| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Original Article |
1 Sydney Melanoma Unit and Melanoma and Skin Cancer Research Institute, Royal Prince Alfred Hospital, Camperdown, New South Wales 2050, Australia
2 Department of Surgery, University of Sydney, Camperdown, New South Wales 2006, Australia
3 Institute for Biodiagnostics, National Research Council of Canada, 435 Ellice Avenue, Winnipeg, Manitoba R3B 1Y6, Canada Manitoba Canada
4 Institute for Magnetic Resonance Research and Department of Magnetic Resonance in Medicine, University of Sydney, Block 3, Level 3, Royal North Shore Hospital, St. Leonards, New South Wales 2065, Australia
5 Department of Anatomical Pathology, Royal Prince Alfred Hospital, Missenden Road, Camperdown, New South Wales 2050, Australia
Correspondence: Address correspondence and reprint requests to: Cynthia L. Lean, PhD; E-mail: cynth{at}imrr.usyd.edu.au.
| ABSTRACT |
|---|
|
|
|---|
Methods: FNAB samples were obtained from 118 biopsy specimens from 77 patients during SN biopsy and regional lymphadenectomy. The specimens were histologically evaluated and correlated with MRS data. Histopathologic analysis established that 56 specimens contained metastatic melanoma and that 62 specimens were benign. A linear discriminant analysisbased classifier was developed for benign tissues and metastases.
Results: The presence of metastatic melanoma in lymph nodes was predicted with a sensitivity of 92.9%, a specificity of 90.3%, and an accuracy of 91.5% in a primary data set. In a second data set that used FNAB samples separate from the original tissue samples, melanoma metastases were predicted with a sensitivity of 87.5%, a specificity of 90.3%, and an accuracy of 89.1%, thus supporting the reproducibility of the method.
Conclusions: Proton MRS of FNAB samples may provide a robust and accurate diagnosis of metastatic disease in the regional lymph nodes of melanoma patients. These data indicate the potential for SN staging of melanoma without surgical biopsy and histopathological evaluation.
Key Words: Melanoma Lymph node metastases Magnetic resonance spectroscopy Cancer Statistical classification strategy
| INTRODUCTION |
|---|
|
|
|---|
Proton MRS reflects the chemical composition of cells and the biochemical changes associated with the disease process.3 MRS can be performed on tissue or FNAB-derived material to determine tissue pathology with sensitivity and specificity rates of approximately 95% when an automated pattern-recognition method is used.35 Similarly, MRS studies of urine6 have been successful in predicting pathology and transplanted organ rejection.7
Simple methods of discrimination between spectra characterizing different tissue types have achieved reasonable predictive accuracy for some pathologic processes. For example, measurement of the relative intensities of the citrate and creatine/choline resonances in in vivo spectra of prostate tissue is reported to predict the presence of prostate cancer with approximately 85% accuracy.8 We have previously demonstrated that in melanoma patients, proton MRS of solid tissue or FNAB samples can determine the presence of metastatic melanoma in excised lymph nodes (P < .012) by a simple measurement of the relative intensities of the two broad resonances from lipid/metabolite compounds and choline-containing compounds.9 In this earlier study, only visually obvious peaks were measured in the spectra from a small sample database. A more sophisticated analysis of a larger sample database would be expected to improve predictive accuracy.
For clinical databases, it is usually the case that the number of spectral data points generated by each sample measurement greatly exceeds the number of samples available to be measured. A wide range of automated pattern-recognition methods have been developed for spectroscopy and other information-dense analytical techniques (see reviews1012). These methods have in common the ability to select from very large data sets the often very subtle features that best discriminate between classes (e.g., pathologic processes) of interest.
In the study reported here, we applied proton MRS and an automated linear discriminant analysis (LDA)-based pattern-recognition method termed the statistical classification strategy (SCS) to a large database of FNAB samples from resected lymph nodes. The robustness of the SCS method has been demonstrated by the analysis of proton magnetic resonance (MR) spectra from several tissues, including thyroid, ovary, prostate, esophagus, liver, breast, and brain tumors.5,13
| PATIENTS AND METHODS |
|---|
|
|
|---|
Patients, Specimen Collection, and Specimen Storage
FNAB samples (involving a single cortical puncture followed by multiple passes through each quarter of the specimen by using a 25-gauge needle attached to a 3-mL plastic syringe) were obtained from 118 tissue specimens from 77 melanoma patients undergoing regional node operation for suspected metastatic melanoma (SN biopsy or complete node dissection) in accordance with ethics protocol X97-0103 (Central Sydney Area Health Service, Sydney, Australia). All FNAB samples were placed in polypropylene vials containing 300 µL of phosphate-buffered saline (PBS; .27 mM of KCl, 13.69 mM of NaCl, 1.52 mM of KH2PO4, and 15.2 mM of Na2HPO4; pH 7.2) made up in perdeuterated water (PBS/D2O) and immediately snap-frozen in liquid nitrogen and stored at 70°C (for <6 weeks) until thawed for spectroscopy. Metastatic melanoma (22 microscopic metastases and 34 macroscopic metastases) was identified by histopathologic analysis in 56 of the specimens. These were obtained from 22 SNs, 23 non-SNs, and 11 specimens in which no residual recognizable lymph node component could be identified. Sixty-two specimens were benign by histopathologic analysis. These were obtained from 50 SNs, 11 non-SNs, and 1 biopsy sample in which no residual recognizable lymph node component could be identified.
Magnetic Resonance Spectroscopy
FNAB samples were thawed and transferred directly to 5-mm susceptibility-matched MRS tubes. Proton MRS analyses were performed on a Bruker Avance 360-MHz wide-bore spectrometer (Bruker Biospin BmbH, Rheinstetten, Germany) equipped with a 1H/13C 5-mm probe head. Samples were spun at 20 Hz, and the temperature was maintained at 37°C. One-dimensional spectra were acquired with acquisition parameters as follows: frequency, 360.10 MHz; flip angle, 60° (6- to 7-microsecond pulse length); interpulse delay, 1 second; acquisition time, .8 seconds; 8000 complex pairs of data points; 256 transients; and spectral width, 3600 Hz, with a total acquisition time of 5 minutes. The field was locked to D2O. Water suppression was effected by a selective excitation field gradient method (double-pulse field gradient spin echo14). Chemical shifts of resonances were referenced to water at 4.64 ppm (nominal position with respect to tetramethylsilyl propionate in PBS/D2O at 37°C). Magnitude spectra were created by Fourier transformation of the free induction decay data without application of window functions.
Histopathologic Evaluation
Histopathologic evaluation for the presence of melanoma metastasis was performed on all biopsy samples. Each SN was cut in its longitudinal axis in 3-mm-thick slices and embedded in entirety in paraffin blocks after tissue processing. The histopathology protocol for the detection of metastatic melanoma was based on the protocol for the Multicenter Selective Lymphadenectomy Trial (MSLT) coordinated at the John Wayne Cancer Institute (Santa Monica, CA) and funded by National Cancer Institute Grant PO1 CA29605-12. Four sequential 5-µm-thick tissue sections were cut from each block and stained with hematoxylin and eosin (H&E), S100 protein, or HMB-45. The sections were examined microscopically, initially at a scanning magnification of x100. Positive tissue samples were identified by the presence of metastatic melanoma cells on H&E, by immunohistochemically stained tissue sections, or both. This technique has a reported failure rate of .8%.15 Non-SNs were assessed histopathologically for the presence of metastatic melanoma by assessment of single H&E-stained sections.
Pattern-Recognition Method
Diagnostic correlation was performed between the MRS and histopathology data by using the SCS method,4,5 designed specifically for biomedical spectroscopy databases that contain many fewer spectra than data points in each spectrum. Magnitude spectra consisting of 4096 data points over the spectral width of 10 ppm were reduced to 1500 points between .35 and 4.00 ppm. Outside this region, all resonances have a very low signal-to-noise ratio. The spectra were normalized to the total integral in this region. Both the first derivatives and the rank-ordered first derivatives (i.e., replacing the actual first-derivative values by their positions in the sorted derivative list: the smallest derivative value would receive rank 1, and the largest, rank n, n being the number of derivative values) of the MR magnitude spectra were used for the analysis. Both processed versions of the spectra (first derivative and rank-ordered first derivative) were analyzed by a genetic algorithm-based optimal region selection algorithm (ORS_GA) to identify maximally discriminatory subregions in the spectra. The ORS_-GA used as its objective function the results of a leave-one-out cross-validated LDA classification. Fifteen subregions were chosen by ORS_GA for each of the 2 types of spectral preprocessing (first derivative and rank-ordered first derivative), giving a total of 30 subregions as features for classifier production. From these 30 features, all possible 7-feature subsets were found by exhaustive search, and each was used to produce LDA classifiers. The probabilities produced by the best-balanced eight of these classifiers were used as the input features for the third stage of the SCS: classifier fusion. All LDA classifiers were made robust by bootstrap cross-validation, and the process was repeated 10,000 times. The bootstrapping process minimizes "overtraining" over the developed classifier by randomly selecting half of the sample data, deriving a classifier from these data, and testing the accuracy of this classifier on the remaining half of the sample data. The final LDA classifier was based on the accuracy-weighted average of the 10,000 different bootstrap classifier coefficient sets. Samples are classified according to class-assignment probability. Classification is termed crisp if the assignment probability is
75% and fuzzy if it is <75%. Note that development of the classifier is based on repeated validation with data independent of the training data. The overall performance of the final classifier is assessed by classification of the entire primary data set and a secondary validation set.
The primary data set comprised MR spectra of FNAB samples from 56 tissue samples containing metastatic melanoma and 62 benign samples. A secondary validation set comprising duplicate FNAB samples from a subset of the same tissue samples used for the primary data set (24 melanoma containing and 38 benign) were also classified with the classifier developed from the primary data set. This secondary validation set tests the variability of FNAB samples from the same tissue, variability in sample handling and storage, and variability within the proton MRS measurement procedure.
| RESULTS |
|---|
|
|
|---|
|
75%. Samples that could not be assigned with this probability were termed fuzzy. The crispness of the data was 93.2% for the primary data set (8 of 118 samples were fuzzy) and was 92.7% for the secondary validation set (4 of 62 samples were fuzzy). These results are listed in Table 1
|
| DISCUSSION |
|---|
|
|
|---|
The combination of FNAB and MRS illustrates the potential use of new techniques to determine a patients SN status. It is reasonable to consider the future development of minimally invasive assessment of SNs by using ultrasound-directed needle aspiration. One potential strategy might consist of conventional lymphoscintigraphy followed by percutaneous ultrasound-guided FNAB of the identified SNs. Each FNAB could be initially assessed by MRS for spectral features associated with the presence of metastatic tumor. Ultrasound guidance would provide the primary validation of the correct localization of the SN. If mapping agents containing antimony were used, assay of antimony levels could be used to further validate the biopsy results of the appropriate node. We have recently established that ion coupled plasma-mass spectroscopy assay of antimony, a rare metal derived from one of the available lymphoscintigraphy agents, can be used to confirm the accuracy of SN identification.32,33
A rapid, accurate, cost-effective, nonsurgical method of evaluating nodes for their tumor-harboring status would greatly improve the treatment of melanoma patients by obviating the need for invasive SN excision and histopathologic assessment. This would not only reduce patient morbidity, but also produce significant cost savings.
The use of an automated pattern-recognition analysis of MRS data provides a more accurate method for the diagnosis of metastatic melanoma than was possible by the relatively simple comparison of resonance-intensity ratios.9 This was also the case for MRS-based diagnosis of breast cancer.19
In this study, four false-negative results were identified. On review of the histopathologic data, three of the four false-negative nodes were found to harbor only small amounts of metastatic disease that comprised <2% of the nodal volume. In addition, two of the nodes were of "horseshoe" morphology, containing large fatty hila and slender peripheral lymphoid components. This morphology may have resulted in suboptimal sampling of the nodal tissue, thus causing a false-negative result. It is also appropriate to note that horseshoe nodes present a similar problem when assessed by conventional histologic analysis. The slender peripheral rim of nodal tissue is inevitably not comprehensively examined, because it is impractical to examine any SNs by serial sectioning of the entire node.
Six false-positive SNs were also identified in this study. Because the protocol used for histopathologic assessment involves the microscopic examination of only a small portion of the lymph node tissue (<1%), these cases could have contained metastatic disease that was not detected by histopathologic analysis. Alternatively, MRS may have identified metastatic disease not discernible by histopathologic analysis. In a previous study16 using a rat model for breast cancer metastasis, MRS detected lymph node metastases before they could be identified by histopathologic analysis. The presence of these metastases was confirmed by xenografting the nodal tissue into nude mice and documenting tumor growth.
The SN status of a melanoma patient has been established as the single most important determinant of survival, ranking above the Breslow thickness of the primary tumor, the presence of ulceration, and the mitotic rate.34 Although a definitive answer to the question of whether subsequent early completion regional node resection benefits patients with positive SNs is still awaited with the results of the Multicenter Selective Lymphadenectomy Trial, knowledge of the SN status is critical to the accurate stratification of patient risk cohorts in adjuvant therapy trials. This study demonstrates that MRS combined with an automated pattern-recognition analysis can reliably identify metastatic melanoma in the regional lymph nodes by analysis of FNAB samples.
MRS on lymph nodes from melanoma patients reported in this series was performed on FNAB samples obtained from surgically harvested tissues. However, MRS on FNAB samples obtained during conventional investigation of clinically palpable nodes is also under way at our institution. It may be feasible to develop clinical techniques that combine conventional lymphoscintigraphy and MRS of ultrasound-guided percutaneous FNAB samples of SNs to avoid surgical node biopsy and its potential complications.
| CONCLUSIONS |
|---|
|
|
|---|
| ACKNOWLEDGMENTS |
|---|
Received for publication April 19, 2004. Accepted for publication June 29, 2005.
| REFERENCES |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
A. Beavis, M. Dawson, P. Doble, R. A. Scolyer, R. Bourne, L.-X. L. Li, R. Murali, J. R. Stretch, C. L. Lean, R. F. Uren, et al. Confirmation of Sentinel Lymph Node Identity by Analysis of Fine-Needle Biopsy Samples Using Inductively Coupled Plasma-Mass Spectrometry Ann. Surg. Oncol., March 1, 2008; 15(3): 934 - 940. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |