Computerized planning of prostate cryosurgery with pullback operation.

This study is a part of an ongoing project to develop computerized tools for cryosurgery planning. Prostate cryosurgery is frequently performed as a two-stage process in which the first-stage targets are frozen with the cryoprobes at a greater insertion depth, while the second stage of freezing is performed after some cryoprobes are pulled back to new locations, this process being known as the pullback procedure. This paper proposes a method to generate computationally a preferred cryoprobe layout for the pullback operation, using a previously proposed bubble-packing method for cryosurgery planning. For this purpose, additional constraints are imposed to align packed bubbles along the same insertion path between the different stages, and to vary the number of bubbles at each stage. The method is demonstrated on 3D patient models from four individuals, using cryoprobes with a diameter of 1.5 mm and an active length of 15 mm. Results are verified with bioheat transfer simulations, and compared with the non-pullback operations using cryoprobes with longer (25 mm) active lengths. When compared with results of the non-pullback procedure, results with pullback suggest some improvement in freezing in the apex region of the prostate, with a slight increase in overall freezing damage to surrounding healthy tissues.