Interstitial photodynamic therapy (PDT) and photoimmunotherapy (PIT) are frequently based on the use of cylindrical light distributors (CLDs). PDTs and PITs of large tumors require treatment planning to maximize the therapeutic dose spatial distribution while minimizing the number of CLDs. This is, in particular, the case when CLDs are inserted in parallel to treat head and neck squamous cell cancers. In this presentation, we will describe how to position CLDs to maximize the necrosed volume of such cancers for different CLDs insertion geometries. In addition, we will describe the influence of different tissue optical parameters on these positions, in particular when head and neck squamous cell cancers are treated by interstitial PIT with cetuximab–IR700. The descriptions of the light propagation around CLDs were performed using Monte-Carlo simulations with tumor optical properties derived from the literature, when CLDs were inserted perpendicularly to the air-tissue interface. These simulations enabled to determine the tumor volume receiving light doses larger than a therapeutic threshold. An optimization algorithm was then developed to calculate and maximize the necrosed tumor volumes.
Our results indicate that the values of the absorption and reduced scattering coefficients have the most significant influence on the optimal CLD positions. In contrast, the tissue anisotropy factor, the CLD insertion patterns and length, as well as the angular dependence of their radiances have minimal influences. Finally, at first approximation, the variations of the optimal CLD-CLD distances due to changes of their length, optical properties of the tissue, and choice of geometry, are decoupled.