A model to estimate the dose to tumour following intracavity administration of radioimmunoconjugates to patients with malignant gliomas.

Patients who have relapsed following primary treatment for malignant glioma and have undergone further surgical debulking have been treated with an anti-human neural cell adhesion molecule (NCAM) MoAb linked to either Iodine-131 or Yttrium-90. These reagents are introduced into the tumour resection cavity via an Ommaya reservoir. Pharmacokinetic and imaging studies indicate that the radioimmunoconjugate remains within the cavity for a protracted period of time. In this manuscript we develop a dosimetric model to predict the dose delivered to the rim of tissue surrounding the resection cavity. The model takes into account variables such as the diameter of the cavity and the degree of antibody binding which is achieved. Whilst the calculated doses to the wall of the cavity are relatively inaccurate due to our inability to measure factors such as diffusion and heterogeneity in antibody uptake, the model illustrates the potential benefits and pitfalls that can result from targeting the two radionuclides. It is hoped that as increasing interest is shown in this type of "liquid brachytherapy" other groups will find it useful to apply the model to allow comparisons to be made between our targeting strategy and those developed by other individuals.