The Canary Foundation Retrospective Prostate Tissue Microarray Resource is a carefully constructed TMA cohort designed to both definitively validate candidate biomarkers of aggressive disease at the time of radical prostatectomy and to discover new biomarkers for non-recurrent disease that can be used to help select patients for active surveillance. Our intent is to test these candidate biomarkers in an established a prospective, multi-institutional cohort, the Canary Prostate Active Surveillance Study (PASS), to determine whether these prognostic biomarkers can be used for selection of men at low risk for progression on active surveillance [8
]. By carrying out both discovery and validation studies in tandem, we attempt to address the critical question of which patients with localized prostate cancer can be safely watched and which patients require immediate therapy.
Unlike other large prostate TMA cohorts, patients have been selected according to a strict protocol, using design features similar to a clinical trial. This design offers significant advantages in decreasing potential biases inherent in many tissue microarray studies. First, by selecting patients randomly from institutional radical prostatectomy cohorts, we minimize spectrum bias. Second, by distributing patients across institutions we make our results more generalizable by decreasing the influences engendered by local patient selection biases, differences in treatment and variations in follow-up and endpoint assessments. Third, the prospective involvement of statistical experts allowed careful definition of study endpoints and power calculations that will render positive and negative findings of tested biomarkers clinically meaningful. Our objective was to design a study in which tissue based biomarkers could be assessed using methods that were up to standards necessary for regulatory approval for use in the clinic. Given these strengths, the statistical design of this study may serve as a model for future outcomes-based studies in other diseases that employ tissue-based biomarkers. In addition, our tissue microarray is a resource available to the cancer research community for the evaluation of prognostic biomarkers with sufficient preliminary data to justify testing.
Constructing tissue microarrays using the approach and standards we have detailed entails challenges and costs. The time from initially planning to final construction and use of the microarrays was much longer than anticipated. A significant portion of that time was spent in the design of the study. However, in the long-term, we anticipate that investigators using and adapting our study design can save significant time and the output in terms of confidence in the performance of a given biomarker is enhanced. Even with our methods, study planning requires significant input from a dedicated statistician, as well as assessment of data quality from sites, and direct participation in quota sampling. In addition, use of this study design relies on the availability (or creation) of patient databases at participating institutions. These data must be transferred to the statistician(s) at a central data site in a secure and blinded fashion which requires a database manager at each site. There are also significant challenges in the construction of the TMAs. Obtaining appropriate blocks on specific selected cases from pathology archives can be rate limiting. Furthermore, as in all tissue microarray studies, our study required significant time and commitment on the part of the study pathologists, who had to review all cases, select the dominant tumor, mark the blocks for core harvesting, supervise array construction and perform quality control on the final microarrays. While these challenges can be substantial, we have demonstrated that they are surmountable.
The study design imposes certain limitations. Definitive validation of biomarkers of non-recurrent disease requires biopsy tissue taken at the time of diagnosis, i.e. when a patient would be evaluated for entry into an active surveillance program. Biopsies produce a much smaller volume of cancer for biomarker discovery and validation. This study will select a small set of candidates for further validation with those precious biopsy samples. Other limitations include effects associated with sampling tissue blocks to construct TMAs. By using cores to represent the entire tumor, we may miss an ‘index’ lesion that is actually responsible for disease progression.
As we embark on assessment of prognostic biomarkers in this cohort, we will continue to test and refine the use of this resource. We anticipate that the quality of the resource will be sufficient to allow definitive testing of tissue biomarkers that they may be translated to clinical use. We encourage use of this resource by the prostate cancer research community for evaluation of mature prognostic biomarkers.