Fast Neutron Dosimetry, Microdosimetry and Relative Biological Effectiveness (RBE)
In the University of Washington Medical Center (UWMC) cyclotron, energetic (50.5 MeV) protons collide with a beryllium target to produce an intense beam of fast neutrons. The beam of fast neutrons is collimated and then shaped to the (beams eye view) tumor contours using a multi-leaf collimator (MLC). For salivary gland tumors, fast neutron treatments can achieve a 5-year local control rate of 75% compared to 32% for x-rays (Huber et al. 2001). At the UWMC, Douglas et al. (1999) achieved a 9-year local control rate of 78% for lesions less than or equal to 4 cm. For salivary gland tumors with skull base invasion, local control can be increased from 39% to 82% through the use of a stereotatic (photon) boost to the skull base (Douglas et al. 2008).
Although fast neutrons have been used to successfully treat salivary gland and selected other tumors for over 30 years (Laramore 2009 and references therein), the physical and biological mechanisms underpinning this form of cancer treatment are not fully understood. To improve the clinical care of patients receiving fast neutron therapy, we are using advanced Monte Carlo simulation techniques to improve the accuracy of neutron dose estimates in patients. We are also developing a multiscale system of biological models (Carlson et al.2008, Stewart et al. 2011, Frese et al. 2012) to gain insight into the mechanisms that make fast neutrons so uniquely effective for the treatment of some cancers. Through more accurate patient dosimetry and an improved understanding of biological mechanisms, we will be able to more fully individualize and exploit fast neutrons in the treatment of cancer.