—  SHORT COURSE  —

USES AND LIMITATIONS OF ANCILLARY TECHNIQUES
APPLIED TO CYTOPATHOLOGY

Jeffrey S. Ross, M.D.




USES AND LIMITATIONS OF
IMMUNOCYTOCHEMISTRY IN CYTOPATHOLOGY

The applications of immunocytochemistry to diagnostic cytology including consideration of limitations and errors associated with the technique has been the subject of numerous reviews.1-9  Although both immunofluorescence and immunoperoxidase methods have been utilized, the immunoperoxidase technique has been preferred in that a fluorescence microscope is not required and the stained slides can be permanently stored for re-review and subsequent research studies. Immunohistochemistry has been performed on both cytologic smears and cell blocks obtained from fluids and fine needle aspirate specimens. When significant diagnostic material is available in paraffin embedded cell blocks, these have been the favored specimens for immunohistochemical studies.10  However, significant new technical errors can be introduced by the use of formalin fixation and paraffin embedding which can denature antigens and cause false negative staining as well as the production of false positive staining by over-aggressive antigen retrieval techniques (see below).11-12  Antigen expression can be lost on prolonged storage of paraffin slides.13  In that different methods are needed to optimize the identification of different antigens, it has not been possible to standardize immunocytochemistry for all specimens.14, 15  The three major uses of immunocytochemistry in cytology are: the differentiation of malignant from benign cells; the phenotyping of malignant cells; and prognosis assessment of malignant lesions. Although consensus holds that immunohistochemistry (IHC) is of significant value as an adjunct to diagnosis in cytologic specimens (Table 1), there has been considerable variation in the success rate of using the technique as an ancillary procedure.16  The so-called useful rate for IHC in cytologic specimens has varied from 50 to 80%.16-17  High utility rates have been described even for specimens that have been previously stained by another method.18  Immunocytochemistry has also been utilized in the confirmation and characterization of infectious organisms present in cytologic specimens. When both cytopathology and surgical pathology are considered together, the cost-effectiveness of IHC is substantial.19 

Table 1. Contribution of Immunocytochemistry in Cytology (Lai et al)

 CONTRIBUTORY 78(54%)
       ESSENTIAL 41(28%)
             EFFUSIONS 36
             CSF 4
             OTHER 1
       CONFIRMATORY 37(26%)
             FNA 20
             OTHER 7
             EFFUSIONS 8
             CSF 2
  
 NON-CONFIRMATORY67(46%)
       EFFUSIONS 45
       OTHER 9
       CSF 5
       FNA 8
  
 TOTAL 145

The introduction of IHC to cytologic diagnosis and tumor prognosis assessment has also introduced pitfalls and errors in interpretation that must be taken into consideration when the technique is utilized. General pitfalls and errors associated with IHC in cytology specimens include: technical factors affecting staining results, biologic factors concerning expression of cellular markers, and interpretation factors concerning the application of the method to a given specimen.