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Pathology Reports for GIST

 

Biopsy Samples and Reports for GIST

As discussed earlier, biopsies are done on tumors suspected to be gastrointestinal stromal tumor (GIST) only when advance knowledge of the diagnosis would affect decisions about what treatment to use first (surgery, pre-surgery drugs to shrink the tumor, etc.).

What is different about testing biopsy samples versus surgical samples?

A biopsy contains only a small amount of tissue.  Sometimes the sample is too small for the pathologist to describe tissue structure (how the cells are organized), but the individual cells can be described.  Sometimes a needle biopsy will withdraw only a few useful tumor cells along with blood and necrotic fluid (dead cells and decomposing residue). The pathologist will not attempt to estimate prognosis on such a small sample, but the sample will be described and a diagnosis will be made, if possible.  Unfortunately, sometimes the biopsy misses the tumor and only samples tissues next to the tumor.  In this situation, the biopsy must be repeated.

Microscopic Description

Cell shape varies among different tumors.  Most GISTs are composed of "spindle" cells that are long and skinny, but some GISTs contain "epithelioid" cells that are round or polygonal in shape.  Some GISTs contain a mixture of both cell types.

Once the diagnosis is established, the mitotic rate is the most important microscopic factor for prognosis.

See illustrations and more details about this in the section below on surgical pathology reports.

Immunohistochemical Results

The pathologist will select the most definitive tests to use on the limited biopsy sample in order to give a diagnosis, as there may not be enough tissue for every desired test.  This list may include immunohistochemistry for KIT (CD117), CD34, smooth muscle actin, S100 protein, desmin, and pan-cytokeratin.  For details about these markers see the section Pathology Analyses.
 

Sample Biopsy Report on a GIST

Here is a sample report on a biopsy.

 

GASTRIC TUMOR, BIOPSY:

GASTROINTESTINAL STROMAL TUMOR, spindle cell type.
Mitoses number 1 per 20 high power fields.
No necrosis identified.

Immunohistochemistry shows the following staining profile in tumor cells:
  Positive - KIT, CD34
  Negative - desmin, S100 protein
These findings support the above diagnosis.

 

 

Surgical Resection Samples and Reports on GIST


The surgical pathology report will contain much more information because the pathologist has adequate tumor tissue to work with.  After describing the entire surgical specimen (everything that was removed), the pathologist samples areas of the tumor that appear to be growing most rapidly.

In addition to your pathology report, you should read the surgeon's operative report for a description of your tumor's location and how it was removed (lifted away from adjacent organs, removed with adherent parts of adjacent organs "en bloc," etc.)

 

Sample Surgical Pathology Report for GIST

Here is a sample report on a resected tumor. 

SMALL BOWEL RESECTION:

GASTROINTESTINAL STROMAL SARCOMA, mixed epithelioid and spindle cell type (9 cm in greatest dimension),

HIGH RISK (per 2007 NCCN guidelines).
Mitoses number 8 per 50 HPF.
No necrosis identified.
Tumor involves the full-thickness of the bowel wall and penetrates the serosa.
The proximal and distal small bowel resection margins are negative for tumor.

Immunohistochemistry performed on paraffin sections reveal the following staining profile in tumor cells:
  Positive - KIT, DOG1
  Negative - SMA, desmin, S100 protein
 

 

Reports on Treated GISTs

If your report describes a GIST removed after treatment with imatinib or sunitinib or other drugs, the report will differ in several ways from a pre-treatment report.  This applies both to primary tumors pre-treated with imatinib to shrink the tumor, and to resection of metastases after drug treatment.

  • Mitotic count may be listed, but it is not comparable to the mitotic count of an untreated tumor since the drug usually reduces cell proliferation.
  • Risk classification is not possible (and is irrelevant for metastatic tumors since the tumor has already spread).

Percent viable cells

One of the most informative facts about a treated tumor is the percent of viable (living) tumor cells remaining, in contrast to necrotic (dead) cells, and areas where tumor cells have been destroyed and only scarring (often described as "hyalinization") remains.  The smaller the percentage of viable tumor cells, the better the response to treatment has been.  However, even if there appears to be 100% tumor response (no viable tumor cells found), there may still be viable tumor cells in the tumor or elsewhere in the body.  In addition, sometimes a successfully treated tumor will become less cellular ("hypocellular"), meaning the tumor cell nuclei are farther apart with more intervening stroma and/or scar tissue, although the tumor cells are still "viable". Even after removal of all visible metastatic disease, treatment is usually continued.

Some treated tumors that have grown resistant to targeted drug therapies like imatinib may show unusual changes that the pathologist can detect.

Loss of KIT protein expression

Tumor cells may no longer express KIT protein if the tumor has developed another pathway for survival and growth.  For example, some treated GISTs have shown a "kinase switch" to AXL, an alternate growth factor.  If the tumor stops testing positive for KIT protein, the pathologist may be uncertain whether this tumor is GIST or whether it is a new cancer.

Change in morphology

Rarely, treated tumor cells may show a change in appearance such as differentiation toward a different cell type (such as rhabdomyosarcomatous, which is skeletal muscle) or dedifferentiation toward more primitive cell forms.  Such changes make it more difficult to be sure the tumor is GIST.  Fortunately, these changes are very rare.

 

Sample Report on a Treated GIST

Here is a sample report on a tumor treated with drugs before resection.

 

LIVER, PARTIAL HEPATECTOMY:

METASTATIC GASTROINTESTINAL STROMAL SARCOMA, spindle cell type (4.5 cm in greatest dimension) with extensive (>95%) hyalinization, consistent with treatment effect.
Mitoses number less than 1 per 50 HPF.
The surgical resection margin is negative for tumor.

 

Go to next section, GIST Pathology Questions and Terminology



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