A 68-year-old man from Delhi presents with lower urinary tract symptoms. Transrectal ultrasound-guided biopsy shows a Gleason score 3+3 adenocarcinoma in one core, while another core shows atypical small acinar proliferation (ASAP). Which feature best distinguishes ASAP from Gleason 3+3 adenocarcinoma?

## ASAP vs. Gleason 3+3 Adenocarcinoma: Diagnostic Distinction ### Clinical Context **Key Point:** ASAP (Atypical Small Acinar Proliferation) is a diagnostic category that represents a borderline lesion with uncertain malignant potential. It requires careful morphological and immunohistochemical assessment to distinguish from low-grade adenocarcinoma. ### Comparative Morphology | Feature | ASAP | Gleason 3+3 Adenocarcinoma | |---------|------|---------------------------| | **Nuclear size** | Mildly enlarged, but not clearly abnormal | Distinctly enlarged, hyperchromatic | | **Nucleoli** | Absent or inconspicuous | Prominent, visible at low power | | **Chromatin pattern** | Fine, normal | Coarse, irregular | | **Basal cell layer** | Usually present but may be focally absent | Absent or severely disrupted | | **Architectural pattern** | Small, crowded acini; may mimic benign | Infiltrative glandular pattern | | **Cytologic atypia** | Minimal to mild | Moderate to marked | ### Key Discriminating Feature **High-Yield:** The presence of **prominent nucleoli and increased nuclear size** is the single best morphological discriminator. In ASAP, nucleoli are absent or inconspicuous, whereas in Gleason 3+3 adenocarcinoma, nucleoli are readily visible and nuclear enlargement is evident. ### Diagnostic Algorithm ```mermaid flowchart TD A[Small acinar proliferation on biopsy]:::outcome --> B{Nucleoli prominent?}:::decision B -->|Yes| C{Basal cells lost?}:::decision B -->|No| D[Likely ASAP]:::outcome C -->|Yes| E[Adenocarcinoma - Gleason 3+3]:::action C -->|Uncertain| F[IHC: p63/CK34βE12]:::action F --> G{Basal layer intact?}:::decision G -->|Yes| H[ASAP - Recommend rebiopsy]:::outcome G -->|No| I[Adenocarcinoma confirmed]:::outcome ``` ### Clinical Pearl **Clinical Pearl:** ASAP is a provisional diagnosis that warrants close follow-up and often repeat biopsy in 6–12 weeks, as approximately 40–50% of ASAP lesions are upgraded to adenocarcinoma on subsequent sampling. The absence of prominent nucleoli helps clinically reassure that this is not yet frankly malignant. ### Immunohistochemical Confirmation When morphology is ambiguous: - **p63 and CK34βE12 staining:** Intact ring in ASAP; absent/fragmented in adenocarcinoma - **High-molecular-weight cytokeratin (HMWCK):** Preserved in ASAP; lost in cancer - **Alpha-methylacyl-CoA racemase (AMACR):** Negative in ASAP; positive in adenocarcinoma **Mnemonic:** **NUCA** — **N**ucleoli **U**nconspicuous = **CA**ncer absent (ASAP); **N**ucleoli **U**biquitous = **CA**ncer present (adenocarcinoma). [cite:Robbins 10e Ch 20]