ISUP Grade Groups: Translating Gleason Architecture into Prostate Cancer Risk

What it is and where it sits in current classification

The ISUP Grade Group system is not a distinct tumor entity but a grading system—the pathology output that most powerfully stratifies prostate adenocarcinoma by risk. It is fully endorsed in the current fifth-edition WHO classification of urinary and male genital tumours [1], where it runs alongside the traditional Gleason score that clinicians have used for decades.

Historically, prostate cancer was graded by the Gleason score, which sums the two most prevalent architectural patterns (each scored 1–5). The Grade Group system, formalized at the 2014 International Society of Urological Pathology (ISUP) consensus conference [2], distills the practically relevant range of Gleason scores into a more intuitive five-tier scale. Crucially, it corrects a longstanding source of patient anxiety: under the old scheme, the lowest score encountered in practice was 6 on a scale that appeared to run to 10, implying a mid-range malignancy when in fact it represents the least aggressive category. Grade Group 1 restores the sense that this is, in fact, grade one.

How a pathologist recognizes it: morphology first

Prostate grading is an architectural exercise. Unlike many tumors, where immunohistochemistry (IHC) or molecular assays drive classification, Grade Group assignment rests almost entirely on the pattern of glandular growth seen on hematoxylin and eosin sections. The pathologist evaluates how tumor glands are organized in tissue space.

Gleason pattern 3 consists of discrete, individually recognizable glands—infiltrative but still forming separate, well-defined lumina. Gleason pattern 4 represents a loss of that individual glandular architecture: fused glands, cribriform structures (a single mass perforated by multiple lumina, like a sieve), glomeruloid formations, and poorly formed glands lacking clear lumina. Gleason pattern 5 reflects near-total loss of glandular differentiation: solid sheets of cells, single infiltrating cells, cords, or comedonecrosis (central necrosis within tumor masses) [1,2].

IHC plays a supporting, not grading, role. This is an important teaching point: there is no immunostain that assigns a Grade Group. IHC (for example, basal-cell markers to confirm that a suspicious focus is truly invasive rather than a benign mimic) helps establish that cancer is present and can help distinguish invasive carcinoma from intraductal spread, but the grade itself is read off the architecture. In the diagnostic sequence—morphology, then IHC, then molecular—grading lives firmly in the first step.

Molecular findings currently inform prognostic understanding rather than routine grade assignment. The evidence suggests that cribriform and intraductal patterns carry adverse genomic features, which is part of why the field increasingly flags them separately (discussed below). This remains an active area of correlation between morphology and genomics.

Grading and classification

The five-tier Grade Group system maps onto Gleason scores as follows [2]:

• Grade Group 1 = Gleason score 6 (3+3): only discrete, well-formed glands.
• Grade Group 2 = Gleason score 7 (3+4): predominantly well-formed glands with a lesser component of pattern 4.
• Grade Group 3 = Gleason score 7 (4+3): predominantly pattern 4 with a lesser component of well-formed glands.
• Grade Group 4 = Gleason score 8 (typically 4+4, also 3+5 or 5+3).
• Grade Group 5 = Gleason score 9–10.

The distinction between Grade Group 2 and Grade Group 3 turns on which pattern dominates—both are Gleason 7, but the proportion of pattern 4 differs, and this proportion carries prognostic weight. Reporting the percentage of pattern 4 has therefore become standard practice for intermediate-grade tumors.

Beyond the core grade, the current standard is to report adverse architectural subpatterns separately. Cribriform pattern 4 and intraductal carcinoma of the prostate are called out explicitly because they behave more aggressively than their numeric grade alone would predict [1]. The evidence suggests these features are independently associated with worse outcomes, and the practice of flagging them reflects a broader shift toward capturing biologically meaningful morphology that a single grade number can obscure. Intraductal carcinoma in particular is a marker of aggressive disease and should be documented even when it accompanies invasive cancer.

Why the distinction matters clinically

Grade Group is the single most consequential piece of information a pathologist provides about a prostate cancer. It drives risk stratification and, in turn, informs the category of management a patient may be eligible for—ranging from active surveillance considerations at the lowest grades to definitive local or systemic approaches at higher grades. (These are eligibility categories determined by clinicians in context; grading itself is educational and diagnostic, not a treatment prescription.)

The clinical logic is straightforward. Grade Group 1 disease is, by definition, composed only of discrete glands and has a very favorable natural history—which is precisely why the renaming from "Gleason 6" matters, since it reframes patient and clinician expectations. As the pattern shifts toward fusion, cribriforming, and eventually sheets and necrosis, the probability of extraprostatic extension, recurrence, and progression rises across the groups.

The separate flagging of cribriform and intraductal patterns is where contemporary practice has moved most notably. Two tumors could share a Grade Group yet differ in outcome depending on the presence of these features. Because they are independently adverse [1], omitting them from a report deprives the clinical team of prognostically decisive information. This is a fast-evolving area, and the precise weight given to these features continues to be refined as morphologic and genomic data accumulate.

For trainees, the take-home is that prostate grading rewards disciplined pattern recognition: know the three patterns cold, understand how proportion converts Gleason score into Grade Group, and never let a numerically reassuring grade suppress the reporting of cribriform or intraductal architecture.

References

1. WHO Classification of Tumours Editorial Board. Urinary and Male Genital Tumours, 5th ed. Lyon: IARC; 2022. ISBN 978-92-832-4512-4.

2. Epstein JI, et al. The 2014 ISUP Consensus Conference on Gleason Grading / Grade Groups. Am J Surg Pathol. 2016. doi:10.1097/PAS.0000000000000530.