Microdosimetry of irregular field paraffin collimation in fast neutron therapy

The King Faisal Specialist Hospital and Research Centre is the only fast neutron therapy facility to utilize paraffin collimation for irregular beam ports. The application of this treatment modification to clinical practice required the microdosimetric characteristics of paraffin to be established. Measurements were made with a tissue equivalent (Rossi) proportional counter at different spatial locations in a water phantom and in air. The neutron beam was shaped by a 10 x 10 cm paraffin collimator. The analytically derived effective relative biological effectiveness (RBE) for 80% survival was approximately 3.5, which is similar to the standard 10 x 10 cm Benelex collimator and is in good agreement with empirical biological observations for this neutron beam.