Optical treatment planning for interstitial photodynamic therapy

By Tim Baran
Tim Baran

A number of research groups, including Thomas Foster’s lab at the University of Rochester, have become interested in using photodynamic therapy (PDT) to treat cancers that are located deep within the body.

PDT is an emerging cancer therapy that uses a combination of light-sensitive drugs, known as photosensitizers, and targeted illumination to create photochemical reactions that result in the destruction of cancer cells. Since the treated area is limited by the penetration depth of the treatment light, PDT has typically been used to treat superficial malignancies of the skin and other easily accessible regions.   

To make it work within the body, the photosensitizer is delivered systemically or locally, and allowed to accumulate in the tumor. Under image guidance, cylindrical diffusing fibers are then inserted into the tumor to deliver the treatment light. This treatment light is usually delivered by a laser with wavelengths varying from 630-700 nm (and beyond), depending on the photosensitizer used. As the diffusers used can be as long as 5 cm for large tumors, the required laser power at the source can approach or exceed 1 W.

As interstitial PDT is often performed in regions where there is sensitive healthy tissue nearby, there is a need for careful treatment planning. Towards that end, we have developed a Monte Carlo simulation space that allows for patient optical properties and anatomy to be incorporated into a rigorous treatment plan. 

Unlike radiation therapy, in which radiation doses can be directly computed from CT scan data, calculations of optical dose require knowledge of the patient’s optical properties, which can vary among patients and even within a single patient. Therefore, spectroscopic determination of optical properties is required before a treatment plan can be formulated.
Simulated patient data showing the insertion of four cylindrical diffusing fibers for photodynamic therapy of head and neck cancers
A number of techniques exist to do this. In our case, we use a custom optical probe that is inserted into the treatment region and a Monte Carlo based fitting algorithm in order to extract optical properties. These extracted optical properties are then combined with CT image data from the patient in order to build an optical and anatomical map of the patient in our simulation space. The number of diffusers, and the amount of light delivered by them, can then be optimized using a constrained nonlinear optimization algorithm. This ensures that tumor tissues receive a physician-prescribed light dose, while damage to healthy tissue is minimized.

My work thus far has been preclinical, using simulated data sets and animal models. We are actively seeking to translate interstitial PDT into the clinic at the University of Rochester Medical Center for treatment of cholangiocarcinoma, cancers of the head and neck, and deep-tissue microbial infections.

TIM BARAN is a PhD candidate in the Instituteof Optics at the Universityof Rochester (Rochester, NY).  His research in the Foster lab is related to optical dosimetry and treatment planning for interstitial photodynamic therapy, with an emphasis on the simulation of light propagation in tissue.

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