Effect of Imaging Frequency on PTV Margins and Geographical Miss during Image Guided Radiotherapy for Prostate Cancer

Publication date: Available online 7 October 2017
Source:Practical Radiation Oncology
Author(s): Meetakshi Gupta, Poonam Gamre, Sadhana Kannan, Ganesh Rokde, Rahul Krishnatry, Vedang Murthy
BackgroundThe relationship between frequency of imaging during Image Guided Radiotherapy (IGRT) and Planning Target Volume (PTV) margin remains unclear. This issue is of practical significance given resource and time intensive nature of image guided radiation therapy (RT). The purpose of this study was to evaluate PTV margins with predefined and commonly used less-than-daily IGRT schedules using data obtained from patients treated with daily IGRT for prostate cancer.Methods and materialsDaily set up error and 3 dimensional daily alignment data for a total of 108 consecutive patients with prostate cancer treated with 2700 fractions of daily image guidance on Tomotherapy was retrospectively analysed. Five IGRT scenarios were simulated, namely alternate day, twice weekly, once weekly, first 3days only and no image guidance. The daily alignment data was modelled to simulate the 5 predefined scenarios by applying appropriate corrections to determine the PTV margin for each image guidance scenario. The data was also analysed to predict possible geographical miss in any direction using two frequently used PTV margins of 7mm and 5mm, for all the scenarios.ResultsDecreasing frequency of image guidance increased the mean systematic error and the standard deviation of the systematic error. With decrease in image guidance frequency, an increase in PTV margins was required to achieve adequate coverage of the clinical target volume. With reduction in image guidance from 50 to 12%, a gradual increase in percentage of fractions with predicted geographical miss using an isotropic PTV margin of 7 or 5mm was seen. With every 15% decrease in imaging, a 5% increased risk of geographical miss was estimated.ConclusionThe use of less than daily image-guided RT requires larger PTV margins for patients treated with IMRT for prostate cancer. With every 15% reduction a 5% increased risk of geographical miss was estimated.



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