Monte carlo simulations of light distributions in an embedded tumour model: studies of selectivity in photodynamic therapy.

It is well known that photosensitisers in photodynamic therapy (PDT) tend to localise in greater concentrations in tumours. This attractive feature may help confer on PDT the potential to selectivity destroy tumours while sparing the surrounding normal tissue. In this paper Monte Carlo simulations were used to study light distributions in a simple model consisting of tumour embedded in surrounding normal tissue subjected to superficial irradiance. The Monte Carlo model was coded to allow modelling of arbitrary geometries and multiple tissue types. This permitted the use of different optical properties for tumour and normal tissue. Two simulations were run using optical coefficients appropriate to breast carcinoma in adipose tissue and liver tumour in liver. Contours of equal fluence were plotted against depth for both simulations. Contours of equal photodynamic dose (fluence×drug concentration) were plotted for various tumour/normal drug ratios. By assuming a threshold for necrosis it was possible to estimate the depth of damage in the normal tissue and tumour simultaneously. A greater depth of selective tumour damage was observed in the breast tissue simulation for a given drug ratio due to the higher penetration of light compared to the liver. For a tumour to normal ratio of 4:1 selective damage to a depth greater than 4 mm was observed in the breast simulation compared to almost 3 mm in the case of the liver model.