title: A new finite element approach for near real-time simulation of light propagation in locally advanced head and neck tumors.
abstract: OBJECTIVE
  Several clinical studies suggest that interstitial photodynamic therapy (I-PDT) may benefit patients with locally advanced head and neck cancer (LAHNC). For I-PDT, the therapeutic light is delivered through optical fibers inserted into the target tumor. The complex anatomy of the head and neck requires careful planning of fiber insertions. Often the fibers' location and tumor optical properties may vary from the original plan therefore pretreatment planning needs near real-time updating to account for any changes. The purpose of this work was to develop a finite element analysis (FEA) approach for near real-time simulation of light propagation in LAHNC.
  
  
  METHODS
  Our previously developed FEA for modeling light propagation in skin tissue was modified to simulate light propagation from interstitial optical fibers. The modified model was validated by comparing the calculations with measurements in a phantom mimicking tumor optical properties. We investigated the impact of mesh element size and growth rate on the computation time, and defined optimal settings for the FEA. We demonstrated how the optimized FEA can be used for simulating light propagation in two cases of LAHNC amenable to I-PDT, as proof-of-concept.
  
  
  RESULTS
  The modified FEA was in agreement with the measurements (P = 0.0271). The optimal maximum mesh size and growth rate were 0.005-0.02 m and 2-2.5 m/m, respectively. Using these settings the computation time for simulating light propagation in LAHNC was reduced from 25.9 to 3.7 minutes in one case, and 10.1 to 4 minutes in another case. There were minor differences (1.62%, 1.13%) between the radiant exposures calculated with either mesh in both cases.
  
  
  CONCLUSIONS
  Our FEA approach can be used to model light propagation from diffused optical fibers in complex heterogeneous geometries representing LAHNC. There is a range of maximum element size (MES) and maximum element growth rate (MEGR) that can be used to minimize the computation time of the FEA to 4 minutes.
authors.affiliation: Photodynamic Therapy Center, Roswell Park Cancer Institute, Buffalo, New York.
authors.forename: Emily
authors.initials: E
authors.lastname: Oakley
authors.forename: Brian
authors.initials: B
authors.lastname: Wrazen
authors.forename: David A
authors.initials: DA
authors.lastname: Bellnier
authors.forename: Yusef
authors.initials: Y
authors.lastname: Syed
authors.forename: Hassan
authors.initials: H
authors.lastname: Arshad
authors.forename: Gal
authors.initials: G
authors.lastname: Shafirstein
mesh_terms: D000970:Antineoplastic Agents / Q000627:therapeutic use; D020342:Finite Element Analysis; D006258:Head and Neck Neoplasms / Q000188:drug therapy*; D006801:Humans; D054025:Lasers, Dye / Q000627:therapeutic use*; D008962:Models, Theoretical; D055100:Optical Fibers; D010778:Photochemotherapy / Q000295:instrumentation / Q000379:methods*; D017319:Photosensitizing Agents / Q000627:therapeutic use
chemical_list: D000970:Antineoplastic Agents; D017319:Photosensitizing Agents
keywords: finite element; head and neck cancer; intersititial photodynamic therapy