NK3 homeobox 1 (NKX3.1) has been increasingly used to diagnose metastatic prostatic carcinoma in histologic samples. However, its utility and reliability in cytologic direct smears have not been studied.
A total of 59 fine-needle aspiration (FNA) cases with a definitive diagnosis of metastatic carcinoma from the prostate were included. The cases were grouped based on different Gleason score in their corresponding primary tumors and morphologic variants. For each case, tumor cells were immunostained with NKX3.1, prostate-specific antigen (PSA), and prostatic acid phosphatase (PAP) on cell-transferred smears.
NKX3.1 was strongly and diffusely positive in all 40 metastatic prostatic adenocarcinomas, including those with ductal features, but negative for the 19 small cell carcinoma (SmCC) cases. NKX3.1 had a better detection rate than PSA (13/50, 26%) and PAP (0/47, 0%).
NKX3.1 immunostaining on FNA smears is highly reliable for detecting metastatic prostatic carcinomas of conventional and ductal types but not for SmCC.