Photothermal Therapy Nicholas Ellens MBP September 2010

Outline Context Photothermal mechanisms and delivery Absorption and power deposition in tissue

Context Delivery Absorption and Power Deposition Hyperthermia has been used for thousands of years Photothermal therapy emerged shortly after the invention of the laser Laser treatments range in orders of magnitude in both treatment time and intensity

Boulnois, J-L, “Photophysical Processes in Recent Medical Laser Developments: a Review” in Lasers in Medical Science, Context Delivery Absorption and Power Deposition

Driving limitation in phototherapy is the absorption, is quantified by a, the absorption coefficient (units of cm -1 ) – Depends on medium and frequency – Penetration depth, a -1, ranges from fractions of m m (UV) to a few mm (near IR) As such, therapy is suitable for topical applications or deeper via fibres delivered an endoscope or percutaneously Context Delivery Absorption and Power Deposition

Further control provided by pulsing the light – If the pulse length is less than the thermal relaxation time, the treatment can be made more localised Feedback usually involves direct observation – Smoke, blanching – Where this is not achieved, other methods are required Context Delivery Absorption and Power Deposition

Boulnois, J-L, “Photophysical Processes in Recent Medical Laser Developments: a Review” in Lasers in Medical Science, 1986.

Thermal treatment is a three stage process: 1.Conversion of heat to light 2.Thermal transfer 3.Tissue Response Context Delivery Absorption and Power Deposition

Brunetaud, J et al., “Non-PDT Uses of Lasers in Oncology” in Laesrs in Medical Science, Covered elsewhere in course

Reflection: – An air-tissue (n ≈ 1.41) interface causes substantial internal reflection, further depositing energy Scattering: – Scattering length is typically times less than absorption length – Upon entering tissue, photons are scattered many times before being absorbed – Effectively, this increases the light intensity close to the tissue surface Context Delivery Absorption and Power Deposition Burgholzer, P. “Photoacoustic tomography: Sounding out fluorescent proteins” in Nature Photonics 3, 2009.

Conversion of light to heat – Absorption: A + h u  A* – Deactivation: A* + B(E)  A + B’(E+ D E) Efficacy depends on high probability of collision and high number of accessible vibrational states Though sensitive to wavelength, less so than chemical processes Context Delivery Absorption and Power Deposition

Heat diffusion – As per the bioheat transfer equation – Affected by thermal properties of tissue, timescale, blood perfusion Thermal effects – Hyperthermia – Coagulation – Vaporisation Context Delivery Absorption and Power Deposition

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