Cryosurgical technique: assessment of the fundamental variables using human prostate cancer model systems

Cryosurgery offers a promising therapeutic alternative for the treatment of prostate cancer. While often successful, complete cryoablation of cancerous tissues sometimes fails due to technical challenges. Factors such as the end temperature, cooling rate, duration of the freezing episode, and repetition of the freezing cycle have been reported to influence cryosurgical outcome. Accordingly, we investigated the effects of these variables in an in vitro prostate cancer model. Human prostate cancer PC-3 and LNCaP cultures were exposed to a range of sub-zero temperatures (-5 to -40 degrees C), and cells were thawed followed by return to 37 degrees C. Post-thaw viability was assessed using a variety of fluorescent probes including alamarBlue (metabolic activity), calceinAM (membrane integrity), and propidium iodide (necrosis). Freeze duration following ice nucleation was investigated using single and double freezing cycles (5, 10, and 20 min). The results demonstrated that lower freezing temperatures yielded greater cell death, and that LNCaP cells were more susceptible to freezing than PC-3 cells. At -15 degrees C, PC-3 yielded approximately 55% viability versus approximately 20% viability for LNCaP. Double freezing cycles were found to be more than twice as destructive versus a single freeze-thaw cycle. Both cell types experienced increased cell death when exposed to freezing temperatures for longer durations. When thawing rates were considered, passive (slower) thawing following freezing yielded greater cell death than active (faster) thawing. A 20% difference in viability between passive and active thawing was observed for PC-3 for a 10 min freeze. Finally, the results demonstrate that just reaching -40 degrees C in vitro may not be sufficient to obtain complete cell death. The data support the use of extended freeze times, multiple freeze-thaw cycles, and passive thawing to provide maximum cell destruction.