Radiation detector probes for tumor localization using tumor-seeking radioactive tracers.

Abstract

Accurate tumor staging requires that all sites of disease be identified. Imaging studies using CT, MA, or sonography may identify abnormal anatomic sites, but these studies may be unable to distinguish between different causes of an abnormality. For example, enlarged lymph nodes may represent metastases or inflammation. Lymph nodes that are not enlarged still may harbor micrometastases, however, and in this circumstance no abnormality will be detected by anatomic imaging studies. Tumor-seeking radiopharmaceuticals have the potential to show neoplasm in both enlarged and normal-sized lymph nodes, but results with external gamma-camera imaging of nodal metastases have been disappointing. An early study of 67Ga-citrate imaging in Hodgkin disease showed that no tumor-bearing lymph nodes smaller than 1 cm in diameter were imaged, and the size most easily detected was 4 cm [1]. More recent imaging studies using radiolabeled antibodies have shown that few deep tumor sites smaller than 1.5-2.0 cm are detected [2]. Several factors impede external gamma-camera detection of small, deep tumors. Most tumor-seeking tracers are not truly tumor specific, and tracer uptake in tumor is seldom more than a few times higher than that in adjacent normal tissue. Soft tissue interposed between gamma camera and tumor acts as both a background source and an attenuating medium. An alternative to external imaging is to bring the detector close to the tumor, where it will intercept a large fraction of the gamma rays emitted by the tumor and where there is less interposed soft tissue to cause attenuation. Radiation-detector probes can be used to detect and localize tumors. The potential for the use of probes with tumor-seeking tracers has been recognized for nearly 50 years. Recent improvements in both detector materials and radiopharmaceuticals have made probes a practical option for use in tumor staging and resection. We provide here a brief review of uses of probes inside the body for tumor detection and of design considerations for such probes.