Localization of malignant melanoma using monoclonal antibodies

Nuclear probes and intraoperative gamma cameras

Gamma probes are now an important, well-established technology in the management of cancer, particularly in the detection of sentinel lymph nodes. Intraoperative sentinel lymph node as well as tumor detection may be improved under some circumstances by the use of beta (negatron or positron), rather than gamma detection, because the very short range (∼ 1 mm or less) of such particulate radiations eliminates the contribution of confounding counts from activity other than in the immediate vicinity of the detector. This has led to the development of intraoperative beta probes. Gamma camera imaging also benefits from short source-to-detector distances and minimal overlying tissue, and intraoperative small field-of-view gamma cameras have therefore been developed as well. Radiation detectors for intraoperative probes can generally be characterized as either scintillation or ionization detectors. Scintillators used in scintillation-detector probes include thallium-doped sodium iodide, thallium- and sodium-doped cesium iodide, and cerium-doped lutecium orthooxysilicate. Alternatives to inorganic scintillators are plastic scintillators, solutions of organic scintillation compounds dissolved in an organic solvent that is subsequently polymerized to form a solid. Their combined high counting efficiency for beta particles and low counting efficiency for 511-keV annihilation γ-rays make plastic scintillators well-suited as intraoperative beta probes in general and positron probes in particular Semiconductors used in ionization-detector probes include cadmium telluride, cadmium zinc telluride, and mercuric iodide. Clinical studies directly comparing scintillation and semiconductor intraoperative probes have not provided a clear choice between scintillation and ionization detector-based probes. The earliest small field-of-view intraoperative gamma camera systems were hand-held devices having fields of view of only 1.5-2.5 cm in diameter that used conventional thallium-doped sodium iodide or sodium-doped cesium iodide scintillation detectors. Later units used 2-dimensional arrays (mosaics) of scintillation crystals connected to a position-sensitive photomultiplier tube and, more recently, semiconductors such as cadmium telluride or cadmium zinc telluride. The main problems with the early units were their very small fields of view and the resulting large number of images required to interrogate the surgical field and the difficulty in holding the device sufficiently still for the duration (up to 1 min) of the image acquisition. More recently, larger field-of-view (up to 5 × 5 cm) devices have developed which are attached to an articulating arm for easy and stable positioning. These systems are nonetheless fully portable and small enough overall to be accommodated in typical surgical suites.

Advances in FDG PET probes in surgical oncology

Whole-body positron emission tomography (WB-PET) has been developed as a sensitive, cost-effective method for imaging malignant disease. WB-PET provides complete body imaging with a single scanning approach for a variety of malignancies. With increasing clinical experience, the indications for its use have broadened. This article reviews current uses of the technology and discusses some potential applications, particularly the utility of a commercially available surgical gamma probe for detecting the gamma particles emitted in the decay process of FDG.

Application of an [(18)F]fluorodeoxyglucose-sensitive probe for the intraoperative detection of malignancy

BACKGROUND

Whole-body positron emission tomography (PET) has been shown to be a highly sensitive method for detecting malignancy not imaged by conventional modalities. We have adapted a hand-held gamma-ray-sensitive probe to detect the radiation emission from the [(18)F]fluorodeoxyglucose (FDG) used in PET imaging. This pilot study was devised to examine the feasibility of using a hand-held probe to intraoperatively differentiate normal from tumor-bearing tissue.

MATERIALS AND METHODS

A commercially available gamma probe was adapted to detect the radioactivity released from FDG and examined to determine the in vitro sensitivity for localization of a FDG point source. Eight consecutive patients underwent resection of metastatic colon cancer or melanoma; each received a preoperative injection of 7--10 mCi of FDG. The gamma probe was used to determine radioactive counts per second from tumor and normal tissue, and ratios of tumor to adjacent normal background were calculated.

RESULTS

In vitro studies with a FDG point source demonstrated the probe could identify the source with a 50% reduction in maximum counts 1.7 +/- 0.1 cm from the source (full-width half-maximum measurement). Based on the results of their preoperative PET scans 17 tumors were identified from the 8 patients. Of the 17 tumors assessed the in vivo tumor-to-background ratios varied from 1.16:1 to 4.67:1 for the melanoma patients (13 tumors) and from 1.19:1 to 7.92:1 for colon cancer patients (4 tumors). Thirteen tumors were resected; four (2 patients) were unresectable.

CONCLUSIONS

This study demonstrates the use of a hand-held gamma-ray-sensitive probe to intraoperatively differentiate the radioactivity released from FDG from tumor-bearing and adjacent normal tissue. While further studies are necessary for us to optimize the use of this probe, the intraoperative detection of FDG-avid malignancies may ultimately improve our ability to completely resect patients with metastatic disease.

Novel antibody drug products

Recent developments in protein and genetic engineering methods have allowed the production of antibody-derived molecules that have important potential as therapeutic agents. Although monoclonal antibodies of murine origin have been used for therapeutic purposes, limitations due to anti-antibody responses and suboptimal effectiveness for some indications, such as tumor cell killing, have led to the development of human monoclonal antibodies, chimeric and complementarity determining-region grafted antibodies, immunotoxins, and other engineered products. These novel antibodies are being tested for the treatment and prevention of infectious diseases and for the diagnosis and treatment of cancers, as well as for indications considered nontraditional for antibodies (e.g., as antithrombotics or inhibitors of neutrophil adherence). The availability of antibody drug products raises a number of issues for clinicians. Among these are new patterns of adverse effects, immunogenicity (development of anti-antibody response), important questions regarding administration and dosage, and substantial cost implications.