Background Differentiating Spitz nevus (SN) from Spitzoid malignant melanoma (SMM) is one the most difficult problems in Dermatopathology. Specific Aim To identify differences on proteomic level between SN and SMM. Methods We performed Imaging Mass Spectrometry (IMS) analysis on formalin-fixed, paraffin embedded tissue samples (FFPE) to identify differences on proteomic level between SN and SMM. The diagnosis of SN and SMM was based on histopathologic criteria, clinical features, and follow up data, which confirmed that none of the lesions diagnosed as SN recurred or metastasized. The melanocytic component (tumor) and tumor microenvironment (dermis) from 114 cases of SN and SMM from the Yale Spitzoid Neoplasm Repository were analyzed. After obtaining mass spectra from each sample, classification models were built using a training set of biopsies from 26 SN and 25 SMM separately for tumor and for dermis. The classification algorithms developed on the training data set were validated on another set of 30 samples from SN and 33 from SMM. Results We found proteomic differences between the melanocytic components of SN and SMM and identified five peptides that were differentially expressed in the two groups. From these data, 29 out of 30 SN and 26 out of 29 SMM were recognized correctly based on tumor analysis in the validation set. This method correctly classified SN with 97% sensitivity and 90% specificity in the validation cohort. Conclusions IMS analysis can reliably differentiate SN from SMM in FFPE tissue based on proteomic differences.

P75 nerve growth factor receptor (p75 NGF-R) is a low-affinity receptor expressed on the surface of neural crest-derived cells and in a variety of neural tumors. Strong p75 NGF-R expression has been found in spindle cell melanoma (SCM). We studied spindle cell neoplasms of sun-damaged skin to determine whether this marker can reliably distinguish between SCM and other spindle cell malignancies. We evaluated the staining of p75 NGF-R, S100, and HMB-45 in 11 cases of SCM, 16 cases of spindle cell squamous cell carcinoma (SCSCC), 19 cases of spindle cell atypical fibroxanthoma, 6 cases of cutaneous leiomyosarcoma, and 20 scars. Staining with p75 NGF-R was positive in all 11 of 11 (100%) cases of SCM, whereas S100 stained 10 of 11 (91%) cases, and HMB-45 was negative in all SCMs. In addition, there was superior intensity of the staining for p75 NGF-R in comparison to S100. P75-NGF-R showed focal positivity in 3 of 16 (19%) cases of SCSCC. None of the rest of the cases of SCSCC, and none of the cases of spindle cell atypical fibroxanthoma, cutaneous leiomyosarcoma, and scars expressed p75 NGF-R, S100, or HMB-45. P75 NGF-R is a useful marker to distinguish SCM from other spindle cell neoplasms of sun-damaged skin. This marker exhibits greater sensitivity than S100 in identifying SCM and may be a useful diagnostic and ancillary stain especially in the setting of an S100 negative spindle cell neoplasm.