A summary of the changes to PCa grading from the original Gleason grading system to the latest prognostic grade grouping

THE 2005 AND 2014 MODIFIED GLEASON GRADING SYSTEMS One of the biggest changes to the Gleason grading system was the classification of Grades 1 and 2. Grade 1 tumors are generally benign, and Grade 2 tumors do not appear to differ from those classified as Grade 3. In 2005, Grade 2 was recommended to be used “rarely, if ever,” and in the 2014 modified Gleason grading system, grading started from 3. This modification accounted for some of the observed rises in Gleason scores. A second change causing an increase in Gleason scores was the narrowing of the definition of Gleason 3 and concomitant expansion of Gleason 4. From the 2005 to the 2014 consensus conferences, the histologic criteria for Gleason patterns 3 and 4 changed, resulting in the reduction of pattern 3 and expansion of pattern 4.2 In the original system, pattern 3 included some cribriform as well as poorly formed glands.5 Only well‐formed discrete glands are included in pattern 3 in the 2014 modified Gleason grading system. In particular, cribriform glands lacking basal cells, independently of their morphology and size, are considered as pattern 4 in the 2014 modified Gleason system.1,2,6–15 Fused, poorly formed, and glomeruloid glands are part of the morphologic spectrum of the current Gleason pattern 4. As some patterns that were previously included in Gleason pattern 3 are now considered pattern 4, PCas with a GS of 3 + 3 = 6 based on the ISUP 2014 modified system have a far better prognosis than PCas INTRODUCTION Treatment of prostate cancer (PCa) is based on the clinical and pathological features that predict the course of the disease. The risk of local or systemic recurrence is usually based on data obtained from prostate needle biopsy or radical prostatectomy (RP) specimens. The Gleason grading system is one of the most important prognostic factors.1–3 In 1966, DF Gleason created a grading system for PCa based on tumor architectural patterns.4 Dr. Gleason recognized the heterogeneity of PCa by assigning two grades to the two most common architectural patterns, reported as the Gleason score (GS).4 Dr. Gleason reported that the presence of more than two architectural patterns was quite rare to allow for an accurate evaluation of the prognostic role of the third most prevalent pattern (i.e., the tertiary pattern).5 The management of PCa has changed since the original system was proposed. In particular, patients in the 1960s and 1970s were not treated with RP because they presented with advanced disease and because of the greater morbidity associated with surgery; therefore, grading of RPs with multiple tumor foci and tertiary patterns was not fully investigated by Dr. Gleason. With the PSA screening and 18‐gauge needle biopsies, pathologists faced new issues, such as how to report multiple cores with PCa of different GSs and how to grade small amounts of PCa. Pathologists needed guidance for applying the grade to newly described histological patterns and variants of PCa, and modifications of the original Gleason system were needed to reflect the modern practice. The Gleason grading system has undergone changes as a result of two International Society of Urological Pathology (ISUP) consensus conferences held in 2005 and 2014.4,6 INVITED REVIEW