Photothermal Therapy

Photothermal Therapy

Nicholas EllensMBP1028

28 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, 1986.


• 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 mm (UV) to a few mm (near IR)• As such, therapy is suitable for topical

applications or deeper via fibres delivered an endoscope or percutaneously


• 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



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• Thermal treatment is a three stage process:1. Conversion of heat to light2. Thermal transfer3. Tissue Response



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• Reflection:– An air-tissue (n≈1.41) interface causes substantial

internal reflection, further depositing energy• Scattering:– Scattering length is typically 100-1000 times less than

absorption length– Upon entering tissue, photons are scattered many

times before being absorbed– Effectively, this increases

the light intensity close tothe tissue surface


Burgholzer, P. “Photoacoustic tomography: Sounding out fluorescent proteins” in Nature Photonics 3, 2009.

• Conversion of light to heat–Absorption:• A + hu A*

–Deactivation:• A* + B(E) A + B’(E+DE)

• Efficacy depends on high probability of collision and high number of accessible vibrational states

• Though sensitive to wavelength, less so than chemical processes


• Heat diffusion–As per the bioheat transfer equation–Affected by thermal properties of tissue,

timescale, blood perfusion• Thermal effects–Hyperthermia–Coagulation–Vaporisation


Photothermal Therapy

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