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External beam


The process of radiation treatment for cancer patients involves many steps. One crucial step in this process is the preparation of  the optimal treatment plan that will deliver a high dose to the diseased volume and an acceptable dose to  healthy tissues which may be at risk for complications. Treatment planning includes the use of 3-dimensional imaging to determine the precise location of the malignant disease as well is the critical structures to be avoided. Usually this involves CT scanning but other imaging modalities such as magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission tomography (SPECT), or ultrasound may aid this process as well. Once the target volume and normal tissues have been delineated, the optimum beam arrangement is determined using a treatment planning computer with sophisticated radiation dose calculation software.

Important Principles

An important aspect of treatment planning includes accurate patient positioning such that the treatment set-up is reproducible from the imaging device to the therapy machine and then for each daily treatment fraction. In the treatment planning computer, the dose calculation algorithm must be able to predict the dose delivered to the tumour and different normal tissues accurately. Furthermore, with modern therapy machines that are capable of beam intensity modulation, the beam directions need to be determined and automated optimization routines must be capable of providing the best multileaf collimator configurations. Plan evaluation is usually performed with the use of dose-volume histograms (DVH).

Introduction to References

Chapter 7 of the IAEA Handbook for Teachers and Students gives a good summary of the various activities involved in the treatment planning process. IAEA TRS-430 and in AAPM TG-53 have treatment planning process summaries and contain the details of the acceptance, commissioning and quality assurance of radiation treatment planning systems.