57 Study of the camera’s limit of detectability with Iodine-131

Abstract

Introduction In differentiated thyroid cancer, scintigraphic images are performed after therapeutic administration of iodine-131. Several courses are proposed in case of thyroid markers arise due to thyroid’s cell regrowth or metastatic disease appearance. As disease evolves, in some cases, thyroid cells no longer absorb iodine-131 and thus, remain unseen on scintigraphic images. This situation leads to the end of iodine-131 treatments for the patient. Nevertheless, lack of detection does not equally correspond to absence of thyroid tissues or metastasis. In this context, we have evaluated the limit of detectability of our gamma-camera. Methods Gate (v7) Monte-Carlo code was used to model our gamma-camera ( Discovery NM / CT 690, GEHC). Acquired and modelled spatial resolution, sensitivity and contrast (SR, S, C) were compared. Collimator Angular response function (ARF) was used to speed simulations up and get images of a cylindrical object in different geometrical conditions close to clinical settings (radioactive concentration, tumor’s volume and depth) used to conduct a Receiver Operating Characteristic (ROC) study with several physicians. We have also studied different models of automatic observers in order to get closest results as possible to human observer ROC study. Results Simulated and acquired images have identical SR, S and C characteristics. Simulations were 20 times faster with ARF table than without. ROC/LROC human study shows an important variability in detection performances. Some automatic observers such as “2–Alternative Forced Choice” method present close performances to human observers whereas others like CHO outperformed them. Those different results allow us to derive an empirical law, relating the minimal radioactive concentration to detect a lesion as function of its depth and size. Regardless of lesion depth, a radioactive concentration of 25 kBq/mL is necessary to detect all lesions superior to 15 mm. Conclusions We performed a ROC/LROC study on simulated images to evaluate our camera detectability limit and we confirmed that lesions might stay unseen on scintigraphic images as long as radioactive concentration is below some threshold.