Physiologic Reactions After Proton Beam Therapy in Patients With Prostate Cancer: Significance of Urinary Autoactivation

Purpose Proton therapy is a sophisticated treatment modality for prostate cancer. We investigated how physiologic factors affected the distribution of autoactivation as detected by positron emission tomography (PET) after proton beam therapy. Methods and Materials Autoactivation was evaluated in 59 patients treated with a 210-MeV proton beam. Data acquisition for autoactivation by PET started 5minutes after proton irradiation to assess activation. In the first 29 patients, five regions of interest were evaluated: planning target volume (PTV) center, urinary bladder inside the PTV, urinary bladder outside the PTV, rectum (outside the PTV), and contralateral femoral bone head (outside the PTV). In the remaining 30 patients, urine activity was measured directly. In a phantom study autoactivation and its diffusion after proton beam irradiation were evaluated with water or an ice block. Results Mean activities calculated by use of PET were 629.3Bq in the PTV center, 555.6Bq in the urinary bladder inside the PTV, 332.5Bq in the urinary bladder outside the PTV, 88.4Bq in the rectum, and 23.7Bq in the femoral bone head ( p < 0.001). Mean urine activity was 679.4Bq, recorded 10minutes after therapy completion, and the half-life for urine autoactivation was 4.5minutes. Conclusions Urine is a major diffusion mediator of autoactivation after proton beam therapy. Our results indicate that physiologic factors can influence PET images of autoactivation in the context of proton beam therapy verification.