The dynamics of prostate-specific antigen after definitive radiation therapy for prostate cancer

We report the use of an exponential model for capturing the dynamics of ser ial measurements of prostate-specific antigen (PSA) made just before and af ter definitive radiation therapy of localized prostate cancer. Our study pa tients consisted of 164 men treated at a community hospital and without use of adjuvant hormonal therapy, and we had a mean of 5 years follow-up. We f ound that the model fits allowed us to condense PSA dynamic information int o four parameters, including the initial pretreatment value of PSA, and thr ee of these related significantly to subsequent outcome. The model also pro vided greater understanding of the prognosis of men with rising PSA after r adiation therapy. Specifically, two of the model's parameters allowed us to compare the PSA status of these men to those with hormone-refractory disea se, and we discovered that at the time of "biochemical relapse," there is a broad spectrum in expected probability of imminent death as well as in tim e to an adverse outcome. Thus, the model provides information that allows o ne to stratify men with rising PSA into a continuous spectrum from low to h igh risk for an adverse outcome. We believe these results show that exponen tial models have the potential for providing useful clinical information ab out men with rising PSA after definitive radiation therapy and that they co uld help us decide when further therapy is needed. Therefore, we recommend further study and development of these models as part of clinical research protocols involving radiation therapy of localized prostate cancer.