Monte Carlo computation of unwanted patient dose from a Medical Linear Accelerator

The principal radiation modality for treatment of deep seated tumours is x-rays of high energy and penetrating power. These x-rays are produced when high energy electrons are stopped in a target material such as tungsten. Alternatively, the electrons themselves may be used directly to treat superficial tumours. Radiation protection measures at an accelerator for external beam radiotherapy are aimed at protecting both the patient and the staff. Protection of staff is achieved by housing the accelerator in a room provided with adequate shielding barriers. During treatment, when a collimated beam of photons is directed towards a selected area (tumour) of the patient's body, the remaining portions of the body invariably receives a small amount of stray radiation. In the case of accelerators operating below 10 MV, the main contributor to the stray radiation is the photon radiation leaking through the protective barriers of the accelerator head. It is necessary that the leakage radiation is limited as per the limits specified by the Competent Authority. In India, these limits are based on the recommendations of the International Electrotechnical Commission (IEC). This paper deals with the computation of the unwanted photon dose to the patient due to leakage radiation from a 4 MV Medical Linear Accelerator designed and fabricated in India, using Monte Carlo Techniques. (author)