Treatment planning for the second stereotactic radiosurgery utilizing the previous target reconstructed by image fusion

Abstract

Purpose: The intracranial recurrence of brain metastases often manifests in patients treated with stereotactic radiosurgery (SRS) alone. Although such patients would be retreated with SRS for their new lesions, the precise dose distributions of prior irradiation have been difficult to take into account, and often ignored, in the reirradiation planning. We present a methodology for fusing the volumetric images of the previous and current targets into a 3D image, by which the accumulated doses to the normal structures during repeated SRS can be minimized. Materials and Methods: From October 1997 through February 1999, 13 lesions of recurrent metastatic brain tumors were retreated with SRS. Treatment planning was conducted with the X-Knife 3-D planning system (Radionics) in the following manner: (1) The previous MR and current CT (with stereotactic frame) images were fused; (2) the current MR and current CT images were fused; (3) the bone data were contoured from the current CT images and the internal brain structures were acquired from the current fused images; (4) the previous and current targets were contoured in the respective fused images; (5) the previous targets were transferred into the current fused images; and finally (6) optimum beams were selected by Beam's-eye-View method not to overlap with the previous beams. Results: By fusing the previous and current targets in a 3D image, it has become feasible to carry out safe treatment planning for the second SRS. For all the 13 lesions, volume doses between the current and the previous targets were 10-15% either of the current or the previous prescribed dose. The number of beams and total arc degrees selected for each new target were 3 to 7 (median 5) and 220-440 degrees (median 305 degrees), respectively. Only an extra 15 minutes was needed for this method. Conclusion: Accurate reproduction of the previous dose distribution in the current 3D image could facilitate a safe and effective reiiradiation. We hope that this method will be widely used for the SRS retreatment planning.