Characterization of the Biomechanical Properties of Skin Using Vibrational Optical Coherence Tomography: Do Changes in the Biomechanical Properties of Skin Stroma Reflect Structural Changes in the Extracellular Matrix of Cancerous Lesions?

Abstract

Early detection of skin cancer is of critical importance since the five-year survival rate for early detected skin malignancies is 99% but drops to 27% for cancer that has spread to distant lymph nodes and other organs. Over 2.5 million benign skin biopsies (55% of the total) are performed each year in the US at an alarming cost of USD ~2.5 B. Therefore there is an unmet need for novel non-invasive diagnostic approaches to better differentiate between cancerous and non-cancerous lesions, especially in cases when there is a legitimate doubt that a biopsy may be required. The purpose of this study is to determine whether the differences in the extracellular matrices among normal skin, actinic keratosis (AK), basal cell carcinoma (BCC) and squamous cell carcinoma (SCC) can be assessed non-invasively using vibrational optical coherence tomography (VOCT). VOCT is a new diagnostic technology that uses infrared light and audible sound applied transversely to tissue to measure the resonant frequencies and elastic moduli of cells, dermal collagen, blood vessels and fibrous tissue in skin and lesion stroma without physically touching the skin. Our results indicate that the cellular, vascular and fibrotic resonant frequency peaks are altered in AK, BCC and SCC compared to those peaks observed in normal skin and can serve as physical biomarkers defining the differences between benign and cancerous skin lesions. The resonant frequency is increased from a value of 50 Hz in normal skin to a value of about 80 Hz in pre- and cancerous lesions. A new vascular peak is seen at 130 Hz in cancerous lesions that may reflect the formation of new tumor blood vessels. The peak at 260 Hz is similar to that seen in the skin of a subject with Scleroderma and skin wounds that have healed. The peak at 260 Hz appears to be associated with the deposition of large amounts of stiff fibrous collagen in the stroma surrounding cancerous lesions. Based on the results of this pilot study, VOCT can be used to non-invasively identify physical biomarkers that can help differentiate between benign and cancerous skin lesions. The appearance of new stiff cellular, fragile new vessels, and stiff fibrous material based on resonant frequency peaks and changes in the extracellular matrix can be used as a fingerprint of pre- and cancerous skin lesions.

Keywords: actinic keratosis; basal cell carcinoma; collagen; epithelial cells; epithelial–mesenchyme transition; extracellular matrix; fibrous tissue; squamous cell carcinoma; stroma; vascular mimicry.

Figure 1

(A) Block diagram of the setup of an OCT device modified to do vibrational optical coherence tomography (VOCT). The speaker shown, which is 2 inches in diameter, provides audible sound through a computer-driven app to vibrate the sample between 30 and 300 Hz. The displacement of the sample at each frequency is obtained from amplitude data collected and stored from raw images created by the OCT. (B) Schematic drawing of the specimen holder and the OCT handpiece used to collect the VOCT data at each frequency on skin lesion biopsies. The biopsy sample is placed on saline-wetted gauze to prevent dehydration during testing.

Figure 1

(A) Block diagram of the setup of an OCT device modified to do vibrational optical coherence tomography (VOCT). The speaker shown, which is 2 inches in diameter, provides audible sound through a computer-driven app to vibrate the sample between 30 and 300 Hz. The displacement of the sample at each frequency is obtained from amplitude data collected and stored from raw images created by the OCT. (B) Schematic drawing of the specimen holder and the OCT handpiece used to collect the VOCT data at each frequency on skin lesion biopsies. The biopsy sample is placed on saline-wetted gauze to prevent dehydration during testing.

Figure 2

Schematic diagram of OCT color-coded images describing the differences between normal skin (A), AK, BCC and SCC (B,C) derived from previous study results [20,21]. Normal skin (A) contains an epidermis composed of a stratum corneum (SC) on top, an epithelial layer that derives from the basal cell layer (BE), rete pegs that undulate at the basal cell–papillary dermal layer interface (PD). The papillary dermis contains dermal collagen with a resonant frequency of 100 Hz. Pre- (AK) (B) and cancerous lesions (C) (BCC and SCC) contain nodular and linear tumors. Cancerous tumors (BCC and SCC) contain reduced amounts of stratum corneum and lack the undulating rete pegs. The dermal collagen fibers with a resonant frequency of 100 Hz are replaced by lesions with dense fibrous tissue (black spots) that reflect more light back to the detector. The stiffness of the tumor collagen can be determined by measuring the resonant frequency of the material in the black spots. Resonant frequency peaks in cancerous lesions occur at 260 Hz as a result of stiffening of the collagen deposited by tumor cells.

Figure 3

Comparison of the color-coded OCT images (A–D), pixel intensity versus depth plots (E–H), and weighted displacement in μm versus frequency plots in Hz (I–L) for the areas marked by the arrows for normal skin, AK, BCC and SCC obtained using VOCT. The arrows in (A) through (D) mark the locations at which the measurements were made on biopsies with the results presented in panels (E) through (L). Note the plateau marked with a horizontal line in (F) through (H) in the pixel intensity versus depth plots occurs at a depth of about 0.2 mm. This is approximately at the interface between the epidermis and the papillary dermis. In normal skin and AK there is no fibrous tissue peak at 260 Hz (I,J) while in the cancerous lesions ((K), BCC and (L), SCC) note the large peak at 260 Hz suggesting that this peak is where stiff fibrous tissue is deposited. The 260 Hz peak is one of the markers that can be used to differentiate between benign and cancerous skin lesions.

Figure 3

Comparison of the color-coded OCT images (A–D), pixel intensity versus depth plots (E–H), and weighted displacement in μm versus frequency plots in Hz (I–L) for the areas marked by the arrows for normal skin, AK, BCC and SCC obtained using VOCT. The arrows in (A) through (D) mark the locations at which the measurements were made on biopsies with the results presented in panels (E) through (L). Note the plateau marked with a horizontal line in (F) through (H) in the pixel intensity versus depth plots occurs at a depth of about 0.2 mm. This is approximately at the interface between the epidermis and the papillary dermis. In normal skin and AK there is no fibrous tissue peak at 260 Hz (I,J) while in the cancerous lesions ((K), BCC and (L), SCC) note the large peak at 260 Hz suggesting that this peak is where stiff fibrous tissue is deposited. The 260 Hz peak is one of the markers that can be used to differentiate between benign and cancerous skin lesions.

Figure 3

Comparison of the color-coded OCT images (A–D), pixel intensity versus depth plots (E–H), and weighted displacement in μm versus frequency plots in Hz (I–L) for the areas marked by the arrows for normal skin, AK, BCC and SCC obtained using VOCT. The arrows in (A) through (D) mark the locations at which the measurements were made on biopsies with the results presented in panels (E) through (L). Note the plateau marked with a horizontal line in (F) through (H) in the pixel intensity versus depth plots occurs at a depth of about 0.2 mm. This is approximately at the interface between the epidermis and the papillary dermis. In normal skin and AK there is no fibrous tissue peak at 260 Hz (I,J) while in the cancerous lesions ((K), BCC and (L), SCC) note the large peak at 260 Hz suggesting that this peak is where stiff fibrous tissue is deposited. The 260 Hz peak is one of the markers that can be used to differentiate between benign and cancerous skin lesions.

Figure 4

Comparison of color-coded OCT images for (A) normal skin, (B) AK, (C) BCC and (D) SCC lesions. In normal skin (A) the stratum corneum (SC), basal epithelium (BE) and papillary dermis are (PD) are characteristic of the skin along with the undulating rete pegs. In the AK lesion (B) normal skin is seen on the left while the pre-cancerous lesion is seen on the right. In the region of the AK lesion on the right, the basal epithelium and papillary dermis are truncated and replaced with a tumor containing fibrous tissue that reflects light. However, the resonant frequency of the fibrous collagen deposited and the peak height are different than what is seen in cancerous lesions (see Figure 3). The stratum corneum (SC) is thickened and is hypercellular. In BCC (C) and SCC (D) the stratum corneum (SC) is reduced in size or fragmented and the lesion is seen as either circular or elongated nodules. The rete pegs are missing and black lesions are embedded in the epidermis. There are some basal epithelium (BE) and the papillary dermis (PD) that are reduced in size in the lesion on the right.

Figure 4

Comparison of color-coded OCT images for (A) normal skin, (B) AK, (C) BCC and (D) SCC lesions. In normal skin (A) the stratum corneum (SC), basal epithelium (BE) and papillary dermis are (PD) are characteristic of the skin along with the undulating rete pegs. In the AK lesion (B) normal skin is seen on the left while the pre-cancerous lesion is seen on the right. In the region of the AK lesion on the right, the basal epithelium and papillary dermis are truncated and replaced with a tumor containing fibrous tissue that reflects light. However, the resonant frequency of the fibrous collagen deposited and the peak height are different than what is seen in cancerous lesions (see Figure 3). The stratum corneum (SC) is thickened and is hypercellular. In BCC (C) and SCC (D) the stratum corneum (SC) is reduced in size or fragmented and the lesion is seen as either circular or elongated nodules. The rete pegs are missing and black lesions are embedded in the epidermis. There are some basal epithelium (BE) and the papillary dermis (PD) that are reduced in size in the lesion on the right.