Teleradiotherapy is a form of tumour treatment using ionising radiation with the source of radiation outside patient’s body. It is important to understand physical basis and principles that guide various interactions of ionising radiation, its effect on tissues, as well as the energy deposition in tissues. A dose is delivered to a patient using high energy x-rays and/or electrons created by linear accelerator - a machine for the treatment of cancer with ionising radiation. Dose delivery in radiotherapy is preceded by the process of planning which combines and modifies radiation fields so as to deliver the proper dose to a specific site. Once the available dose volume histograms and isodose distribution fulfil the prescribed requirements, the optimal plan is obtained. Periodic quality assurance of the linear accelerator is necessary for the proper operation of the machine. Weekly procedures assure that the internal dosimetry system is equivalent to the external dosimetry system whose readings are corrected with correction factors determined by the protocol. By means of this, one monitor unit read off by the machine is made equal to 1 cGy dose. A computer isodose plan and weekly quality assurance of linear accelerator were made at Clinic for oncology and radiotherapy KBC Split.