The next evolution in radioguided surgery: breast cancer related sentinel node localization using a freehandSPECT-mobile gamma camera combination

Abstract

Accurate pre- and intraoperative identification of the sentinel node (SN) forms the basis of the SN biopsy procedure. Gamma tracing technologies such as a gamma probe (GP), a 2D mobile gamma camera (MGC) or 3D freehandSPECT (FHS) can be used to provide the surgeon with radioguidance to the SN(s). We reasoned that integrated use of these technologies results in the generation of a "hybrid" modality that combines the best that the individual radioguidance technologies have to offer. The sensitivity and resolvability of both 2D-MGC and 3D-FHS-MGC were studied in a phantom setup (at various source-detector depths and using varying injection site-to-SN distances), and in ten breast cancer patients scheduled for SN biopsy. Acquired 3D-FHS-MGC images were overlaid with the position of the phantom/patient. This augmented-reality overview image was then used for navigation to the hotspot/SN in virtual-reality using the GP. Obtained results were compared to conventional gamma camera lymphoscintigrams. Resolution of 3D-FHS-MGC allowed identification of the SNs at a minimum injection site (100 MBq)-to-node (1 MBq; 1%) distance of 20 mm, up to a source-detector depth of 36 mm in 2D-MGC and up to 24 mm in 3D-FHS-MGC. A clinically relevant dose of approximately 1 MBq was clearly detectable up to a depth of 60 mm in 2D-MGC and 48 mm in 3D-FHS-MGC. In all ten patients at least one SN was visualized on the lymphoscintigrams with a total of 12 SNs visualized. 3D-FHS-MGC identified 11 of 12 SNs and allowed navigation to all these visualized SNs; in one patient with two axillary SNs located closely to each other (11 mm), 3D-FHS-MGC was not able to distinguish the two SNs. In conclusion, high sensitivity detection of SNs at an injection site-to-node distance of 20 mm-and-up was possible using 3D-FHS-MGC. In patients, 3D-FHS-MGC showed highly reproducible images as compared to the conventional lymphoscintigrams.

Keywords: Sentinel node; breast cancer; freehandSPECT; mobile gamma camera; navigation; nuclear medicine; radioguided surgery.

Figure 1

FreehandSPECT acquisition using the declipseSPECT-mobile gamma camera combination. A. Different components of the 3D-freehandSPECT-mobile gamma camera (3D-FHS-MGC) setup. A patient reference tracker (ReT) is placed on the patient. Additional ReTs are placed on the MGC and on the navigation tool (gamma probe (GP)). The position of the patient ReT as well as that of the MGC and the GP are optically tracked by the navigation system. An overhead camera records video-feed. This allows projection of the acquired 3D-FHS-MGC image on the patient thereby generating an augmented-reality image (part B of the figure). B. Generation of a 3D-FHS-MGC scan consists merely of three steps: I) Placement of the volume of interest (VOI) of the MGC over the sentinel node (SN) area; II) Scanning the VOI in three orthogonal directions; and III) Reconstruction of the acquired data results in the generation of a 3D-FHS-MGC augmented-reality image allowing the surgeon to navigate to the SN.

Figure 2

Examples of resolvable and unresolvable nodes. A. Test setup 1 at 4 mm source-detector depth. Eight out of ten hotspots (white arrows) are clearly separable. B. Test setup 1 at 60 mm source-detector depth. No separable hotspots were seen (black arrows). In the scale bars, a blue color represents a low signal intensity and a red color represents a high signal intensity.

Figure 3

Phantoms. A. Picture and schematic overview of the phantom used for the detection sensitivity and resolution experiments. Diameter of the holes is 5 mm with a hole-hole space of 10 mm. Dimensions of the phantom: 250×170 mm. On the bottom right the reference tracker is drawn. B. Picture and schematic overview of the phantom used for the clustered node experiment. Diameter of the holes is 5 mm with a hole-hole spacing 7 mm. Dimensions of the phantom: 150×40 mm.

Figure 4

Sensitivity of the 3D-freehandSPECT-mobile gamma camera combination compared to 2D-mobile gamma camera and conventional 2D-flatbed imaging. A. Schematic representation of test setup 1 with all dilutions placed within the VOI. B. Schematic representation of test setup 2 in which only the five highest dilutions were placed in the VOI. C. Schematic representation of test setup 3 in which the five highest dilutions were placed at a respective distance from each other. For all the setups shown: I) Schematic overview of the setup; II) Conventional 2D-flatbed imaging of the various setups at a source-detector depth of 60 mm (acquisition time: five min); III) Corresponding 3D-freehandSPECT-mobile gamma camera (3D-FHS-MGC) image at 60 mm Perspex depth; and IV) Resulting sensitivity graphs. The green line represents 2D-MGC imaging. 2D-flatbed imaging is represented by the blue line and 3D-FHS-MGC by the red line. Note to setup 3: To make sure all sources fitted inside the VOI during 3D-FHS-MGC scan acquisition the 1:128 source had to be moved so that it was in the centre of the other four sources, so the source seen in the middle in CIII is actually the 1:128 source.

Figure 5

Sentinel node resolvability. A. Schematically representation of the phantom setup. The injection site was simulated using 4×25 MBq ^99mTc-pertechenetate sources placed in a diamond. The sentinel node (SN) was simulated with a 1 MBq ^99mTc-pertechenetate source. Injection site-to-SN distance was varied in steps of 10 mm (range 10-80 mm). B. 2D-flatbed images at a source-detector depth of 60 mm (acquisition time: five min). SNs were placed at 20, 40, 60 and 80 mm injection-site to SN distance at 60 mm depth (upper panel) and 10, 30, 50 and 70 mm at 12 mm depth (lower panel). C. SN resolvability for SNs at various injection site-to-SN distances and various source-detector depths for 2D-flatbed imaging (blue), 2D-MGC imaging (green), and 3D-FHS-MGC (red).

Figure 6

Cluster node resolvability. A. Four ^99mTc-pertechnetate sources of 0.5 MBq placed at 10 mm center-to-center distance from each other. B. Four ^99mTc-pertechnetate sources of 1 MBq placed at 10 mm center-to-center distance from each other. C. Four ^99mTc-pertechnetate sources of 25 MBq placed 10 mm center-to-center distance from each other. The images show: I) 2D-flatbed imaging; II) 2D-mobile gamma camera (MGC) image; III) 3D-freehandSPECT-mobile gamma camera (3D-FHS-MGC) augmented-reality overlay; and IV) virtual-reality navigation to the hotspots. 2D-flatbed, 2D-MGC and 3D-FHS-MGC were only able to separately visualize (and navigate) the 4×25 MBq ^99mTc-pertechentate sources. In the scale bars, a blue color represents a low signal intensity and a red color represents a high signal intensity.

Figure 7

Clinical sentinel node resolvability. A. Patient presenting with a solitary SN in the axilla. I) 2D-flatbed image showing a clear SN. II) 3D-FHS-MGC overlay showing the same SN in the axilla. III) 3D navigation pointing towards the SN in the axilla. IV) Localization of the SN in the axilla. B. Patient with two axillary SNs located 11 mm apart from each other. I) 2D-flatbed imaging showing an intense and a weak hotspot in the axilla. II) 3D-FHS-MGC overlay shows one elongated hotspot. III) 3D navigation shows the same elongated hotspot. IV) Localization points to a spot between the two spots marked by the nuclear medicine physician. C. Patient presenting with an intra-mammary and an axillary SN located at 33mm from each other. I) 2D-flatbed imaging clearly identified two SNs. II) 3D-FHS-MGC overlay also shows two SNs. III) 3D navigation points towards both SNs separately. IV) Successful localization of both SNs. D. Patient with a cluster of 2-4SNs in the axilla. I) 2D-flatbed image showing one hotspot. II) 3D-FHS-MGC shows a hotspot with a protrusion to one side indicating a cluster of SNs. III) 3D navigation shows the protrusion more clearly. IV) Localization points towards the center of the hotspot. In the scale bars, a blue color represents a low signal intensity and a white color represents a high signal intensity.

Figure 8

Improved reconstruction algorithms help improve the data that is shown in the 3D-freehandSPECT-mobile gamma camera image. The images show an example of test setup 2 of the sensitivity experiment at a source-detector depth of 24 mm. A. Summation of volume data in z-direction when a linear transfer function is used. This data is currently displayed by the declipseSPECT system after reconstruction of the 3D-freehandSPECT-mobile gamma camera (3D-FHS-MGC) scan. B. Summation of volume data in z-direction when a logarithmic transfer function is used. Clearly in this setting more information is displayed than in A. C. A single volume slice from test setup 2. In Osirix medical imaging software (Pixmeo) the logarithmic transfer function is shown with the window leveling set to display all hotspots. In the scale bars, a blue color represents a low signal intensity and a red color represents a high signal intensity.