External radiation beam treatment of cancer tumours involves delivery of invisible radiation beams through the body where internal structures can not be seen. Beam targeting of patient anatomy has to very accurate to achieve the desired therapeutic result. Good understanding of radiotherapy treatment (RT) concepts is essential to training. This paper presents a virtual environment simulator developed by the authors for training and education of intensity modulated radiotherapy (IMRT) treatment of cancer. This simulator employs immersive visualization to provide a high fidelity spatial awareness of the complex relationships between tumour, organs at risk, treatment beam and radiation dose. All these visualization are provided by a 3D virtual environment based on the patient in a RT treatment room. Immersive visualization using this simulator is being used to train radiation oncologist and radiation physicists about radiotherapy treatment.