Summary 201 The incidence of prostate cancer is high: approximately 1 in 9 men are diagnosed with prostate cancer over the course of their lifetime. However, when prostate cancer is localised and contained within the prostate, the chance of curation is exceptionally high and – in selected patients – does not seem to depend on the type of treatment. Furthermore, prostate cancer survival is relatively high. Generally, patients with primary localised prostate cancer are treated with either surgery (prostatectomy) or radiotherapy. With prostatectomy, the entire prostate is surgically removed. Nowadays, this type of surgery is often performed using a surgical robot (i.e. Da Vinci robot). In case of radiotherapy treatment, patients can be treated with internal radiotherapy (brachytherapy) or with external beam radiation therapy (EBRT). With brachytherapy, radioactive sources are transported into the prostate via hollow needles. This way, the tumour is irradiated from the inside. With EBRT, the radiation enters the patient through the skin and travels through the body, with the highest intensity focused on the tumour. Radiotherapy is sometimes combined with androgen deprivation therapy (ADT), depending on the risk classification. Conventional image-guided radiotherapy and hypofractionation Radiotherapy treatment has seen some major advancements over the last decades. This all started with the introduction of image-guided radiation therapy (IGRT). Together with developments in treatment delivery, such as intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), IGRT has led to significant improvements in toxicity outcomes of radiotherapy treatment, due to improved accuracy of dose delivery. Besides improvements in imaging and treatment delivery, there has also been a shift towards delivering radiotherapy for prostate cancer in less fractions, i.e. hypofractionated radiotherapy. Nowadays, low- and intermediate-risk prostate cancer patients are often treated in only five treatment fractions, with fractional doses beyond 7.0 Gy. This is also known as ultra-hypofractionated radiotherapy or stereotactic body radiation therapy (SBRT). Ultra-hypofractionated radiotherapy is attractive, since prostate cancer seems sensitive to a high dose per fraction. Furthermore, ultrahypofractionated radiotherapy reduces the number of visits and improves departmental logistics. With the strive for shorter treatment schedules with higher fractional doses, the accuracy of treatment delivery becomes more and more important. Between fractions and during radiation delivery, changes in size, shape, and location of anatomical structures occur that affect the delivered dose to the target and the organs-at-risk (OARs). With conventional radiotherapy systems, a single treatment plan is created based on the anatomy prior to the actual treatment. During treatment, low-quality computed tomography (CT) scans are used to verify the position of the prostate. However, with conventional systems, no corrections can be performed for rotations of the prostate or for deformation and/or displacement of the OAR. This leads to differences between the planned dose and the actual delivered dose over the course of treatment. To make sure the target receives an adequate dose, an error margin (Planning Target Volume or PTV) around the target is applied. However, healthy tissues, such as the bladder and rectum, lie within this margin. 10
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