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Clinical Investigations |
1 Department of Nuclear Medicine, Tel-Aviv Sourasky Medical Center, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel
2 Department of Otolaryngology and Head and Neck Surgery, Tel-Aviv Sourasky Medical Center, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel
3 Department of Plastic and Reconstructive Surgery, Tel-Aviv Sourasky Medical Center, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel
4 Radio-Isotope Guided Surgery Unit of Surgery A, Tel-Aviv Sourasky Medical Center, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel
| ABSTRACT |
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Key Words: malignant melanoma head and neck tumor lymphoscintigraphy sentinel node fusion
| INTRODUCTION |
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SN mapping has been suggested in the management of various human malignancies, including cutaneous malignant melanoma, squamous cell carcinoma (SCC) of the head and neck region, vulvar and penile malignancies, and breast cancer (2,68). At surgery, the search for SNs may be performed either by blue dye injection coloring of the nodes or by injection of labeled particles before surgery and by the use of a hand-held probe during surgery to locate the nodes with the highest counts. The importance of scintigraphic SN mapping before surgery is enhanced in tumors located in body parts with ambiguous lymph node drainage, such as the trunk, shoulder, and the head and neck. Drainage from these regions is unpredictable and is individual for each tumor (9,10). Presurgical lymphoscintigraphic SN mapping may, therefore, assist the surgeon in tailoring the field of surgery and provide guidance at the incision site (2,11,12).
Previous reports have discussed the technical details required for accurate scintigraphic SN mapping. These include the use of various radiopharmaceuticals, acquisition protocols, markers, and transmission images with a 57Co flood source to facilitate localization of lymphatic nodes and also skin marking to reduce operating time for SN identification and minimize the surgical incision (1,3,13-18). The aim of this study was to assess the potential role of fusion of the SPECT lymphoscintigraphy data with CT in the clinical setting of scintigraphic SN mapping using a hybrid gamma-camera/low-dose CT system that allows SPECT and CT to be performed at the same setting without changing the patients positioning (19).
| MATERIALS AND METHODS |
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Scintigraphic Technique
Seventy-four MBq (2 mCi) 99mTc-rhenium colloid (TCK-17, CIS International) divided into 4 equal aliquots of 0.4 mL each were injected at the borders of the primary tumor site using 25-gauge insulin syringes. If the primary tumor had been removed, the injections were performed on either side of the excisional scar. In the case of melanoma, injection was intradermal whereas, in the case of SCC of the mouth and tongue, the injection was submucosal. The rhenium colloid preparation has a particle size of 50100 nm, larger than the 99mTc-antimony trisulfide colloid and 99mTc-sulfur colloid particles. The rhenium colloid product also contains gelatin. Therefore, it was characterized by a slow migration from the injection site as well as a prolonged accumulation in the SNs. The lymph node level shows binding of 3.06% ± 0.10% of the injected dose at the first hour and 3.83% ± 0.16% at the third hour (manufacturers information).
Lymphoscintigraphy was performed 1 d before surgery, and the injected doses at the time of scintigraphy were larger than with other radiopharmaceuticals to allow a probe reading 1 d later.
Planar imagesincluding anterior, lateral, posterior, and, occasionally, oblique projectionswere obtained within minutes after injection and continued until the SNs were identified, up to 24 h after injection if necessary. Most SNs are detected 13 h after injection. The more delayed images also include areas remote from the primary lesion. A 57Co flood source was placed between the patient and the camera to define the body contour. A SPECT/CT emission/transmission study was performed using a hybrid system composed of a dual-head gamma camera with a low-dose x-ray tube installed in its gantry (Discovery VH consisting of a VG8 gamma camera and a Hawkeye x-ray system [General Electric Medical Systems]). This system allows both transmission and emission acquisitions to be performed without changing the patients position, and fused images overlying the transmission (CT) and emission (SPECT) data are generated. SPECT acquisition parameters for SN detection include a matrix size of 128 x 128, 180° in the anterior L-mode rotation, and a 3° angle step with a 20- to 25-s time frame. The CT detector consists of 384 crystals and photodiodes mounted on the gantry rotating module. For transmission of data, a "half-scan" acquisition was performed over 220°, 16 s for each transaxial slice. Multiple slices are obtained by moving the table by a slice step before acquiring the next slice. The full field of view consisting of 40 slices is completed in 10 min. Transmission data of the patient are corrected and reconstructed using a filtered backprojection to produce cross-sectional attenuation images in which each pixel represents the attenuation of the imaged tissue. Fusion of the SPECT and CT images is performed on the eNTEGRA workstation (General Electric Medical Systems). On the basis of the scintigraphic findings on both planar and SPECT/CT images, the skin is marked to guide the surgeon to the location of the SNs.
Scintigraphic Interpretation
SN localization was interpreted separately on planar images with and without a 57Co flood source and on fused SPECT/CT images. The location of lymph nodes of the neck was categorized as submental, submandibular, preauricular, jugular chain, occipital, posterior cervical, retroauricular, jugulodigastric, or supraclavicular. Nodes in the axilla nodes were categorized as anterior (pectoral), central, or posterior (prescapular) (20,21).
In the analysis of the results, fused SPECT/CT images data were concluded to be clinically relevant to SNB if it identified SNs that were missed on planar images, if it excluded a SN suspected on planar images, or if it localized the SNs in additional or different basins than those suggested by planar images.
SN Identification at Surgery
The surgeon is notified of the findings on both the planar and SPECT/CT images. The patient is taken to the operating room the next day after lymphoscintigraphy. Skin marking done at the Department of Nuclear Medicine are the first guide to the supposed location of an SN. An intraoperative hand-held probe (Neoprobe 2000; Neoprobe Corp.) is used before incision to identify the site with the highest counts along the lymphatic basin. The direction of incision is chosen, keeping in mind that a lymph node dissection might be indicated. A patent blue dye (2.5% in aqueous solution containing 0.6% sodium chloride and 0.05% disodium hydrogen phosphate; Laboratoire Guerbet) is injected similarly to the earlier colloid injection. Dye is injected immediately before surgery because the dye fades quickly after injection. At this point, a blue duct or even a blue node can direct the probe in identifying the SN and assist in its dissection. After excision of the node, ex vivo counting is performed followed by probing of the lymphatic basin in search of additional nodes with high counts. Dissection is considered complete only after all "hot" nodes have been removed.
| RESULTS |
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In 9 of the study patients, SPECT/CT fused images identified SNs that were missed on planar images, including 3 nodes that were located close to the injection site and were hidden by its scattered radiation (Fig. 1) and 2 in-transit nodes. The other 4 nodes that had been identified only on fused images were located in basins other than or in addition to the basins identified on planar images (Fig. 2). In 2 patients with head and neck tumor, the SN identified only on fused images was involved with the tumor.
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| DISCUSSION |
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Preoperative lymphoscintigraphy provides the road map for the surgeon by defining the drainage basins and the SN localization of an individual tumor site. This preoperative mapping is particularly important if the tumor is located in body regions with ambiguous lymph node drainage, such as the head and neck and the trunk. Lymphatic drainage in the head and neck region is especially complex. There are >350 lymph nodes and a rich lymphatic network as well as a wide variability in the patterns of lymphatic drainage (27). In a study conducted on 30 patients with head and neck melanoma, only 53% of the nodes were found to be both dyed blue and radioactive (10). Determination of the accurate number of draining basins is of major importance. A group of lymph nodes that belong to a single basin must be differentiated from multiple lymph nodes of several basins. In the latter case, each of the nodes may be an SN. Identifying >1 draining basin indicates the need to identify, remove, and examine the SN of each of the basins separately (3).
An effort should be made to accurately define all draining basins before surgery and to localize all SNs. Aberrant SNs outside the recognized lymphatic fields have been reported in up to 21% of the patients. These aberrant SNs may be sites of metastasis (21,28,29). In-transit lymph nodes located between the tumor site and the basin may also be recognized (Fig. 3). These nodes are equally at risk for metastatic disease and are, by definition, SNs.
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Previous reports have discussed the methodologic details for optimizing lymphoscintigraphy, recommending acquisition of early images, the use of high-resolution collimation, multiple projections, and the use of markers and transmission images with a 57Co flood source to facilitate anatomic localization (1,3,1318). Even then, there was a discrepancy between the lymphoscintigraphic and the actual number and location of SNs in 23% of the patients. Lymphoscintigraphy has been found to accurately predict the number of nodes in only 81% of the basins, overlooking nodes that were superimposed and could not be separated from other nodes or from the injection site or lymphatic channels and nodes that were beyond the resolution of the planar images (30). In patients with mucosal SCC or melanoma of the head and neck, the distance between the primary tumor injection site and the lymphatic basin is usually short. This proximity may result in obscuring of the SN by the scatter radiation arising from the injection site (27,30,31).
The better anatomic definition and improved resolution that characterize SPECT images may overcome the above limitations of planar images. Localization of hot lymph nodes on SPECT images without anatomic landmarks is not possible. However, in this study it became possible by fusing the SPECT image with the anatomic data obtained by performing low-dose CT at the same setting as with the SPECT acquisition. SPECT/CT images were found to add data that were clinically relevant in patients with a tumor in the head and neck region and in the trunk. Multiple drainage basins were found in 33% of patients with a head and neck tumor and in 50% of patients with melanoma in the trunk. In patients with melanoma in the extremities, SPECT/CT did not add to planar images. Unpredicted SNs were found, including anterior nodes in patients with cutaneous melanoma in the back and a deep supraclavicular SN in a patient with SCC of the tongue. SPECT/CT clarified the number of draining basins in 4 of 5 patients in whom the accurate number of basins could not be determined on planar images. In 9 patients, SPECT/CT identified nodes that were overlooked by planar images: 3 nodes were obscured by the injection site, 2 were in-transit nodes, and 4 nodes belonged to additional basins or were located in unexpected sites. Two of the nodes detected only by SPECT/CT images were positive on histopathologic examination. Potential explanations for identification of additional nodes by SPECT/CT could have been the effect of time, inadequate planar technique, or improved tomographic imaging technology. Planar images were obtained immediately before or after SPECT acquisition and as long as 24 h after injection. The planar imaging protocol was not altered by the addition of SPECT/CT but, rather, was identical to the multiprojection, multiple time-points protocol that has been used in our institution for many years. It appears, therefore, that the improved lesion detectability of tomographic images and the anatomic landmarks of CT would be the more likely explanation for the additional data provided by SPECT/CT. It should be borne in mind that SPECT/CT is not without disadvantages, as it involves a higher radiation dose because of the addition of CT and requires a longer acquisition time.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Einat Even-Sapir, MD, PhD, Department of Nuclear Medicine, Tel-Aviv Sourasky Medical Center, 6 Weizman St., Tel-Aviv, 64239 Israel.
E-mail: evensap{at}tasmc.health.gov.il
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