Thermal Ablation of Canine Prostate Using Interstitial Temperature Self-Regulating Seeds: New Treatment for Prostate Cancer

Abstract

An interstitial temperature self-regulating implantable thermal seed for ablation of the prostate was investigated for the treatment of cancer. The technique is analogous to brachytherapy implants with three important advantages: highly localized temperatures may reduce complications, the thermal seed can be activated for retreatment at any point in time, and the seeds pose no radiation hazard to the clinical staff. Thermal seeds were implanted in the left lobe of the prostates in four dogs; the right lobe was a control to evaluate undesired heating outside the seed array. Linear-array thermometry probes were placed in both lobes, and the induction field for heating the implants was activated for 1 hour. After treatment, biopsies were taken from both lobes at 4-hour intervals up to 28 hours to evaluate thermal damage and thermotolerance as measured by expression of heat shock protein (hsp) 70. Only 5 minutes was required to heat the left lobe to 45° to 55°C. The maximum and minimum cumulative equivalent minutes at 43°C (CEM₄₃) thermal doses in the treated lobe were 174 and 4.4 hours, respectively; less than a 1-minute CEM₄₃ was observed in the control lobe. Elevated hsp70 expression was detected in tissue of the treated lobe between 12 and 24 hours after treatment; minimal increases occurred in the control lobe. The thermal seed system was effective at heating the prostatic volume without damage to normal tissues outside the implant array, and subsequent treatments were simplified in comparison with other hyperthermia devices. Expression of hsp70 implies that retreatment of the prostate at intervals as short as 48 to 72 hours may avoid thermotolerance making weekly treatments an appropriate regimen.