Spectral analysis enhances tumor delineation

Jan 01, 2008, 5:00am

Dye-enhanced reflectance and fluorescence polarization imaging (RFPI) for noninvasive delineation of nonmelanoma skin cancers are feasible. Findings of an initial study indicate spectral analysis of the reflectance images may improve its performance for discriminating cancerous and benign skin structures.

Key Points

Dr. Yaroslavsky conducted a study in which fresh specimens of cancerous tissues were processed and then imaged using RFPI alone or with spectral analysis.

Compared with RFPI, spectral analysis of the cross-polarized reflectance images improved differentiation of malignant tissue from all benign structures and skin appendages (sebaceous glands, epidermis, dermis and subcutaneous fat), although the spectral signatures of the tumors and hair follicles were found remarkably similar. Nevertheless, morphologically malignant lesions could be readily distinguished from hair follicles.

Similarly, it appears spectral analysis may be added to that system without making it more complex, while enabling the surgeon to differentiate between normal cutaneous structures and pathology," Dr. Yaroslavsky tells Dermatology Times.

Explaining the motivation for developing these noninvasive techniques, Dr. Yaroslavsky notes that although most nonmelanoma skin cancers are curable by surgical excision, if diagnosis is delayed, patients may present with more advanced tumors with poorly defined margins.

In that situation, Mohs micrographic surgery represents the best method for surgical management, and it has been shown in multiple studies to result in a very high cure rate. However, the technique is time-consuming.

"An optical diagnostic tool that could be used for in vivo imaging might be advantageous for guiding surgical tumor removal and, ultimately, save time and cost while improving surgical efficacy," Dr. Yaroslavsky says.

The study evaluating spectrally-resolved RFPI was performed using clinical cancer specimens from patients undergoing Mohs micrographic surgery. Freshly excised samples were briefly stained in 0.02 percent aqueous solution of methylene blue and imaged with a wide-field (33 mm x 25 mm) RFPI macro-imaging system. En face frozen H&E (hematoxylin and eosin) sections were also processed from the imaged tissues.

"A simple staining technique is used to enhance the contrast of the lesions with this method, and methylene blue is an FDA (Food and Drug Administration)-approved nontoxic stain that could be used in vivo," Dr. Yaroslavsky says.

Comparison of the emission spectra from the imaging technique to the gold standard histological samples showed the spectral analysis was very effective for delineating tumor margins of basal cell carcinomas, squamous cell carcinomas, and also infiltrative tumors that are typically difficult to define using standard histopathology.

"While we know that the very heavily stained parts of the tissue are tumor, the dye is not 100 percent specific and there is some residual staining of other skin structures.

"Since the RFPI images obtained are lower resolution compared to microscopic examination of histological specimens, it can be more difficult to discriminate the cancerous lesion from benign skin structures.

"Analyzing every pixel's spectral response revealed there are differences between tumor and other skin structures, indicating the spectral analysis adds diagnostic value," Dr. Yaroslavsky explains.

A clinical study is presently ongoing, evaluating the performance of the optical diagnostic system. In that trial, imaging is being performed both in situ and of excised tissue specimens.

"Each approach has advantages and limitations. By evaluating the two methods and comparing them, we can determine which might be more practical for clinical use," Dr. Yaroslavsky says.

She explains that in situ imaging allows the surgeon to immediately determine if there is any residual tumor. However, patient movement and tissue distortion resulting from excision could confound the accuracy of further excision if it is needed.

There is no problem with manipulating excised tissue for imaging, but care must be taken to orient it properly in order for the surgeon to know the site where further excision is needed if the margins are positive.

"The same challenge exists when performing Mohs surgery, and so it is nothing new or particularly complex, but still an issue that needs to be considered," Dr. Yaroslavsky says.

"The best practical thing about the technique," Dr. Yaroslavsky concludes, "is that physicians used to interpreting histopathology do not need any additional training, as the appearance of all skin structures is remarkably similar to that in standard H&E slides."