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OCT-GUIDED BIPHASIC PDT 

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SUMMARY: Sub-surface imaging technology (OCT) combined with biphasic PDT makes it possible for skin cancer diagnosis and removal on the same day, without any needles, without any cutting, without any stitches and with minimal (if any) scarring.

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THE DETAIL  

Photodynamic treatment (PDT) of skin cancers has been around for over 20 years (see a demonstration video here). The treatment involves loading the skin cancer cells with a photoactive drug (applied in a cream) and then activating the drug with a strong light source.

 

PDT can be a remarkable non-invasive treatment but recurrences are an issue with the conventional treatment protocol. The light dosage used in this protocol has no published scientific basis and is suboptimal. Several studies have demonstrated that higher doses of light are more effective, yet there has been no modification of the protocol since it was introduced in 2004.

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Red light is used for conventional PDT as it penetrates skin slightly deeper compared to other colours. Wavelengths in the green-yellow spectrum however are far stronger at activating the drug. Some modification of how the light is delivered (see below) can achieve deeper penetration of these wavelengths leading to higher 'photodynamic dosage' and enhanced efficacy.

 

Incomplete treatment (recurrence) will also occur when inappropriate tumours are selected for PDT. Too often tumours are deeper than they appear to the naked eye or on biopsy. This problem can be overcome by screening lesions with high-resolution sub-surface imaging (OCT) prior to selection. Apart from measuring tumour depth, an OCT scan can assess the optical properties of tumour, the surrounding tissue and can also assess blood flow - these properties are very important with regard to determining suitability of PDT.  The depth measurement obtained by biopsy is far from being accurate*.

 

An OCT scan can also be performed following treatment to verify the tumour has been removed**.

 

 

* Stephens R. Histologic depth measurement of basal cell carcinoma should not be considered as a standard to judge the accuracy of depth measurement made using optical coherence tomography. Australas J Dermatol. 2023 Jan 30. doi: 10.1111/ajd.13989. Epub ahead of print. PMID: 36716201.

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** Wolswijk T, Adan F, Nelemans PJ, Defauwes A, Mosterd K. 'Optical coherence tomography for diagnosing recurrent or residual basal cell carcinoma after topical treatment: a diagnostic cohort study'. J Am Acad Dermatol. 2023 Jun 28:S0190-9622(23)01196-9. doi: 10.1016/j.jaad.2023.06.033. Epub ahead of print. PMID: 37391069.

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The treatment protocol developed at our clinic is described below:

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1. ASSESSMENT

An OCT scan is performed to confirm a diagnosis of skin cancer - a biopsy is generally not required. The scan will show the precise depth of the cancer down to 1/100th of a millimetre (see the image above).  Other aspects of the tumour (that are recognisable on OCT) will determine if biphasic photodynamic treatment is suitable.

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2.  INCUBATION WITH PHOTOSENSITISER

A special photosensitising cream is applied to the tumour and this is left under a dressing for a few hours. The ingredient in the cream will be converted to a light-activated molecule which selectively accumulates in skin cancer cells. 

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3. ACTIVATION WITH LIGHT

Two types of intense light will then be used to illuminate the area ("biphasic activation"):

 The first illumination is of high intensity red light (wavelength: 630nm), delivered by an LED lamp for 5 - 8 minutes. The red light will begin activation, inflammation then occurs and the skin often becomes flushed. Flushing is both advantageous and a problem! It is advantageous as more blood will deliver more oxygen to tumour and oxygen is absolutely essential for successful activation. A problem however is that more haemoglobin absorbs more light! This includes light which would otherwise have bounced off the underlying collagen and been redirected back to tumour. 

This problem is made worse because oxy-haemoglobin is converted to deoxy-haemoglobin which is a much stronger absorber of 630nm light.

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After 8 minutes of red light, or, at the point when flushing is noticeable, the light source is switched to an intense pulsed light device (IPL). Additional high fluence polychromatic light is then delivered with mechanical pressure onto the skin. The mechanical pressure is in order to drain the skin of blood (haemoglobin) whilst light is being delivered. Removing blood at the tumour site will dramatically increase the recycling and the delivery of green + yellow + red light to tumour cells. It is our premise that removing blood will not result in inadequate oxygen supply because much oxygen has already dissociated from haemoglobin. This second phase of activation takes less than 30 seconds to complete. This is demonstrated in the video further below.

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It is during the illumination process that a chemical reaction occurs within the tumour cells. They are destroyed. The normal healthy cells are spared. This means tumour removal with faster healing and less scarring.

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If the tumour is not suitable for biphasic PDT then we can still excise it in our rooms or refer you for excision by a plastic surgeon if you prefer. Some tumours are best treated by a combination of biphasic PDT followed by spot cautery, laser or minor surgery.

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