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Impact of Imaging Quality On Interfraction Patient Setup Errors and Autocorrelation in Image Guided Head-And-Neck Radiotherapy

S Qi

S Qi*, A Hu, UCLA School of Medicine, Los Angeles, CA

WE-G-141-2 Wednesday 4:30PM - 6:00PM Room: 141

Purpose: To assess the impact of imaging quality of three commonly used on-board image-guidance systems on interfraction patient setup errors for head-and-neck (H&N) cancer irradiation. Serial correlation patterns and trend of the setup errors over the treatment course were investigated via time-series analysis.
Methods: 117 H&N patients imaged with different IGRT modalities: 29 kilovoltage cone beam CT (KVCBCT, Synergy, Elekta Inc.), 35 megavoltage fan beam CT (MVFBCT, TomoTherapy, Accuray Inc.), and 53 megavoltage cone beam CT (MVCBCT, MVision, Siemens Inc.) were studied retrospectively. The daily displacements in mediolateral (ML), craniocaudal (CC) and anteroposterior (AP) dimensions were investigated. The CTV-to-PTV margins were calculated using 1.96σp, where σp was random error for the pooled standard deviations in each cohort. Time series analysis was performed to further investigate possible autocorrelations and time trend of the setup errors serially for each patient during the course of the treatment.
Results: Based on a total of 3302 pre-treatment CT scans, including 632 KVCBCT, 974 MVFBCT and 1696 MVCBCT scans, the maximal random errors in three translational directions for H&N cases were 1.6 (KVCBCT), 1.7 (MVFBCT) and 2.1 mm (MVCBCT), the calculated CTV-to-PTV margins that would encompass 95% of the cohort were 3.1, 3.3 and 4.2 mm respectively. Approximately 24%, 29% and 52% of the shifts were auto correlated in one of the three translational directions with KVCBCT, MVFBCT and MVCBCT, approximately half of the autocorrelations were found in AP direction.
Conclusion: CTV-to-PTV margin for H&N IMRT may be a function of the imaging modality if determined based on IGRT data. Superior image quality in KVCBCT, compared to MVFBCT and MVCBCT, results in smaller random errors and smaller CTV-to-PTV margins. Coincidentally, modality with superior image quality also resulted in less autocorrelation serially in setup errors. IGRT modalities with better image quality are encouraged in clinical practice.

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