Prostate Seed Implants

Much of the thrust of new developments in radiation therapy over the past decade has been aimed at increased conformality, i.e., reduction of the volume receiving high dose to exactly conform with the desired target region containing the tumor and any suspected subclinical disease. Increased conformality of the high dose region directly results in lower dose to nearby normal tissues, thereby reducing the likelihood of treatment related complications. This in turn allows the tumor dose to be further increased in many cases, offering the potential of improved local tumor control without a corresponding increase in probability of complications.

A venerable technique in radiotherapy is brachytherapy, where radioactive sources are directly implanted in or around the tumor itself. By its nature, brachytherapy offers a high degree of conformality, since the dose can in principle be given to the tumor with no entering or exiting beams to irradiate normal tissues. A negative aspect of brachytherapy is that surgery is required to insert the sources, in contrast to external beam radiation therapy. In recent years, developments have occurred which have led to so-called "minimally invasive" procedures for performing many surgical procedures, including some types of brachytherapy. A key aspect of these techniques is the use of medical imaging technologies both for planning of radioactive implants based on detailed three-dimensional anatomical models of an individual patient's anatomy, and image-guided placement of sources in some instances.

One such procedure is ultrasound-guided implantation of radioiodine (I-125) seeds in the prostate. Over the past two years we have implemented the procedures and technology needed to plan and deliver these treatments at three of our clinical sites (Lagrange Memorial, Michael Reese, University of Chicago) and Jaishanker Nautiyal, MD has performed over sixty image-guided implants. Franca Kuchnir, Ph.D. and Jolanta Dabrowski, MS have led the development of quality assurance, logistics, technology and dosimetry standards for dissemination of this technique to the various clinics. Charles Pelizzari, Ph.D. and Danny Spelbring, Ph.D. have developed software for ultrasound image-based treatment planning (computer aided design) and CT-based postimplant evaluation. With support from Picker International, Inc., Dr Pelizzari has developed a new method for precisely identifying the positions of implanted seeds in CT images, aided by three-dimensional computer-generated views of the implant. These positions are required to make accurate calculations of the dose delivered to the prostate and to nearby sensitive normal tissues such as the rectum and bladder. Since there are frequently more than 100 seeds in an implant, and the size of the prostate is typically only 5 cm along its maximum dimension, localization of the individual seed positions from CT slices is a tedious and error-prone task. Using three-dimensional views of the entire implant generated from the CT scan, the seeds can readily be identified and their positions calculated. This allows analysis of the dose distribution actually delivered. In addition, the three-dimensional geometry of the overall pattern of seeds actually achieved can be compared with the ideal distribution as was planned.

Figure 1: Computer-generated views of an Iodine-125 seed implant. Information from approximately 75 CT slices was combined to produce these 3D views.

Figure 2: Interactive localization of seeds from 3D view.

Figure 3: Display of dose distribution calculated from seed positions identified in 3D views.