Intensity Modulated Imaging?: Clinical Workflow For Fluence Field Modulated Ct On A Tomotherapy System

Purpose: This work presents a new approach that uses the multi leaf collimator present on TomoTherapy devices to modulate the imaging beam. This allows for targeted imaging in which only regions of clinical importance for IGRT (patient positioning confirmation) receive a high level of image quality. This allows the total imaging dose to decrease, especially for healthy tissue. Methods: A clinical TomoTherapy machine was programmed to deliver imaging dose such that small pre-defined volumes of interest (VOI) received relatively higher levels of image quality with respect to surrounding regions. Four different size ROIs were placed at varying distances from isocenter. The noise and mean CT number were compared between VOI and un-modulated scans. Dose distributions were generated using a treatment planning dose calculator. A clinically feasible workflow was developed to implement this technique that used treatment planning contours to define VOI positions. Results: The VOI-FFMCT technique produced an image noise within 5% of an unmodulated scan for all size VOIs. The VOI technique required a total imaging dose of 0.61, 0.69, 0.60, and 0.50 times the “full dose” acquisition dose for VOI sizes/locations of 10/13/10/6 cm in diameter and located 0/2/5/6 cm from isocenter respectively. Mean CT numbers for the VOI scans were within 1% of the unmodulated case. Conclusion: This approach, which combines state-of-the-art radiation therapy fluence control with a novel imaging approach has been implemented on a clinical system with no hardware or software changes. A clinically feasible method for implementing this technique was developed which requires no additional user input relative to the current procedures in our clinic. The method allows physicians to choose between: (1) better image quality at no dose penalty or (2) equal image quality while reducing dose levels relative to today’s standard of care. Research grants from GE HealthCare and TomoTherapy.