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Keywords:

  • PSA;
  • PCA3;
  • prostate cancer;
  • biopsy;
  • decision

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES

The decision to take a prostate biopsy is traditionally guided by a digital rectal examination and measurement of serum total prostate-specific antigen (tPSA). However, both techniques are subject to inherent weaknesses. The prostate cancer gene 3 (PCA3), a gene-based marker, specific for prostate cancer, supplements the predictive power of tPSA to improve diagnosis of disease. Including this new marker in the standard of care for men at risk of prostate cancer should be considered, as it presents marked potential for better decision making for a prostate biopsy and for improving overall patient care.


Abbreviations
tPSA

total PSA

%fPSA

percentage free/tPSA

ROC

receiver-operating characteristic

AUC

area under the curve

PCPT

Prostate Cancer Prevention Trial.

INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES

Certainty: freedom from doubt; a total security from error. What if urologists were able to make diagnostic and treatment decisions in a state of complete certainty? Patients would no longer face the risks of unnecessary invasive techniques, with their inherent risks, or of adverse events related to avoidable treatment. In diagnosis of prostate cancer, the urologist would make decisions about prostate biopsy based on noninvasive or minimally invasive indicators indicating the presence of the disease. No patient would have a biopsy who did not have prostate cancer. No patient would risk the anxiety and comorbidities of biopsy without good cause. Further, what if those noninvasive or minimally invasive indicators quantified tumour aggressiveness? The urologist could then determine the urgency of biopsy and treatment in the light of patient desires and life-expectancy based on other health status indicators. Unfortunately, today’s urologists and their patients enjoy no such security in diagnosis and management of the most prevalent disease that they manage.

However, biopsy decisions have not been uninformed. In addition to clinical variables, demographics and the presence or absence of other risk factors, a DRE of the prostate and measurement of serum total (t) PSA have traditionally been used to assist in biopsy decision-making. The use of these techniques combined has improved the detection and treatment of prostate cancer over the last few decades. However, the improvement has been limited due to intrinsic weaknesses in both methods. The DRE is subjective and has marginal predictive value [1–3], while PSA is subject to various inherent flaws, primarily driven by nonspecificity for prostate cancer. Thus, poor survival in prostate cancer results from a current lack of specific, highly predictive methods for early detection, and for differentiation of aggressive and indolent cancers.

Recently, a noninvasive urinary test for the prostate cancer gene 3 (PCA3) has been developed. PCA3 is an emerging gene-based marker that is highly specific for prostate cancer. In this review we examine the current diagnostic dilemmas, the weaknesses of traditional testing, and the potential of PCA3 to complement existing diagnostic methods.

THE DILEMMA

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES

We will illustrate the current clinical dilemma with an example: a 55-year-old Caucasian man consults his urologist for an initial appointment after referral from primary care. He reports a family history of prostate cancer in second-degree relatives, he complains of frequent urination, and his latest annual physical indicated a serum tPSA level of 2.7 ng/mL. The patient has a DRE, which reveals no suspicious nodule, and subsequently confirms the tPSA value. Should the urologist then recommend a prostate biopsy? Despite more than two decades of research on PSA, clinical experience, and establishment of institutional procedures, urologists still cannot make this decision with great confidence.

This example is neither unrealistic nor uncommon [4]. While patients present with more obvious risks for prostate cancer, still others present with more apparently benign indications and who are subsequently found to harbour prostate cancer (the prostate cancer rate is ≈ 20% in men with a tPSA level of 2.5–4 ng/mL, inclusive of indolent and clinically relevant cancers [5]). Even taking a more compelling example, wherein the hypothetical patient presents with a tPSA level of 4–10 ng/mL, there is still no certainty that prostate cancer will be detected on biopsy. In fact, up to 60% of men with a tPSA level in this range will have a negative biopsy.

PSA: CONTEMPORARY USE AND INHERENT FLAWS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES

Beginning when aged ≈ 50 years [6], men with a life-expectancy of ≥10 years have an annual measurement of their serum tPSA level. If the tPSA results are higher than their previous results but no other symptoms indicate a risk for prostate cancer, a DRE or tPSA testing can be repeated at appropriate intervals to observe and confirm any trends. If the tPSA level continues to increase or subsequent DRE results are suspect, the clinician might attempt to exclude various benign conditions using imaging techniques, cystoscopy, and measuring the percentage free PSA (%fPSA). If the results of these analyses indicate a sufficient risk of prostate cancer, a biopsy will be recommended. Of course, the definition of ‘sufficient risk’ will depend on the physician, their interpretation of the data, and established institutional procedures.

Characterizing risk based solely on serum tPSA findings presents inherent difficulties. PSA is specific for prostate tissue but not for prostate cancer. Elevated values of serum tPSA are found in many benign conditions involving enlargement of the prostate [7–11], including BPH [7] and acute prostatitis [8]. Conversely, a high body mass index erroneously lowers tPSA values as a result of haemodilution [12]. Thus, the interpretation of tPSA values is prone to error arising from nonspecific sources.

Furthermore, serum tPSA values are poor indicators of the aggressiveness of prostate cancer, regardless of the threshold chosen [13]. Because PSA does not correlate well with aggressiveness, there is a trend in clinical practice toward over-diagnosis and consequent over-treatment of prostate cancer [14].

In the light of these inherent weaknesses of serum tPSA testing, it is clear that complementary indicators are needed to better inform the decision to biopsy or proceed to radical treatments. Preferably, these new indicators would be insensitive to the nonspecific factors that affect serum tPSA results. Furthermore, emerging indicators would ideally correlate with tumour aggressiveness and provide information independent of and complementary to serum tPSA.

IDENTIFICATION OF PCA3 AND EARLY STUDIES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES

PCA3, also referred to as PCA3DD3 or DD3PCA3, was first identified in 1999 via differential-display analysis in healthy, hyperplastic and cancerous tissue from patients undergoing radical prostatectomy [15]. PCA3 had a high level of expression in prostate tumours and was apparently absent in benign tissue. Further analyses showed low but quantifiable expression in benign prostate tissue but undetectable levels of expression in normal tissues from all major organs. Also, no expression could be detected in tumours from breast, cervix, endometrium, ovary or testis, and cancer cell lines from bladder, breast, kidney and ovaries.

Subsequent studies using quantitative research tests for PCA3 showed a median 66-fold up-regulation in prostate malignancies, and a high sensitivity and specificity for detecting prostate cancer [16]. The ProgensaTM PCA3 test (Gen-Probe, San Diego, CA, USA) was developed soon thereafter. The test uses transcription-mediated amplification technology to quantify PCA3 and PSA mRNA in urine samples collected after a DRE. The DRE is required to release prostate cells into the urine and the quantification of PSA mRNA is required to normalize for the total mRNA present in a sample (PSA mRNA levels in prostate cells released into urine are completely unrelated to PSA protein levels in blood and are essentially unchanged in prostate cancer [16]). Thus, the method measures both PCA3 mRNA and PSA mRNA, and the results are represented as a ratio of the two mRNAs, referred to as the ‘PCA3 score’. Similar to other gene-based tests, the PCA3 assay is comparable in cost and complexity. Samples must be sent to an accredited laboratory experienced in performing molecular testing and PCA3 scores are reported to the urologist.

Sample collection and specimen stability are robust. Informative rates (percentage of urine samples yielding accurately quantifiable mRNAs for assay) are >99%[17], and the assays have good reproducibility with intra- and interassay coefficients of variation of <13% and <12%, respectively, and total variation of <20% for the PCA3 score [18].

CLINICAL APPLICATION OF THE PCA3 ASSAY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES

Determining a PCA3 score could be useful in several clinical scenarios. First, the score can be used to increase confidence in an initial biopsy decision where the serum tPSA results are uncertain (2.5–10 ng/mL). Second, PCA3 testing could be used to increase confidence in a re-biopsy decision, wherein the DRE and serum tPSA results are suspicious and/or family history and other factors indicate an increased risk of prostate cancer. Lastly, when biopsy results are positive but tumour aggressiveness is unknown, PCA3 might be useful in comparing the risks and benefits of radical prostatectomy vs active surveillance management. Thus, the availability of a PCA3 score alone or combined with existing methods might better guide biopsy decision-making than current methods, and might be useful as an indicator of clinical stage and disease significance.

PCA3 clinical performance vs serum tPSA

Comparative research studies have consistently shown a better predictive value for prostate cancer for PCA3 than for serum tPSA. Marks et al.[19] studied 233 men with a previous negative prostate biopsy but with evidence of a serum tPSA level persistently >2.5 ng/mL. Applying receiver-operating characteristic (ROC) curve analysis to PCA3 and serum tPSA results obtained after a re-biopsy of these men yielded a significantly higher area under the curve (AUC) for PCA3 than for serum tPSA (Table 1) [17,19–21].

Table 1.  A summary of comparative studies of serum tPSA and PCA3
StudyNComparison typePCA3Serum tPSAComments
[24]122Prediction of positive initial biopsy79% specificity at 69% sensitivity60% specificity at 69% sensitivityROC-AUC for PCA3, 0.746
[20]583Prediction of positive initial biopsyAUC 0.66 (95% CI 0.61–0.71)AUC 0.57 (95% CI 0.52–0.63) 
[21]570Prediction of positive initial biopsyAUC 0.686AUC 0.547 
[19]233Prediction of positive repeat biopsyAUC 0.678 (95% CI 0.597–0.759)AUC 0.524 (95% CI 0.438–0.610)Difference in AUCs significant (P = 0.008); tPSA AUC 95% CI overlaps random chance
[21]570Correlation with prostate volumeNot correlatedPositively correlated 
[17]463Correlation with prostate volumeNot correlatedPositively correlated 

In a separate study, Groskopf et al.[22] compared PCA3 and tPSA in 70 men who had a prostate biopsy based on pre-existing risk factors, in comparison with 52 apparently healthy men with no known risk factors. At a PCA3 score threshold of 50, the sensitivity was 69% and the specificity 79%. For serum tPSA at the established threshold of 2.5 ng/mL, and with sensitivity held constant at 69%, the specificity for tPSA was 60%.

The foregoing results were recently confirmed using a time-resolved fluorescence-based variant of the PCA3 test by van Gils et al.[20]. In their multicentre study of 583 men with a serum tPSA level of 3–15 ng/mL, the AUC for predicting a positive biopsy was higher for PCA3 than for serum tPSA testing (Table 1). There was also a correlation of increasing PCA3 score with increasing probability of positive repeat biopsy [20].

Associations with prostate volume and tumour volume/tumour aggressiveness

An association of a marker with prostate volume regardless of the presence or absence of prostate cancer is an undesirable characteristic, as it indicates nonspecificity of the marker. Whereas serum tPSA shows such associations, PCA3 did not in an initial study by Deras et al.[21]. In that study the associations of both markers with prostate volume were evaluated in 570 men scheduled for initial or repeat prostate biopsy. Serum tPSA values increased significantly (P < 0.001) with increasing prostate volume, whereas PCA3 scores were unaffected (P = 0.54).

Conversely, associations of a marker with tumour aggressiveness are obviously desirable. Nakanishi et al.[23] assessed the correlation of PCA3 with tumour volume and tumour aggressiveness in an analysis of urine collections before prostatectomy in 96 men with biopsy-confirmed prostate cancer. The PCA3 score increased linearly and significantly with increasing tumour volume (R = 0.27, P = 0.008), and there was a significant difference (P = 0.007) when comparing PCA3 scores for individuals with low-volume and low-grade tumours with PCA3 scores in the subpopulation with ‘significant cancers’ (men with combinations of high dominant tumour volumes and Gleason scores).

Most recently, a study by Haese et al.[17] supported the findings of Nakanishi et al.[23], Deras et al.[21] and Marks et al.[19]. In that multicentre, multinational European study, 463 men with one or more previous negative biopsies were re-biopsied after a DRE and urine collections; the re-biopsy yielded 128 cancers (28%). Detected cancers were classified as indolent if they were stage T1c, had a PSA density of <0.15 ng/mL, a Gleason score at biopsy of ≤6, and had ≤33% positive cores (a standardized minimum of 10 peripheral zone cores were acquired). The median PCA3 scores were higher in significant than in indolent cancer (42.1 vs 21.4, P = 0.006). Also, PCA3 scores were higher for patients with a biopsy Gleason score of ≥7 vs < 7 (P = 0.040) and for patients with stage T2 vs T1c cancer (P = 0.005). Furthermore, the PCA3 score was not affected by age, number of previous negative biopsies, and total prostate volume. Lastly, the PCA3 score corresponded with an increasing probability of a positive repeat biopsy.

PCA3 performance vs serum tPSA range

It is desirable for any new indicator of prostate cancer to maintain its predictive value across the entire range of serum tPSA values in the population. To address the performance of PCA3 in this regard, Deras et al.[21] categorized their study cohort into patients with a serum tPSA of <4, 4–10 and >10 ng/mL, and determined the sensitivity and specificity of PCA3 for detecting a positive biopsy within each category. Across all categories, the sensitivity was 54% at a specificity of 74%. These parameters varied by <10% across serum tPSA range categories. The study of Haese et al.[17] confirmed these findings; PCA3 scores did not vary significantly (P = 0.728) across the same categories of serum tPSA level. Across all categories the sensitivity was 47% at a specificity of 72%, results which are in excellent agreement with Deras et al.[21].

COMPLEMENTARITY WITH SERUM TPSA

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES

If tPSA and PCA3 are destined to be complementary in clinical management and treatment decisions, there must be evidence that the utility of both markers is enhanced when data are analysed as covarying predictors of biopsy outcomes. In the study of Deras et al.[21], urinary PCA3 and serum tPSA were evaluated in univariate and multivariate logistic regression models. The predictive probability relative to biopsy outcome was determined. For tPSA alone, PCA3 alone, and the combination of tPSA + PCA3, the AUCs from ROC were 0.547, 0.686 and 0.752, respectively (Fig. 1) [24]. The increase in AUC in the multivariate model was highly significant (P < 0.001), showing the complementarity of the methods.

image

Figure 1. ROC curve analysis of serum tPSA, PCA3 score, and a multivariate logistic regression model for predicting a positive initial biopsy. The multivariate model included logarithmic transformations of PCA3 score and tPSA in addition to prostate volume and DRE result. (Reproduced from [21], with permission).

Download figure to PowerPoint

Furthermore, the incorporation of PCA3 information into the Prostate Cancer Prevention Trial (PCPT) risk calculator showed additional benefit [24]. Although it was not possible to measure directly the urinary PCA3 scores in the PCPT study population, statistical methods to incorporate data from external populations were developed [24]. PCA3, serum tPSA and DRE data from 521 men undergoing prostate biopsy were included in the original PCPT risk calculator, serum tPSA by itself, urinary PCA3 by itself, and an updated PCPT risk calculator incorporating PCA3. The AUC for the PCPT calculator incorporating PCA3 was 0.703, which was statistically superior to the PCPT calculator without PCA3 (AUC 0.618, P < 0.025). Both the updated PCPT calculator and PCA3 alone were also superior to serum tPSA alone. In addition, the sensitivity, positive predictive value and negative predictive value were all maximized for the PCPT risks with PCA3 incorporated.

The recent study by Haese et al.[17] again supports these earlier findings. In a multivariate logistic regression model for predicting prostate cancer at repeat biopsy, the PCA3 score was an independent predictor (P = 0.006) of outcome after adjusting for age, serum tPSA, %fPSA, DRE and prostate volume. Including PCA3 in the base model containing the other terms improved the accuracy of the model by 4.2%, which was significant (P < 0.001). The PCA3 score was also the most informative univariate predictor in this study.

AVOIDANCE OF UNNECESSARY BIOPSIES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES

The effectiveness of PSA screening on prostate cancer mortality is still a matter for debate, but increasingly many men undergo PSA testing annually. For example, data from the National Cancer Institute indicates that in the USA an additional 1.8 million men aged 40–69 years and 1.2 million men aged >70 years would have an abnormal PSA value if the threshold for PSA was decreased from 4.0 to 2.5 ng/mL [25]. This results in an additional 3.0 million men with an elevated PSA level who would be candidates for biopsy. Such trends show a need for other cancer-specific markers to improve diagnostic and treatment decision-making.

The study of Haese et al.[17] provides some insights into the identification of patients for whom biopsy is unwarranted. At a PCA3 score threshold of 20, repeat biopsies would have been reduced by 44% while missing only 9% of cancers. This finding suggests the potential of PCA3 to reduce the incidence of over-diagnosis, and further studies are ongoing.

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES

Over the past two decades, serum tPSA, together with other indicators, has guided biopsy decisions for urologists. While research during this period has characterized the strengths of tPSA testing, many weaknesses have also been revealed. The discovery and clinical evaluation of PCA3 has shown that the marker supplements tPSA in diagnosis and is insensitive to the nonspecific factors that can affect circulating tPSA levels. The addition of PCA3 to the urologist’s diagnostic tools will not result in a state of certainty; however, diagnostic sensitivity, specificity and predictive value are incrementally improved by its inclusion. In turn, biopsy and management decisions might be better informed. This has the potential to improve the overall level of patient care.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. THE DILEMMA
  5. PSA: CONTEMPORARY USE AND INHERENT FLAWS
  6. IDENTIFICATION OF PCA3 AND EARLY STUDIES
  7. CLINICAL APPLICATION OF THE PCA3 ASSAY
  8. COMPLEMENTARITY WITH SERUM TPSA
  9. AVOIDANCE OF UNNECESSARY BIOPSIES
  10. CONCLUSION
  11. CONFLICT OF INTEREST
  12. REFERENCES
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