Advantages of IHC:
(1) The test is widely available.
(2) The test is relatively simple to perform.
(3) The cells being tested can be visualized.
The most common EGFR mutations involve:
(1) deletion of exon 19
(2) point mutation L858R in exon 21
Sections of the tumor are stained with antibodies to the specific mutations, with detection based on a chromogen attached to a detector antibody.
|
Color Intensity
|
Staining Grade
|
|
none or minimal
|
0
|
|
weak
|
1+
|
|
moderate
|
2+
|
|
strong
|
3+
|
Quantification of the staining intensity may be done using the sum of (intensity times percent of tumor) subscores. This is referred to as the Quickscore (Q score) or Histochemical score (H score).
total score =
= (percent of tumor scoring 1+) + (2 * (percent of tumor scoring 2+)) + (3 * (percent of tumor scoring 3+))
Interpretation:
• minimum score: 0
• maxium score: 300
• A cutoff (such as > 200) is used to detect a tumor positive for the mutation.
Limitations of IHC:
• The test can only detect the specific mutation that is being tested for. This can result in false negative detection of mutations.
• There is considerable variation in testing and interpretation. Results are affected by the tissue sample, fixation, processing, staining and reading.
• The test requires a minimum number of identifiable tumor cells to be present.
• The test is phenotypic and not genotypic. While low and high scores can help to identify the mutation status, a score just below the cutoff may fail to detect a patient who could respond to TKI therapy.
