title: Prostate specific antigen kinetics following robotic stereotactic body radiotherapy for localized prostate cancer.
abstract: BACKGROUND
  Stereotactic body radiotherapy (SBRT) has emerged as an effective treatment for localized prostate cancer. However, prostate specific antigen (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.
  
  
  METHODS
  In total 175 patients were treated definitively for localized prostate cancer to a dose of 35-36.25 Gy in 5 fractions using robotic SBRT in the absence of androgen deprivation therapy (ADT). PSA and testosterone were collected at regular intervals following treatment and patients were assessed for biochemical failure and benign PSA bounce. A PSA nadir threshold of 0.5 ng/ml was used as a predictor of long-term disease-free survival. 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 a multi-component exponential model accounting for benign and malignant sources of PSA.
  
  
  CONCLUSIONS
  At a median follow-up of 3 years, 70% of patients achieved a PSA nadir below 0.5 ng/ml with a median PSA nadir of 0.3 ng/ml at a median time to nadir of 30 months. In our cohort, 36.2% experienced a benign PSA bounce. Absence of PSA bounce, initial PSA, and testosterone at the time of nadir proved to be significant predictors of achieving a PSA nadir below threshold. PSA kinetics after prostate SBRT were well described with a functional volume model with fitted half-lives of 4.4 and 14.8 months for malignant and benign sources of PSA, respectively. Patients treated with prostate SBRT experience an initial period of rapid PSA decline followed by a slow decline which will likely result in lower PSA nadirs after longer follow-up. The long-term disease specific impacts of these results remain to be determined.
authors.affiliation: Department of Radiation Medicine, Georgetown University Hospital , Washington, D.C. , USA.
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Kole
authors.forename: Leonard N
authors.initials: LN
authors.lastname: Chen
authors.forename: Olusola
authors.initials: O
authors.lastname: Obayomi-Davies
authors.forename: Joy S
authors.initials: JS
authors.lastname: Kim
authors.forename: Siyuan
authors.initials: S
authors.lastname: Lei
authors.forename: Simeng
authors.initials: S
authors.lastname: Suy
authors.forename: Anatoly
authors.initials: A
authors.lastname: Dritschilo
authors.forename: Sean P
authors.initials: SP
authors.lastname: Collins
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D018572:Disease-Free Survival; D005500:Follow-Up Studies; D006801:Humans; D007700:Kinetics; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D017430:Prostate-Specific Antigen / Q000097:blood*; D011471:Prostatic Neoplasms / Q000097:blood* / Q000473:pathology / Q000601:surgery*; D016634:Radiosurgery / Q000379:methods*; D065287:Robotic Surgical Procedures; D013739:Testosterone / Q000097:blood; D013997:Time Factors
chemical_list: D013739:Testosterone; D017430:Prostate-Specific Antigen