Evaluation of the subtraction method utilizing posterior view for improving visibility of the sentinel lymph nodes for lymphoscintigraphy in breast cancer

Abstract

Objective: This study aimed to evaluate the usefulness of the subtraction method for improving sentinel lymph node (SLN) visibility by reducing scattering near the injection site.Methods: Images of two phantoms for the injection site and SLNs built using an original design were simultaneously acquired using a dual-head camera equipped with a low-energy high-resolution (LEHR) on the lower detector (posterior view) and a low-energy general-purpose (LEGP) collimator on the upper detector (anterior view). Subtraction method images were created by subtracting the posterior view from the anterior view, the latter of which was designated as the conventional method. Image contrast was calculated from the counts of regions of interest (ROI) placed on the two phantoms of the injection site and SLNs. SLNs visibility to a distance from the injection site and a radioactivity ratio based on the injection site (15 MBq) was evaluated by image contrast and visual interpretation.Results: The best improvement in contrast occurred at a distance of 20 pixels (1.08 mm/pixel) from the injection site, and improved further as the lymph node radioactivity is smaller. The SLNs visibility corresponding to a distance of 20 pixels improved significantly (p < 0.001), from 1/2560 of radioactivity at the injection site (approximately 6 kBq) to 1/640 (approximately 23 kBq), and SLN was only detectable using the subtraction method. SLN (1/5120, approximately 3 kBq) was difficult to detect even with the subtraction method, whereas SLN with a ratio ≥1/320 (approximately 46 kBq) was easily detected even with the conventional method. These visibilities did not differ significantly between the two methods (p = 0.16 and >0.32, respectively). The subtraction method could detect SLNs near the tumor on clinical images.Conclusions: The subtraction method improved SLN visibility near the injection site by reducing scattering from the injection site. Furthermore, an advantage of the subtraction method is that it does not require additional imaging because the posterior view is obtained simultaneously and utilized.