3D deformable registration for monitoring radiotherapy treatment in prostate cancer Borja Rodr´ ıguez-Vila 1 , Johanna Pettersson 2 , Magnus Borga 2 , Feliciano Garc´ ıa-Vicente 3 , Enrique J. G´ omez 1 , and Hans Knutsson 2 1 Bioengineering and Telemedicine Group, Universidad Polit´ ecnica de Madrid, Spain {brvila,egomez}@gbt.tfo.upm.es 2 Department of Biomedical Engineering and Center for Medical Image Science and Visualization, Link¨oping University, Sweden {johpe,knutte,magnus} @imt.liu.se, 3 Medical Physics, Radiotherapy Department, University Hospital La Princesa, Spain fgarcia.hlpr@salud.madrid.org Abstract. Two deformable registration methods, the Demons and the Morphon algorithms, have been used for registration of CT datasets to evaluate their usability in radiotherapy planning for prostate can- cer. These methods were chosen because they can perform deformable registration in a fully automated way. The experiments show that for in- trapatient registration both of the methods give useful results, although some differences exist in the way they deform the template. The Mor- phon method has, however, some advantageous compared to the Demons method. It is invariant to the image intensity and it does not distort the deformed data. The conclusion is therefore to recommend the Morphon method as a registration tool for this application. A more flexible regu- larization model is needed, though, in order to be able to catch the full range of deformations required to match the datasets. 1 Introduction Prostate cancer is the third most common cause of death from cancer in men of all ages and it is the most common cause of death from cancer in men over the age of 75 [8]. External beam radiotherapy has shown to be an effective treatment for localized prostate cancer in early stages. Radiation dose to the prostate was earlier very limited because of concern about normal tissue toxicity. Now three-dimensional conformal radiotherapy has allowed safe dose escalation to treat prostate cancer, which makes it possible to apply higher radiation doses on cancerous tissues and, at the same time, reduce the dose in healthy tissues [7]. There are several side effects associated with radiation therapy, including for example rectal bleeding and hematuria (blood in urine), basically derived from the undesired but unavoidable radiation of rectum and bladder [2]. This implies that careful consideration must be taken to the dose-volume histograms of normal tissues to avoid excessive toxicity in these regions. Before beginning radiotherapy treatment CT scans have to be obtained for computerized treatment planning, to determine the most appropriate way to deliver radiation therapy. The radiotherapy team selects the target volume and