Background: Intraductal carcinoma of the prostate (IDCP) is a pathological pattern that involves atypical cells proliferating within the normal gland where basal cells are preserved. These exhibit cribriform morphology, a finding associated with poor prognosis. In addition to the TMPRSS2-ERG fusion gene, TP53, RB1, and PTEN deletions, and SPOP and HRR gene mutations, such as a BRCA mutations, are frequently detected in patients with IDCP. The National Comprehensive Cancer Network (NCCN) guidelines suggest that early genetic testing should be considered. However, IDCP recognition is notably low, and diagnosis requires skill. Methods: We selected 98 cases with "prostate cancer with high-grade prostatic intraepithelial neoplasia (PIN) in the background" by referring to the pathology reports of cases who underwent total prostatectomy for prostate cancer at our hospital between November 2011 and November 2021. Twelve cases were narrowed down to those that were considered suggestive of IDCP based on the pathology reports and reviewed by three pathologists to determine the presence of IDCP. These were additionally reviewed by three pathologists for the presence or absence of IDCP and a gene test of prostatectomy specimens was submitted for the relevant cases to determine the presence or absence of HRR gene mutations. Results: Five of the twelve cases were determined to be IDCP positive. The Gleason score of background prostate cancer was 4+3 in 3 cases, 4+4 in 1 case, and 4+5 in 1 case. Median postoperative follow-up was 37 months (22- 45 months); 3 of the 5 patients had no recurrence, 1 had recurrence at 2 years and 3 months resulting in additional local radio therapy (RT), and 1 patient died of other causes. Pathology genetic testing identified HRR-related genetic abnormalities, including BRCA mutations. Conclusions: We were able to establish IDCP cases from past high-grade PIN cases. As a number of HRR gene mutations of unknown pathological significance were detected, IDCP cases could be retrieved from past cases and genetic abnormalities could be identified efficiently. The findings of the study provide a possible approach to diagnose prostate cancer cases with genetic mutations at an early stage.