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Inherent Uncertainty in Megavoltage Cone Beam Computed Tomographic Image Guidance System

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Karrthick KP1*, Rajesh Thiyagarajan1 , Tamil Selvan1 , Arunai Nambiraj2 , Shikha Goyal1 , D Manigandan3 , Maragatha Veni1 , T Kataria1 , (1) Medanta The Medicity, Gurgaon, Haryana, (2) VIT University, Vellore, Tamil Nadu, (3) Fortis Cancer Institute, Fortis Hospital, Mohali, Punjab


SU-I-GPD-J-105 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To quantify inherent uncertainty of positional accuracy determined by megavoltage cone beam computed tomographic (MVCBCT) image guidance system

Methods: In this study, Tomo HD Tomotherapy system, anthropomorphic phantom were used. The phantom was placed on the treatment couch in a random position but close to the virtual isocenter of the machine. MVCBCT images were acquired and positional accuracy in translation and rotational directions was estimated by Bone+Tissue technique (BTT) and Full image technique (FIT). The difference in phantom position with respect to the planning position was observed but not corrected. This process was repeated for 15 times consecutively without any gap in a single day. Similar positional shifts were obtained for 25 working days. Everyday’s first set of shift was taken as reference and the deviation of the subsequent 14 sets were calculated. Mean and standard deviation (SD) were calculated for individual days as well as for all 25 days.

Results: Systematic and random error components are calculated from Mean and SD. For BTT Systematic error in lateral, vertical, longitudinal roll, pitch and yaw orientation were 0.21mm, 0.24mm, 0.25mm, 0.17˚,0.21˚ and 0.16˚ respectively. For FIT Systematic error in lateral, vertical, longitudinal roll, pitch and yaw orientation were 0.17mm, 0.14mm, 0.18mm, 0.20˚, 0.21˚ and 0.13˚respectively. The magnitude of random error component in BTT is found to be 0.09mm, 0.07mm, 0.09mm, 0.06˚, 0.08˚ and 0.06˚ in lateral, vertical, longitudinal, roll, pitch and yaw orientation. Similarly random error component in FIT is found to be 0.04mm, 0.07mm, 0.03mm, 0.02˚, 0.02˚ and 0.01˚ in lateral, vertical, longitudinal, roll, pitch and yaw orientation

Conclusion: The inherent uncertainty in MVCBCT positional accuracy is studied and quantified for BTT and FIT. It is found to be acceptable for intensity modulated radiotherapy delivery. Periodical quality assurance estimating systematic and random errors required to ensure localization accuracy.

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