunopositive for cytokeratin. The distinction between neo- and mesothelioma. TEM ultimately provides a level of magnifi-
plastic and nonneoplastic is made on routine H&E light micros- cation to visualize specific and detailed cellular components that
copy based on hallmark features of neoplasia. cannot be appreciated with routine light microscopy.
Evaluation and interpretation of IHC results should be per-
formed with the knowledge of antibody sensitivity and specificity. Flow Cytometry and Polymerase Chain Reaction
For example, both vimentin and cytokeratin are highly sensitive
for sarcomas and carcinomas, respectively, but lack specificity. Both flow cytometry and PCR have become fairly routine pro-
S-100 can be used to support melanomas with good sensitivity, cedures in veterinary oncology and often are combined with
but it lacks specificity because it is expressed in a broad variety of histopathology, cytopathology, IHC, and/or ICC. These tech-
cells, especially neural crest derivatives (e.g., Schwann cell, nerve, niques can be useful for tumor classification and/or confirmation
cartilage, bone, smooth muscle, adipose). Because most tumor (especially hematopoietic tumors), particularly when H&E and/
markers have limitations, the best and most reliable results may or IHC interpretation is ambiguous. 139–141 These techniques are
be obtained using a panel of IHC stains, for which both marker- discussed in detail in Chapter 8.
specific immunopositive and immunonegative results may be
anticipated (e.g., rhabdomyosarcoma should be immunopositive
for vimentin and desmin, but immunonegative for smooth muscle Clinical-Pathological Correlation and Second
actin), rather than relying on a single stain. Additionally, IHC Opinions
stains can be appropriately interpreted only in conjunction with
appropriate species-specific controls. To support the diagnosis or Establishing a definitive histopathologic diagnosis can be pre-
determine the immunophenotype of feline intestinal lymphoma, cluded by the absence of relevant and necessary clinical informa-
appropriate positive control tissue, such as feline LN, spleen, or tion. This may be especially true regarding primary bone tumors
2,5
tonsil, must be run simultaneously. It must be of feline origin and versus secondary tumors involving bone. Diagnosis of a surface
contain normal lymphoid tissue if the pathologist is to confirm or juxtacortical OSA depends both on imaging results and histo-
that the IHC stain was performed successfully and to interpret pathologic features. Similarly, an osteoma may be difficult to dis-
appropriately the immunoreaction of the test tissue. Similarly, a tinguish from reactive bone without a corroborative radiograph.
negative control, which consists of the test tissue treated either Periarticular neoplasia may be difficult to distinguish from other
with nonspecific antibody or omission of the primary antibody, sarcomas, or even inflammatory or immune-mediated joint dis-
must also be run to assist in ruling out background/nonspecific ease, unless radiographic or gross evidence of joint involvement
staining. Laboratories that offer IHC should ensure all tests have and bone invasion is seen. Acanthomatous ameloblastoma may be
been optimized and validated relative to each species for which the difficult to distinguish from benign periodontal ligament tumors
test is offered. unless bone invasion is identified. Confirming bone involvement
IHC can be a powerful tool for providing information that depends on deep biopsy samples that capture underlying bone
could not be otherwise determined on routine microscopy alone or on clinical information that indicates bone involvement. The
(e.g., confirmation of a tumor’s histogenesis based on molecular importance of communicating clinical data might be best illus-
markers). However, an IHC stain should never be interpreted in trated by histologically low-grade yet biologically high-grade FSAs
18
and of itself; rather, it always should be evaluated in conjunction of the canine oral cavity. Histologically these tumors may be
with routine light microscopic findings and the relevant clinical mistaken for benign fibrous tissue, but the clinical presentation is
information. a rapidly growing, invasive and destructive mass that often recurs
18
after conservative surgery. These examples demonstrate the need
Transmission Electron Microscopy to furnish the pathologist with an accurate and thorough clinical
history, all relevant clinical findings, and the results of all other
As other ancillary diagnostic procedures have become more widely diagnostic tests. Ultimately, providing any previous imaging scans
used, TEM is performed less frequently on tumor biopsy speci- (or other) results and reports themselves may be the easiest and
mens, but it is still a notable ancillary resource. TEM requires most accurate means of relaying this information.
specific technical support and equipment and is available only When extensive treatment may be pursued or if a pathology
at a few diagnostic laboratories. Specimen preparation involves diagnosis is not consistent with the clinical impression, a second
preserving very small representative tumor samples (1 × 1 mm) opinion should be considered. 142,143 In human medicine a review
in special fixatives (e.g., glutaraldehyde), processing tissue into of mandatory second opinion surgical pathology at major hospi-
epoxy-based plastic blocks, and sectioning at 1 μm for thick sec- tals revealed 1.4% to 5.8% major changes in the diagnosis that
tions to determine the adequacy of the sample and inclusion of resulted in a change of therapy or prognosis. It was concluded
appropriate tumor cells. Subsequently, sectioning is done at about that despite the extra cost, mandatory second opinions should
600 Å; the samples are stained with heavy metal–based stains and be obtained whenever a major therapeutic endeavor is considered
then examined with the aid of an electron microscope. Samples or if treatment decisions are based primarily on the pathologic
fixed in formalin can be used, although the quality of the subse- diagnosis. 144–146 In veterinary medicine, diagnostic disagree-
quent sections is suboptimal. ment between first and second opinion pathology has been