Responsibilities
It is the responsibility of medical physicists working in the Oncotherapy Department to ensure that all equipment, and especially equipment used for treatment of patients function according to specifications. Thus we ensure that cancer patients receive treatment of a high standard, that a cost-effective service is provided, and that the conditions for patients and staff are safe. New treatment techniques and computer procedures are also developed and verified before being implemented in co-operation with the personnel of the Oncotherapy Department.
Rendering of service
The medical physicist in the Department of Oncotherapy is involved in maintaining and ensuring the correct functioning of equipment employed for the purpose of localising and treating cancer tumours in patients. This includes accurate dosimetry or the measurement of radiation beam intensity and the administration of treatment according to the prescription of the oncotherapist. Regular quality control in respect of equipment is therefore essential. For this purpose we use different types of equipment to measure radiation intensity and dose such as ionisation chambers, semi-conductor radiation detectors, thermo luminescent dosimeters and X-ray film.
During the localisation of tumours we use diagnostic imaging procedures such as computed tomography (CT) and X-ray simulation. Medical physicists are responsible for quality control of the imaging equipment to ensure that tumour size and density is determined correctly for accurate computer planning of the radiation treatment of patients.
We accurately measure the radiation distribution obtained from linear accelerators during the acceptance of the machines. These measurements are transferred to a radiation treatment planning system that is computer based. This system is used to accurately simulate and optimise the distribution of the radiation dose before the patient is treated. Since we, as physicists, have an extensive knowledge of radiation interaction with tissue as well as computers and dose calculation, the algorithms employed by the computer are also tested to ensure that the calculated simulations are valid. We are also responsible for the maintenance of computers and involved in the development of new treatment techniques. Quality control in planning entails our checking each plan and dose calculation for the treatment of a patient before radiation starts.
A variety of radiation sources are used for the treatment of cancer patients. The linear accelerator is used to generate high energy X-rays (up to 20 million electron volt) for the treatment of deep lying tumours. High energy electrons (from 4 to 25 million electron volt) are also generated for the treatment of superficial tumours such as those encountered in patients with breast cancer.
Medical physicists are also involved in the improvement and development of treatment techniques. We optimise treatment by restricting the dose to normal tissue and organs to a minimum, while maximising tumour dose. In addition, new equipment is being evaluated to facilitate the treatment of patients.
Since the cost of equipment used for the treatment of cancer patients is extremely high, we are involved in the drawing up of specifications and the recommendation of purchase of equipment that will be the best for patient treatment. Following the purchase of such equipment, we run well-specified acceptance tests before the equipment is accepted for implementation of patient treatment. After the equipment has been put into service, we do daily quality control on the equipment to ensure that patients do indeed receive the correct prescribed radiation dose during treatment.
Radiation doses administered to individual patients during treatment is also checked regularly by means of thermoluminescent dosimeters. Doses are measured in cases where radiation sensitive organs, such as the eye, are located near the field of radiation, in order to ensure that such organs do not receive an unacceptably high dose.
Sealed radioactive sources are also placed temporarily in body cavities, such as the cervix, for the purpose of local irradiation of tumours. Sealed sources are also used for the treatment of superficial skin tumours. Sealed sources controlled by computerised equipment (after-loading device) are used for administering a very high local tumour dose and can be used in implantations (for instance in the tumour bed) and in natural body cavities (such as the bronchus and cervix). The advantage attached to this equipment is that personnel are not subjected to any radiation during the treatment of patients. Once again medical physicists are very closely involved, in the sense of being responsible for regular quality control of the equipment and planning of the patient's treatment.
Tumours are also irradiated by means of unsealed or liquid radioactive sources. Unsealed sources, such as radioactive Iodine-131 in liquid form, are administered for the treatment of a hyperactive thyroid or cancer of the thyroid, while radioactive phosphorous is used for the treatment of haematological conditions.
Research
We, as medical physicists in oncotherapy, are involved in a variety of research projects, most of which will eventually contribute towards the development and improvement of a computer system for the planning of radiation treatment of patients.
Before any treatment can be administered to a patient, relevant data, such as the outlines of the patient, tumour and radiation sensitive organs, must be known. One of the research projects is aimed at the acquisition of cross sectional tomographic images of patients. An X-ray simulator is used to collect data, which is then processed by means of back projection mathematical techniques in order to obtain a transverse image of the patient in the various planes of interest. The data is collected and processed using an inexpensive personal computer. The images are used in treatment planning to accurately localise organs and the body outline.
- Creation of phantoms to simulate patients
The radiation dose to the tumour and healthy organs of patients during treatment cannot be directly measured. Medical physicists are engaged in the development of tissue-equivalent phantom materials to simulate patient tissue. This will enable us to measure doses at various depths in 'patient-equivalent' phantoms to verify administered patient dose. The material therefore must be equivalent to different types of tissue and must reflect the density of, for instance, bone and muscle. Dosimetry carried out by the planning system before actual treatment can also be verified in these phantoms.
- Dosimetry by means of magnetic resonance imaging
The purpose of this project is to develop a method to measure the radiation dose distribution three-dimensionally by using magnetic resonance imaging equipment after irradiation of iron-sulphate gel. A tissue-equivalent iron sulphate gel is irradiated and the three-dimensional dose distribution is then displayed after imaging. This technique will be employed for radiation dose verification in cases where patients are treated with the use of multiple radiation modalities. Furthermore the technique is suitable for three-dimensional verification of dose distribution calculated by means of commercially available planning systems.
- Monte Carlo simulation technique
Monte Carlo simulations are used to test the accuracy of algorithms employed by planning systems to calculate dose distribution in patients. Monte Carlo calculation techniques are used to simulate three-dimensional dose distributions by using the basic geometry of the accelerator and the patient. These calculations take a long time to complete and are done on a computer workstation.