Terahertz Pulsed Imaging (TPI) is a new medical imaging modality for the detection of epithelial cancers. Over the last two years this technique has been applied to the study of in vitro basal cell carcinoma (BCC). Using time-domain analysis the contrast between diseased and normal tissue has been shown to be statistically significant, and regions of increased terahertz (THz) absorption correlated well with the location of the tumour sites in histology. Understanding the source of this contrast through frequency-domain analysis may facilitate the diagnosis of skin cancer and related skin conditions using TPI. We present the first frequency-domain analysis of basal cell carcinoma in vitro, with the raw power spectrum giving an insight into the surface features of the skin. Further data manipulation is required to determine whether spectral information can be extrapolated at depth. These results highlight the complexity of working in reflection geometry.