Background: IGRT is the latest development for increasing the precision and accuracy in radiation therapy. Usage of specialized multi professional teams for coordinating geometric verification and use of clearly defined protocols are essential for IGRT. Reducing dose to the OARs using IMRT & VMAT and reducing the size of PTV using IGRT enables radiation dose escalation, which in turn improve the treatment outcomes. 

Aims: To evaluate set up errors in patients treated with Intensity modulated radiotherapy / Volumetric modulated arc therapy using Cone beam CT (CBCT) in head and neck and pelvic cancer sites and to study adequacy of PTV margin by applying set up errors. 

Methods and Materials: A total of thirty two patients were enrolled into the study, which included 16 Head and neck site, 16 pelvic sites that were planned with either IMRT or VMAT. CBCT was acquired for each patient on day 1, 2, 3 and thereafter every week until the completion of treatment. The CBCT images were fused with planning CT images. The PTV margin was estimated by calculating the systematic and random errors in X, Y, Z directions. 

Results: Data collected was analyzed with single sample t test. A total of 288 CBCT were taken for this study. The systematic and random errors were calculated in both head and neck and pelvis 16 patients each. The systematic error for H&N site in cranio caudal (CC), medio lateral(ML), anteroposterior (AP) were 0.2cm, 0.2 cm, 0.2 cm respectively. The random errors in H&N sites patients in CC, ML, and AP were 0.2 cm, 0.2cm, 0.2 cm respectively. The PTV margin for H&N site in CC, ML, AP were 0.7cm, 0.5cm, 0.5cm respectively. For pelvic site, the systematic error in CC, ML, AP were 0.3 cm, 0.2 cm, 0.2cm respectively, where as the random errors were 0.2 cm, 0.3cm, 0.2 cm respectively in the same direction. The estimated PTV margin for pelvic site in CC, ML, AP were 0.8cm, 1.0cm, 0.7cm respectively. 

Conclusion: For H & N and pelvic sites, treated with IMRT/VMAT the PTV margin of 5mm is not sufficient. Hence treatment verification with IGRT is very crucial to identify and correct the set up errors in order to deliver the planned treatment. 

Keywords: Image Guided Radiotherapy (IGRT); Intensity Modulated Radiotherapy (IMRT); Volumetric Modulated Arc Therapy (VMAT)