Abstract
Purpose
Sentinel lymph node (SLN) biopsy is instrumental in staging and treatment of cutaneous melanoma. SPECT/CT, single-photon emission computed tomography (SPECT) integrated with computed tomography (CT), increases the accuracy of SLN mapping to improve surgical planning. SPECT/CT can correct for signal scatter to prevent masking, which is especially common in the head and neck. For periparotid lymph nodes SPECT/CT may improve localization of SLNs compared to lymphoscintigraphy.
Materials/Methods
Hospital charts were reviewed for 14 patients with melanoma and suspected lymphatic drainage to the parotid region who received lymphoscintigraphy followed by SPECT/CT prior to surgical excision and SLN.
Results
Overall, SPECT/CT provided data, which changed management in 57% of patients.
Conclusions
Fifty-seven percent of our patients benefited from use of SPECT/CT. The distinction between level II and parotid sentinel lymph nodes was clearly identified through SPECT/CT images. We believe that patients with melanoma draining to the parotid region would benefit from SPECT/CT SLN mapping.
1
Introduction
Metastasis to regional lymph nodes is one of the most important prognostic predictors in malignant melanoma. The presence of lymphatic metastases decreases 10-year survival from 60% to 20% . Sentinel lymph node biopsy allows for diagnosis of nodal staging while sparing patients full node dissection, but this technique is only successful if sentinel lymph nodes are accurately identified. Sentinel node positivity is the single most important prognostic factor in predicting survival in cutaneous melanoma . Additionally, when a positive sentinel lymph node is identified, early nodal dissection may also improve survival . A recent epidemiologic study demonstrates that patients younger than 60, without adverse features of the primary melanoma (less than 2 mm, and no ulceration) benefit significantly from completion neck dissection in 5-year disease-free survival .
Conventionally sentinel lymph nodes are identified preoperatively using 2-D planar lymphoscintigraphy, which uses intravenously injected nuclear tracers to estimate the location of draining lymph nodes. Using this method, at least one sentinel lymph node is successfully identified in 99% of melanoma patients , but only 85% in the head and neck region . Because of the density of lymphatic drainage in this region, masking can occur, where sentinel lymph nodes may be too close to the primary site to be differentiated on planar imaging and no lymph nodes are identified on imaging. In addition, a primary site may drain to multiple lymph node beds, which may not be distinguishable on planar lymphoscintigraphy. Intraoperative tools such as methylene blue Lymphazurin and hand-held gamma probes can aid in identifying these masked lymph nodes, but blue dye techniques are generally less successful in the head and neck due to inconsistent lymphatic drainage .
Another limitation of lymphoscintigraphy is that correlating anatomic landmarks are not imaged. While this imaging modality may be sufficient for identifying inguinal or axillary lymph nodes, in the head and neck region lymphatic drainage is more complex with more than 350 lymph nodes situated adjacent to important and often overlapping structures. This dense network means that masking is more likely to occur because sentinel lymph nodes may be near the primary injection site. In addition, better localization of sentinel lymph nodes in relation to critical anatomic structures can allow for more accurate surgical planning allowing for smaller incisions and less tissue disruption or injury to important structures such as the facial nerve etc.
A recently proposed solution is SPECT/CT, a hybrid of single-photon emission computed tomography (SPECT) integrated with computed tomography (CT) imaging. SPECT/CT provides 3-D visualization of nuclear signals, which is correlated with anatomic landmarks. The use of low-dose CT scan provides low radiation exposure with adequate images for anatomic landmark reference. In addition, SPECT/CT has the ability to correct for signal scatter and attenuation to prevent masking . SPECT/CT is beginning to be used in several other areas of medicine including enhancement of nuclear cardiac imaging, localization of neuroendocrine tumors, mapping of parathyroid activity, and bone scintigraphy .
Preliminary studies using SPECT/CT for identification of sentinel lymph nodes have shown clinical benefit through identification of additional lymph nodes and improved localization in 35%–43% of melanoma patients . The ability to correct for signal scatter allows for identification of lymph nodes that may otherwise have been masked. Better localization and smaller incisions also lead to shorter operating times; one series reported that SPECT/CT information more than halved the operating time (median 40 min vs. 108 min ). SPECT/CT has also shown improved identification and localization of sentinel lymph nodes in other malignancies including breast cancer and oral squamous cell carcinoma .
These studies suggest that head and neck melanoma patients may benefit the most from SPECT/CT for sentinel lymph node localization compared to other anatomic regions. In one series of 85 melanoma patients, 35% patients were determined to have a clear benefit (change in surgical incision or additional nodes visualized) from SPECT/CT images, but among patients with head and neck melanoma, 100% ( n = 14) of patients showed benefit from SPECT/CT. Another series reported that information provided by SPECT/CT images changed the surgical approach in 55% of head and neck melanoma cases .
Even among patients with head and neck melanoma, not all patients may require this additional imaging modality. Identifying which patients would benefit most from the additional information provided by SPECT/CT will provide the best utilization of this new imaging technology. One subset of patients that may benefit is those whose melanoma has potential drainage into the periparotid lymphatics. Removal of lymph nodes from within the parotid gland requires a distinctly different surgical approach than removal of level II lymph nodes, but lymphoscintigraphy images cannot differentiate between lymph nodes in the tail of parotid and level II nodes. Correlation of lymph node location with anatomic landmarks on SPECT/CT images would allow for this differentiation, and offer clear guidance for the best surgical approach. It would also impact the type of preoperative consent and intraoperative monitoring used (facial nerve monitoring for localized intraparotid lymph nodes). For example one patient in the study of van der Ploeg et al. had a left preauricular melanoma with a lymph node localizing to the left lateral neck. Correlation with CT imaging was able to localize the lymph node to the superficial tail of the parotid gland, thus a small incision with partial facial nerve dissection was performed ( Fig. 1 ). In our study, we sought to show that in patients with melanoma with potential lymphatic drainage to the periparotid region, SPECT/CT provides improved identification and localization of sentinel lymph nodes compared to lymphoscintigraphy.
