X Chen*, E Paulson, E Ahunbay, E Omari, W Hall, B Erickson, X Li, Medical College of Wisconsin, Milwaukee, WI

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  X Chen

PO-GePV-M-90 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: The intrafraction anatomic change, especially the air cavity variability in the abdominal region, can be severe which can affect the accuracy of MR-guided adaptive radiation therapy (MRgART). The purpose of this study is to accurately quantify the intrafraction abdominal air cavity change using a dedicated MRI sequence and to study its dosimetric effect on MRgART.

Methods: Abdominal air pockets were imaged using a dedicated, free-breathing air scan MRI sequence (3D FLASH, 1 msec echo time, 9 second scan time). A total of 21 air scans were acquired during pre-treatment imaging study sessions for 5 pancreatic cancer patients, each with 4 or 5 air scans separated by 2-5 minutes. Changes in all air cavities compared to the first air scan for each patient were quantified using the Dice similarity coefficient (DSC). An adaptive plan was generated on the first air scan and was re-calculated on subsequent air scans with the same beam settings. Changes of the dose to a 0.03cc volume (D0.03cc) for stomach, duodenum, colon and small bowel, and PTV coverage by 100% prescription (V100%) from the first air scan were calculated.

Results: The volume of the air cavities ranged from 10cc to 50cc in the treatment field. Average DSC dropped to < 0.2 after 5 minutes. For two patients, the D0.03cc was increased by 15% to 20% in 2 to 5 minutes, while for the other three patients, the D0.03cc increment was <4.7%. The PTV V100% dropped by 17% for one patient.

Conclusion: Intrafraction changes in the position and shape abdominal air cavities during 20-25 minutes may be substantial, and can result in considerable dose increases to critical organs and/or decreases in target coverage. These data suggest the need for developing real-time adaptive planning solutions to account for dynamic intrafraction air cavities changes.

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