CERENKOV LUMINESCENCE IMAGING IN PULMONARY AND HEPATIC METASTASECTOMY

Introduction Cerenkov luminescence imaging (CLI) is an optical imaging modality to detect distributions of radiopharmaceuticals. CLI can be used to visualize surgical margins immediately after resection and to refine surgery in a single procedure [1]. We are planning a clinical study to evaluate the impact of CLI during surgery of lung and liver metastasis from various primary tumors with respect to conventional post-operative histology, and we are performing in-vitro simulation measurements to optimize the clinical protocol in terms of patient inclusion criteria, activity to inject, radiation monitoring. Methods We analyzed PET/CT data of 15 patients performed with [18F]-FDG in pulmonary and hepatic metastases and with 68Ga-DOTATOC in neuroendocrine tumors (NETs) to determine typical injected activities, lesion volumes, uptakes and time delays between injection and imaging. We are collecting data for typical histological margins. Since the Cerenkov signal depends on the spectrum of the beta particles and on the optical properties of the tissue, we prepared phantoms to measure the minimum detectable activity as a function of the type of radiopharmaceutical, the type of tissue and the source depth in tissue. The phantoms were imaged with a LightPath system with acquisition time acceptable for clinical needs. We used two short-pass filters to discriminate the depth of origin of the detected light. Results/Discussion Patient data are summarized in Figure 1. The delay between injection and imaging was ~1 hour. For our clinical study, we expect delays up to 4-5 hours between injection and CLI. The decay-corrected mean uptake values to account for this delay (5-11 kBq/cc) are comparable to the minimum detectable activity level of 8 kBq/cc that we measured for [18F]-FDG [2]. For 68Ga-DOTATOC, the final uptake of 4-7 kBq/cc should be well detectable, because our first tests suggest a 13x signal increase with respect to 18F, but an enhancement up to 22x can be expected [3]. Figure 2a shows a representative phantom image. The attenuation of 68Ga signal in the various animal liver samples is shown in Fig. 2b. Independent data-sets for the same type of tissue suggest good reproducibility. We are finalizing the data analysis to determine the target-to-background ratio in both the patient and phantom data. Conclusions Patient data suggests that CLI can be performed with standard clinical activities and 5-minute exposure times. The typical lesion volumes are suitable for LightPath imaging. Phantom data for signal attenuation in biological tissue show good reproducibility. We are collecting additional data for lung phantoms, and we are studying the target-to-background ratio for 18F and 68Ga and a method to extract the source depth from the spectral images.