Risks posed by neutron contamination in high energy radiotherapy

Keehan, S 2017, Risks posed by neutron contamination in high energy radiotherapy, Doctor of Philosophy (PhD), Science, RMIT University.


Document type: Thesis
Collection: Theses

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Title Risks posed by neutron contamination in high energy radiotherapy
Author(s) Keehan, S
Year 2017
Abstract Radiotherapy is a common cancer treatment which aims to use radiation energy to kill tumour cells without damaging healthy tissue. High energy x-rays penetrate tissue deeply, depositing most of their energy beyond the skin and shallow tissues. X-rays with energies above 8 MeV may interact to produce neutrons, to which the patient is then exposed. Treatment planning systems (TPS), which calculate the amount of energy deposited to biological structures, do not currently account for contaminant neutrons produced in high energy interactions.

Constant improvements in modern radiotherapy techniques have resulted in better patient life expectancy than ever before. The unfortunate corollary of increased life span is increased manifestation of late onset side effects from low dose radiation exposure. This has resulted in an increased drive to reduce the dose to untargeted tissues as much as possible. A perfect treatment plan delivers a prescribed dose to the target volume and no dose to any other tissue. In reality, there is always some tissue through which the radiation must pass in order to reach the target, and some energy which passes beyond the intended target volume. In addition, there is unavoidable scattering of radiation within the patient, which results in out-of-field doses to untargeted tissue.

Dose to untargeted tissues resulting from x-ray energy is relatively well understood and is calculated and reported by TPS. Dose resulting from contaminant neutrons is not currently considered by TPS. The overall number of neutrons produced is relatively low, because high energy x-rays may interact in a variety ways and reactions producing neutrons are generally less likely. However, energy deposited by radiation can produce different biological outcomes depending on the type of radiation which deposits the energy. The biological damage inflicted by neutron radiation depends on the energy of the neutrons, and neutrons with energy around 1 MeV may be up to 20 times more damaging than x-rays.

So, how do we know the risk associated with neutrons produced from high energy x-rays? Measure the number of neutrons produced? Measure the amount of energy they deposit? Neutron radiation is incredibly difficult to quantify, much more so than alpha, beta, x-ray, gamma or proton radiation. Neutrons are neutrally charged particles, which means they are not attracted or repelled by the positive nuclei or negative electrons which make up all matter. Neutrons primarily pass straight through most materials without leaving a trace of evidence. This makes them quite difficult to detect! Neutrons do interact with the nuclei of some materials, in nuclear reactions, which produce energy and secondary particles which can more easily be detected. The extra energy released in these interactions and their complex probability functions make it difficult to determine the number of neutrons or the energy they would release in human tissue.

The high uncertainty in neutron measurement techniques has resulted in some controversy around the use of high energy x-ray beams. There is no question that the deeper penetration of the higher energy x-rays is extremely useful for treatment of some anatomical sites. The source of the contention is the degree of risk posed by the neutrons themselves. Without an accurate and precise method for quantifying the degree of neutron contamination, an accurate determination of the risk cannot be made.

Thermoluminescence dosimeters (TLDs) and activation foils have been used in this work to quantify neutrons produced in high energy radiotherapy. TLDs are a common radiotherapy dose measurement tool and are routinely used in existing clinical protocols. Lithium fluoride is a common TLD material and the two naturally occurring isotopes of lithium, 6Li and 7Li, have very different probabilities for neutron interaction. TLDs made from these materials can be used in pairs to produce signals which in combination can be correlated with the degree of neutron exposure. Activation foils are materials which become radioactive when exposed to neutrons. A first principles calculation can be used to determine the number of neutrons causing activation in a material.

Both these methods for neutron detection are highly dependent on the energy of the neutrons. Both produce a response related to the number of incident neutrons which needs to be corrected for the energy of the neutrons which are to be measured. The energy deposited by the neutrons in the detector material is not directly related to the energy which would be deposited in tissue in a patient exposure. Another energy correction is required to quantify the biological damage which may occur. This is the primary source of uncertainty and the cause of the disagreement between the vast number of existing publications on the topic.

This work determines energy correction values for the calibration of LiF TLDs for neutron measurements. The neutron energy spectrum relevant to high energy radiotherapy is modelled and used to determine energy corrections for activation foils. It is also needed to convert the response of detectors to the energy deposition in tissue and to correct for biological effect.
Additional risk from neutron contamination is also examined. The production of neutrons may induce radioactivity in other materials. The medical linear accelerator (linac) used to produce the treatment x-rays may itself become radioactive when it is operated at high energies. The induced activity is low level and is mainly of concern for radiotherapy staff rather than patients as they spend much greater time in proximity to the linac. Implants within the patient may also interact in unexpected ways. Metallic implants such as prosthetic hips can become radioactive from exposure to neutrons or high energy x-rays. This is primarily of concern for patients, as it potentially induces a low level internal source of radiation.

Many existing peer reviewed publications investigate the degree of neutron exposure to patients undergoing high energy radiotherapy, but there is no consensus amongst experts regarding the risk. This stems from the high degree of uncertainty in neutron measurement techniques. This thesis discusses the challenges of neutron dosimetry and proposes a methodology for correcting for detector energy dependence. An investigation of the indirect risks of neutron production is also presented.
This thesis offers a comprehensive analysis of existing neutron detectors and dosimeters with an in-depth discussion of their properties in relation to their suitability for use in high energy radiotherapy. The energy dependence of LiF TLD response to neutron radiation is carefully examined for calibration sources and for photoneutrons from medical linacs. The energy dependence should be considered for calibration sources, but is shown to be less critical for the energies produced by linacs. The energy dependence of activation foils depends on the material chosen but can be accounted for by calculating an energy spectrum weighted interaction probability, or cross section. Converting neutron measurements to values representing dose to human tissue for a given exposure must be corrected for the energy of the neutrons.

This thesis also presents a summary of neutron dose equivalent values from peer reviewed publications and compares the effects of a number of parameters on the neutron dose. Comparison between studies is made difficult by a lack of detail on the magnitude of energy corrections used in published papers. The indirect risk from secondary activations is considerably lower, however may be reduced by employing the existing collimation devices within linacs as shielding. A small amount of activation in hip prostheses does occur, but does not result in a significant dose to surrounding tissue.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Science
Subjects Nuclear Physics
Medical Physics
Keyword(s) Neutron
Radiotherapy
Neutron detection
Neutron dosimetry
Activation
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Created: Thu, 02 Nov 2017, 10:53:51 EST by Adam Rivett
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