Introduction: Breast cancer is the most common cancer among women. It can be irradiated in different positions, such as supine position on Breastboard. This position is the most usable for patients with small breasts. Breast cancer can also be irradiated in prone position on Sagittilt. Patient with larger breasts (at least bra size C) are irradiated in this position. Not being able lying on patient's abdomen is the greatest contraindication. Purpose: In our diploma work we wanted to figure out in which position during geometric verification before radiotherapy, appear to be bigger isocentric deviation in lateral, longitudinal or vertical direction and how big are those. In addition we have researched if and how patient's age and number of fractions affect size of interfraction movements. Methods: We have used descriptive method and statistical data analysis. We have compared interfraction movements of 50 patients; 25 patients had been irradiated in supine position on Breastboard and 25 patients had been irradiated in prone position on Sagittilt (Orfit). All patients had been irradiated with linear accelerator Elekta Synergy at Onkološki inštitut Ljubljana. Results: From obtained results and statistical analysis we found out that the interfraction movements are bigger in prone position, namely in lateral direction. We have also found out that patient's age and number of fractions do not have any effect on interfraction movements. Discussion and conclusion: To conclude larger movements are bigger in prone position; this can be mostly due to patient's discomfort, but not patient's age or number of fractions. It is important to ensure same position during radiotherapy and to prevent set-up errors, which can be reached using daily geometric verification.
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