Atypical Small Acinar Proliferation

Atypical small acinar proliferation is a pathology descriptor used when a focus on prostate needle biopsy is suspicious for adenocarcinoma but cannot be diagnosed as cancer because the atypical glands are few, small or poorly preserved.

It is reported in about 2 to 5% of prostate biopsies, with a wider reported range of 1.5 to 9%. It is not a premalignant lesion but a marker of diagnostic uncertainty, and the main clinical risk is cancer in the adjacent unsampled tissue.

On repeat biopsy, cancer is found in about 30 to 40% overall in many series (reported range roughly 17 to 70%), more often when MRI shows a high suspicion lesion at the site.

Pathophysiology. The atypical glands show small acini with nuclear enlargement or prominent nucleoli but lack a full set of cancer features. Basal cells are absent or patchy on immunostaining in the atypical glands, which favours cancer, but the focus is too small for a firm call.

Benign mimics include adenosis, partial atrophy, post-atrophic hyperplasia, seminal vesicle or ejaculatory duct epithelium and Cowper gland tissue.

The finding is asymptomatic and arises incidentally in men biopsied for a raised PSA, abnormal digital rectal examination or suspicious MRI. The relevant clinical data are age, PSA, PSA density, family history, previous biopsy results, prostate volume and any use of a 5-alpha-reductase inhibitor.

• Request pathology review: deeper levels, immunostaining and a second uropathology opinion. Immunostains: basal cell markers (high-molecular-weight cytokeratin, p63) absent in the focus support cancer, and alpha-methylacyl-CoA racemase is positive in most cancers but also in some benign mimics, so it supports but never proves cancer.
• Multiparametric MRI before repeat sampling, scored on a 1 to 5 suspicion scale.
• Repeat biopsy within 3 to 6 months, with targeted cores to any MRI lesion plus systematic sampling, concentrating extra cores on the site of the atypical focus and the contralateral lobe.
• Adjunct tests for equivocal cases: prostate health index, kallikrein panel, urinary mRNA assays, or an epigenetic assay on the original biopsy tissue. They refine risk but do not replace repeat sampling.
• Measure PSA again, doubling the value for men on a 5-alpha-reductase inhibitor for over 6 months.

Differential diagnosis.

• Prostatic adenocarcinoma
• Adenosis
• Partial atrophy and post-atrophic hyperplasia
• High grade intraepithelial neoplasia
• Basal cell hyperplasia
• Seminal vesicle, ejaculatory duct or Cowper gland tissue

• The finding itself needs no treatment. Androgen deprivation, 5-alpha-reductase inhibitors and radiotherapy are not indicated.
• Repeat biopsy within 3 to 6 months with MRI-targeted and systematic cores is the central step. If cancer is found, manage by risk group, and the atypical focus in the original core does not change that risk assessment.
• If the repeat biopsy and MRI are negative and PSA is stable, offer PSA with digital rectal examination every 6 months for 2 years, then yearly, with repeat MRI at 12 to 24 months or sooner if PSA rises or the examination changes. This schedule reflects usual practice rather than fixed evidence.
• If the repeat biopsy again shows atypical small acinar proliferation, repeat MRI and consider saturation sampling or a transperineal template approach.

Monitoring. PSA, PSA density and examination as scheduled. A PSA rise or new MRI lesion triggers early repeat biopsy.

Referral and escalation. Urology referral with uropathology review. Urgent referral if PSA rises quickly, a nodule is felt, or MRI shows a high suspicion lesion.