Room: Exhibit Hall | Forum 2
Purpose: With the recently-introduced MR-Linac, MRI-guided online adaptive radiation therapy (MRgOART) has been used clinically to address interfraction variations. A strategy to manage intrafraction motions is to adapt plan based 4D-MRI. This study investigates the necessary margin required to expand clinical target volume (CTV) to the planning target volume (PTV) for adapting plan based on mid-position (MidP) images generated from 4D-MRI.
Methods: Daily 14 4D-MRI datasets acquired for one pancreatic cancer and two liver cancer patients treated with stereotactic radiation therapy on a high-field MRLinac (Unity, Elekta) were analyzed. The MidP images were generated from these 4D-MRI in MIM software and were used to generate adaptive plans. Using the MidP set, motion-incorporated CTV was delineated and was expanded to create the PTV with a margin that is necessary to deliver the prescribed dose to at least 95% of the CTV. This CTV-PTV margin was computed using the Van Herk margin recipe considering inter- and intra-fraction variations, motion amplitude, and beam penumbra. The obtained margins and PTVs along with the resultant adaptive plans were compared with the commonly-used margins and PTVs based on internal tumor volume (ITV) and the corresponding plans.
Results: Compared to the ITV method, the MidP method leads to margin reductions of up to 3 mm, thus, the PTV reductions of up to 66%, for the cases studied with intrafraction motion amplitudes of 6-15 mm. The average reduction of mean doses to the normal liver and kidneys and the maximum doses to small bowel were 22.0%, 19.9% and 14.3%, respectively.
Conclusion: The use of mid-position images from 4D-MRI reduces the PTV as compared to that based on ITV, substantially improving normal tissue sparing in MRgOART for abdominal and thoracic tumors.