ploidy

"Focused on the Prostate since 1996"

DNA Ploidy 101 -A simple explanation

DNA Ploidy Analysis

[Written and Researched May 1996,]

By JR Oppenheimer, M.D., F.C.A.P.
Prostate pathologist on line
DrO@prostatelab.com

Both flow cytometry and and static image analysis have been used to determine DNA content (ploidy) of prostate cancer (CaP). DNA studies have shown that, as a group, patients with diploid cancers have longer disease free intervals and survival times than those with non-diploid tumors (Zinke, 1997). However, they may not be so helpful in predicting stage for an individual patient. Approximately 30% of all organ-confined tumors are non-diploid and almost half of the tumors with region lymph node metastases are diploid (Epstein 1992).

Because diploid tumors are more responsive to hormonal therapy (Zinke, 1992,1997), the Prostate Cancer Working Group (sponsored by the College of American Pathologists) has found that DNA ploidy studies are useful in patients with T3 or node-positive disease. As of June 1995, the Working Group does not recommend DNA analysis in T1, T2, or node-negative disease because of conflicting results in the literature (Grignon 1995). It still might be useful to have this data in case consensus is eventually reached, however.

Studies demonstrating the utility of DNA ploidy status have been performed on whole prostates obtained after RP. Since CaP is often a heterogeneous and multifocal disease, it doesn't necessarily follow that a tiny piece of tissue obtained on needle biopsy is representative of the lesion as a whole. Two reports have confirmed that ploidy status differs in different parts of an involved prostate (Greene,1991;O'Malley,1993). This intratumor variation in DNA ploidy suggests that multiple site sampling (possibly by fine needle aspiration) may be necessary to obtain accurate DNA measurements. Thus it may be misleading to assume than a tumor is entirely diploid when only a small fraction of it is sampled. Nevertheless, three studies have shown correlation between ploidy on needle biopsy and subsequent RP material (Leung,1994; Takai1994, Ross 1994).

The lack of mutually accepted standards limits the usefulness of ploidy analysis. For example, the percentage of non-diploid cells that are necessary to call the test results "non-diploid" varies from study to study (Epstein 1992). Another pitfall is that cells normally found in the seminal vesicle are normally tetraploid and will yield a false positive unless these cells are first recognized as non-cancerous (Hardt,1994). This can be minimized by using the image analysis technique.

Both flow cytometry and image analysis techniques suffer from limitations. Flow cannot be performed when only a small amount of tumor is present on needle biopsy. Neither can it distinguish tumor from non-tumor cells so a small number of non-diploid cells may be diluted to insignificance by larger numbers of benign diploid cells. This may explain why flow is less sensitive than image analysis to non-diploid cell populations (Pindur 1994). Static image analysis allows determination of ploidy in tissue sections with relatively small amounts of tumor, but the interpretation of results is still hampered by the lack of standard methodologies (Grignon,1995; Hardt, 1994; Falkmer,1992). This problem will likely resolve with increased use of image analysis; it is the favored technique of DNA analysis within commercial labs.

Should DNA ploidy be determined from needle biopsy or RP tissue in order to determine prognosis? Two studies using RP material have shown that while DNA ploidy analysis can be used to predict stage, it does not add any additional information to that provided by grade (Epstein 1992, Dejter, 1989). A more recent paper using needle biopsies (Ross 1994) came to the opposite conclusion; ploidy analysis was more important than grade. This last study showed a ten-fold increase in risk for metastasis and a three-fold risk for extracapsular spread if the initial needle biopsy demonstrated non-diploidy. These results have not been duplicated.

In conclusion, it appears premature to place too much emphasis on DNA ploidy analysis performed on needle biopsy cores since a small biopsy demonstrating diploid tumor may be missing a significant underlying non-diploid component.. While groups of diploid patients have better prognoses than groups of non-diploid patients, ploidy status may have uncertain prognostic value in individual patients. Published studies have reached different conclusions concerning prognostic value. Technical standards and methodologies differ from lab to lab. Correlation with histologic criteria remains essential.

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