[Application of image-guided radiotherapy in external beam radiation of cancer patients]

The aim of the study was to present the clinical application of megavoltage cone beam CT (MV-CBCT) for image-guided radiotherapy at different tumour sites in our department. Altogether 2772 CT examinations were performed to verify the accuracy of patient setup before irradiation of 462 patients with pelvic (n=281), thoracic (n=107), head and neck (n=33) and cranial (n=41) tumours. A MV-CBCT with 6 MV photon beam integrated into a linear accelerator was used for imaging. The verification CT images were registered to planning CTs using bony structures, and in the three main directions (lateral, longitudinal, vertical) deviation between treatment and planning isocentres was determined in order to characterise the accuracy of patient setup. The verifications were performed before the first four fractions, and weekly-biweekly thereafter. From data obtained during the first three measurements systematic error of patient setup was calculated "off line", and the setup was corrected with the calculated value. At errors larger than 5 mm "on line" table correction was applied. The measured data were grouped and analysed according to location, and systematic and random errors were determined. From the data safety zone around clinical target volume (CTV) was calculated to create planning target volume (PTV). Following isocentre correction after the first three fractions the patient setup became more accurate at all site locations. At pelvic irradiation the mean error in all the three directions was below 1 mm, and the range of standard deviation was 0.32-0.38. At pelvic and thoracic irradiation the CTV-PTV safety zone calculated without any correction was 9-13 mm depending on direction, while at head and neck and cranial irradiation it was 6-9 mm. After correction of systematic error these data were 7-9 mm and 3-6 mm. After on line correction of setup errors larger than 5 mm the safety zone was 5-6 mm at pelvic and thoracic irradiation, 5 mm at head and neck, and 3-5 mm at cranial irradiation. Verification of patient setup with MV-CBCT at different locations can be easily performed. The initial systematic error can be corrected with a simple verification protocol which results in a few millimeter decrease of the CTV-PTV safety zone. Use of smaller safety zone is possible only with more frequent verifications and on line corrections.