Introduction: Image guided brachytherapy and conformal external beam in the treatment of carcinoma cervix enables calculation and modulation of dose to OARs like the bladder and rectum. In highly conformal techniques, PTV margins are calculated according to the setup error windows of respective individual institutions and it is essential for every institute to evaluate and analyze their setup error. An effective way of producing geometrical margins is by statistical models, which allow for reduction in margin sizes and which are smaller but effective in tumor control. The current study aims to evaluate the various setup errors incurred during treatment and thus calculate the adequate CTV/PTV margins for irradiating cancer cervix patients Objectives: Evaluate setup errors using available immobilization devices. Assess setup imaging and evaluation of interfractional movement. Analyze the total setup error (including setup and interfractional movement) and formation of guidelines for delineation of PTV margins. Materials & Methods: 50 patients undergoing pelvic RT for cancer cervix, using either or IMRT or IGRT technique were included in the study. Immobilization using a four clamp orfit and pelvic base plate, was done. 10 patients were treated by daily image guidance by kVCBCT and 40 patients were treated by weekly kV portal imaging. Translational shift (difference between planned and new coordinates) in all three directions recorded for each patient, separately for IMRT and IGRT groups. Mean and median shift and standard deviation in each direction was calculated. Results: Longitudinal shift was the greatest. Mean Isodisplacement vector calculated for IGRT treatment technique is 0.7 cm and for IMRT treatment technique is 0.96 cm. The difference between the two is statistically significant (p-value = 0.02). The standard deviation of shift in longitudinal direction was found to be 8 mm, 2.5 mm in the vertical direction and 3mm in the lateral direction. PTV margin calculated using the Van-Herk Margin formula for IGRT and IMRT treatments was found to be 0.21 cms and 0.39 cms respectively (Taking into account only random errors and assuming zero systematic errors). (p-value: 0.01).

Keywords: PTV Margin; Cancer Cervix; IGRT Planning.