Improved Sentinel Node Localization in Patients with Newly Diagnosed Melanoma Utilizing a Fully Digital CZT SPECT/CT System Versus a Conventional NaI SPECT/CT System.

328 Purpose: Lymphoscintigraphy with 99mTc Tilmanocept is a proven method of sentinel node localization in patients undergoing sentinel node biopsy for treatment planning. A potential limitation of the technique is the detection of a sentinel lymph node near the site of radiopharmaceutical injection site which may be obscured secondary to high neighboring background activity in the injection site. The purpose of this work is to compare the performance of a fully digital CZT SPECT/CT system (GE Discovery CZT 670 SPECT/CT) to that of a conventional NaI SPECT/CT system (GE Discovery 870DR SPECT/CT) for sentinel lymph node localization with 99mTc Tilmanocept. Methods: Patients underwent Lymphoscintigraphy successively on two SPECT/CT systems; a GE Discovery CZT 670 SPECT/CT with a wide range high resolution (WEHR) collimator and a GE Discovery 870DR SPECT/CT with a low energy high resolution sensitivity (LEHRS) collimator using 99mTc Tilmanocept. The same image acquisition and processing protocols were used on both SPECT/CT systems. Results: In each patient, the sentinel node was not detected on images from the conventional NaI SPECT/CT camera (Discovery NM/CT 870DR). The sentinel node was obscured by blooming due to high background activity from the injection site, where the hot signal from the injection site is smeared out over a larger area, obscuring the weaker signal from the SLN. However, on images from the same patient acquired on the CZT SPECT/CT camera, the sentinel lymph node was well defined, the bloom from the injection site was reduced by the high intrinsic spatial resolution secondary to the one to one coregistration of collimator and detector array inherent to the CZT system as compared to the inherent spatial resolution limitations in a traditional Anger logic dependent system. Conclusions: The purpose of this work is to show how translation of technology can improve patient outcomes in treatment planning for patients undergoing sentinel node biopsy through improved sentinel node localization. In this case, the sensitivity of Lymphoscintigraphy imaging with 99mTc Tilmanocept improved when imaging was performed on a digital SPECT/CT system with solid state CZT pixelated detectors and a registered collimator versus the conventional NaI SPECT/CT system. The Discovery 670 NM/CT CZT utilizes direct conversion detection with Cadmium Zinc Telluride (CdZnTe or CZT) detectors, which directly convert each incident gamma ray photon into an electrical signal. Since each photon is directly converted into an electrical signal, its location and energy are more accurately known, improving point source detection and delineation. Direct conversion on the pixelated detector also eliminates position errors intrinsic to Anger logic XY readout. Each solid state detector represents a single pixel with a size of 2.46 mm2. The detector array is mated with a registered collimator that aligns each collimator hole with a single detector pixel, eliminating light scatter in the scintillator. This provides an intrinsic spatial resolution of 2.46 mm at the face of the collimator. The Discovery NM/CT 870DR is a conventional SPECT/CT camera with detectors containing a large Sodium Iodide (NaI) crystal coupled to 59 PMT’s, with a pixel resolution of 4.3 mm. The 99mTc energy resolution for the CZT is approximately 6.3% FWHM, the 870 is approximately 9.5% FWHM. Both SPECT/CT systems employ a 16 slice CT configuration for attenuation correction.