PSA as a predictor of small cell transformation in prostatic adenocarcinoma: A SEER 2000-2021 study.

e17006 Background: Prostate cancer is the most commonly diagnosed cancer in men, affecting 1 in 8 men in the United States in their lifetime. Adenocarcinoma is the most common subtype of prostate cancer representing > 95% of cases with a 5 year overall survival with localized or regional disease rate of nearly 100%. Small cell carcinoma (SCC) of the prostate is a rare though aggressive subtype with a poor prognosis – representing < 1 % of patients with prostate cancer. Approximately 40-50% of men with prostatic SCC have a history of conventional prostatic adenocarcinoma, and in contrast to prostatic adenocarcinoma, SCCs are usually not responsive to androgen deprivation therapy nor is disease progression associated with rises in serum PSA. Previous analysis of small cell carcinoma of the prostate using the Surveillance, Epidemiology, and End Results (SEER) database between the years 2004-2015 has been reported, with 5-year observed survival rate of 12.5%. Methods: The SEER database was used to identify 1,145,272 patients diagnosed with prostatic adenocarcinoma diagnosed between 2000 and 2021 in the U.S. Patients with adenocarcinoma and small cell carcinoma of the prostate were identified using the ICD-0-3 primary site code C619 and histologic subtype codes of 814, 8041, 8042, 8043, 8044. 70 patients were identified with adenocarcinoma and matched by patient ID with a subsequent diagnosis of small cell carcinoma of the prostate. PSA and Gleason scores available at the time of diagnosis were compared as well as treatment course (i.e. resection vs radiation vs chemotherapy) with relative risk of transformation. Results: Prostate adenocarcinoma with transformation into small cell carcinoma had higher median PSA when compared to prostate adenocarcinoma without transformation (21.9 vs 7.1) and higher median Gleason score (9 vs 7) at the time of diagnosis. Median survival was lower at 41 months compared to 86 months without transformation. Patients with transformation of adenocarcinoma to SCC were more likely to have a PSA of 7 or greater and Gleason score of 8 or greater compared to without transformation. Patients with transformation of adenocarcinoma to SCC were more likely to receive chemotherapy though less likely to receive radiation or resection. Conclusions: Patients with transformation of prostatic adenocarcinoma to small cell carcinoma of the prostate were more likely to have significantly elevated PSA levels of at least 7 (RR = 2.45, p < 0.05) and higher Gleason scores of at least 8 (RR = 6.00, p < 0.05) at the time of transformation.