Neutrons from Radiotherapy Accelerators: Production and Detection

Four years after the discovery of neutrons in 1932 by James Chadwick (Cambridge University) a biophysicist, G. L. Locher of the Franklin Institute in Pennsylvania, introduced the concept of Boron Neutron Capture Therapy (BNCT). The physical principle of BNCT is simple and elegant. It is a two component or binary system, based on the nuclear reaction that occurs when the stable isotope 10 B is irradiated with low energy or thermal neutrons thus producing very energetic helium-4 ( 4 He) nuclei (i.e. alpha particles) and recoiling Lithium-7 ( 7 Li) ions. In 1934 E. Fermi measured the high neutron capture cross section of 10 B, putting the bases of the first medical application for BNCT: the tumour cure. BNCT puts together the targeting principles of chemiotherapy and the localization principles of radiotherapy. The word “cancer” indicates many different malignant tumours arising in different tissues diagnosed at various stages of development. Tumour cells usually are biologically similar to normal cells and at present the therapeutical approaches are strongly limited by this lack of specificity. Local control of the primary tumour is a basic prerequisite condition for cancer cure and this is the radiotherapy goal; the ideal situation in radiotherapy consists in a large amount of energy deposited in the tumour volume and none in the surrounding healthy tissue. In principle, this would result in the absence of sideways spreading out of the beam and the achievement of a very well defined energy distribution in depth. Another crucial aspect is that about 20% of tumours are radio resistant, i.e. they do not respond to treatment with photons and electrons; they are, however, sensitive to beams that deposit their energy in microscopically localized packets (high density energy deposition), a process that increases the probability of giving a lethal dose to every cell (e.g. a beam of light ions). The BNCT method involves the delivery of concentrated doses of radiation directly to tumours while sparing non-cancerous tissues. The technique requires the targeting of malignant tissue with a carrier of elemental 10 B, followed by the exposure of the area to a neutron beam. The result is the release of radiation wherever neutrons are captured by boron.When a neutron collides with a boron atom, an alpha particle and a lithium ion are produced. Being charged and very heavy, they don’t travel farther than the dimension of a cell, ideally leading to selective destruction of the tumour and sparing the neighboring normal tissue. In this way, only cells harboring boron receive high doses of radiation. The treatment success depends on the capability to concentrate boron in the tumour cells. To achieve this, molecular biologists attach 10 B to a “delivery agent” that has an affinity with tumour cells. Right before treatment, patients ingest the agent either orally or intravenously and boron accumulates mainly in the tumour. The concentrations used are non-toxic and the time interval between the drug administration and the irradiation can be chosen to maximize the concentration differential between tumour and normal tissue. The choice of the tumour to cure has focused on a type of brain tumour, the “glioblastoma multiforme”, because still today it is most difficult to deal. Although “glioblastoma multiforme” doesn’t sound nearly as scary as “incurable brain tumour” that’s exactly what it is. The brain support cells start growing out of control and strangle the organ with tentacles of malignant tissue. This type of cancer strikes more than 10000 people each year in the U.S. and kills half of them in 12 months. Within two years, ninety percent of those struck are dead. Sadly, treatment for this condition has improved little in the last 25 years. Glioblastoma multiforme infiltrates the brain so aggressively that surgeons are rarely able to remove all the cancerous tissue. Moreover these types of tumours are resistant to standard radiation treatments and chemiotherapy. The first clinical trials with BNCT were performed between 1959 and 1961 at the Massachusetts General Hospital and at the Brookhaven National Laboratory with no results, but a lot of progress has been made since then. Currently the facilities for BNCT are limited to the only neutron sources available, that are nuclear reactors. To adapt these reactors to the experimentation is of difficult application considering both the practical aspect and the ethic one. It would be much easier from the clinical point of view to perform BNCT in a hospital environment such as the radiotherapy divisions. This thesis describes the role of Physics in the fight against cancer in general, and the physical bases of BNCT (chapters 1 and 2). The work has been done in the framework of the PhoNeS (Photon Neutron Source) project, which is devoted to the development of a neutron moderator to exploit the neutrons photoproduced by a high energy ( 10 MeV) radiotherapy photon beam (chapter 2). Chapter 3 is devoted to a review of the state of the art of neutron detectors. The neutron flux has been measured activating several cylinders of different materials (from Al to Cl) in different positions below the accelerator head, scanning the flux value as a function of PMMA (PolyMethylMethAcrylate) and of the dis-tance from the source (chapter 4). In order to study the feasibility of a real time detection system, the peculiarity of the radiotherapy beam, that is its pulsed nature, has been taken into account. By using a time of flight method, slow neutrons can be counted and their energy spectrum can be extracted (chapter 5).