VAIL, CO USA ( - Objectives: Stereotactic Body Radiotherapy (SBRT) has emerged as an effective treatment for localized prostate cancer.

However, PSA kinetics after prostate SBRT have not been well characterized. The purpose of this study was to analyze the trend in PSA decline following robotic SBRT from a prospective cohort of patients.

24th cap updateMaterials and Methods: One hundred seventy five patients were treated definitively for localized prostate cancer to a dose of 35 to 36.25 Gy in five fractions prescribed to the prostate and proximal seminal vesicles using the CyberKnife Radiosurgical System (Accuray). Patients who received androgen suppression were excluded from this study. PSA and testosterone were collected prior to the start of treatment and then every 3 months for the first year, followed by every 6 months for the next 4 years. Biochemical failure was defined according to the ASTRO Phoenix definition of a 2 ng/mL or greater rise in PSA above nadir. Benign PSA bounce was defined as a 0.2 ng/mL rise in PSA that subsequently returned to or below previous nadir. A threshold value of 0.5 ng/mL was chosen as a PSA nadir cutoff based on previous studies demonstrating its long-­‐term prognostic value after definitive radiotherapy. Multivariate logistic regression was used to assess the effect of disease specific covariates on the likelihood of achieving a PSA nadir less than threshold. PSA kinetics were analyzed using nonlinear least squares regression and a two component exponential model accounting for fast and slow PSA responses. Statistical group comparisons were made using the Wilcoxon rank-­‐sum test.

Results: At a median follow up of 3 years, 12 patients met the ASTRO Phoenix definition of biochemical failure. Of these 12 patients, 6 patient’s PSA have returned to or below previous nadir without intervention. Seventy percent of patients have achieved a PSA nadir below 0.5 ng/mL with a median PSA nadir of 0.2 ng/mL vs. 0.7 ng/mL (p < 0.001), at a median time to nadir of 30 months. Among the patients included in our analysis, 39% experienced a benign PSA bounce. Multivariate analysis revealed absence of PSA bounce, initial PSA, and testosterone at the time of nadir to be significant predictors (p < 0.05) of achieving a PSA nadir below threshold. On average, PSA kinetics after prostate SBRT were well described (rcorr = 0.99) with a two component exponential model with fitted half-­‐ lives of 1.26 and 16.5 months for 62% and 38% of the initial PSA, respectively.

Conclusions: The majority of patients treated with robotic SBRT for localized prostate cancer achieve a PSA nadir less than 0.5 ng/mL. Benign PSA bounces are common and a significant proportion of biochemical failures will return to nadir with continued follow up. Modeling of PSA kinetics suggests that these patients experience an initial period of rapid PSA decline followed by a slow decline which continues at a median follow up of 3 years. This will likely result in lower PSA nadirs after longer follow up. The long-­‐term prostate cancer specific and metastasis free survival impacts of these results remain to be determined.

Presented by Thomas P. Kole, Leonard N. Chen, Joy S. Kim, Rudy Moures, Thomas Young, Siyuan Lei, Simeng Suy, Anatoly Dritschilo, John H. Lynch, and Sean P. Collins at the 24th International Prostate Cancer Update - February 19 - 22, 2014 - Cascade Conference Center - Vail, Colorado USA

Georgetown University School of Medicine, Washington, DC USA



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