Intraductal Carcinoma of the Prostate: To Grade or Not to Grade

SIMPLE SUMMARY: Whether intraductal carcinoma of the prostate (IDC-P) should be factored into prostate cancer (PCa) Grade Group (GG) has been debated vigorously. While there is currently no study that directly compares the two grading approaches with IDC-P incorporated in or excluded from GG, evidence to date suggests that IDC-P should generally be graded to avoid undergrading and to minimize the overuse of basal cell stains, since >95% of IDC-P represents a high-grade PCa with retrograde spread, which has aggressive biological behavior and is also molecularly akin to high-grade PCa. The only exception to this rule is when IDC-P is present in association with GG1 PCa or in pure form without concomitant PCa, which is a rare scenario and accounts for ~2% of IDC-P. IDC-P in such context could represent a precursor lesion with a prognosis similar to GG1 PCa when diagnosed in radical prostatectomy, and hence should not be included in GG to avoid overgrading. ABSTRACT: Intraductal carcinoma of the prostate (IDC-P) is a distinct tumor type characterized by an expansile growth of atypical glandular epithelial cells within pre-existing prostate glands and ducts and has significant implications on clinical outcomes and patient management. There is an agreement that isolated IDC-P should not be graded, and IDC-P should be reported with a comment on its clinical significance. However, whether IDC-P should be factored into Grade Group (GG) in the presence of concurrent prostate cancer (PCa) has been debated vigorously. The contradicting opinions were promulgated when the Genitourinary Pathology Society (GUPS) and the International Society of Urological Pathologists (ISUP) published their recommendations for this issue. When IDC-P is present with PCa, the ISUP recommends incorporating it in the GG for the entire case, whereas the GUPS recommends excluding it from the final GG. Consequently, pathologists and clinicians are faced with the conundrum of conflicting recommendations. In this review article, the authors evaluate the magnitude of discrepant GG between the two grading methods, explore the rationales behind the differing views of the two urological societies, present the current reporting practices for IDC-P, and propose a provisional and pragmatic guide to alleviate the dilemma of which recommendation to follow.